In a politically divided nation, with attitudes among many voters hardened and resistant to change, the 2020 US presidential election could be contested on the narrowest electoral terrain in recent memory.

Just four states are likely to determine the outcome in 2020: Pennsylvania, Michigan, Wisconsin and Florida. Each flipped to the Republicans in 2016, but Donald Trump won each by only a percentage point or less.

Many analysts point to Wisconsin as the single state upon which the election could turn.

Shifting demographics, the growing urban-rural divide and the gap between white voters with and without college educations have helped to create an electoral map unlike those of the recent past. So too have the US president's unique profile, messaging and appeal.

“Because of the partisanship of the country and the partisanship of the president, we are now looking at the smallest map in modern political history,” said Jim Messina, who was the campaign manager for former president Barack Obama's 2012 re-election campaign.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

Both Mr Trump's campaign and that of his eventual Democratic challenger will seek to put other states in play. But those opportunities are fewer than in past campaigns.

Mr Trump has done nothing to expand his base while in office, which Democrats claim will make it extremely difficult for him to win states he lost in 2016. Trump campaign officials disagree.

Democrats' aspirations for expansion rest in part on whether politically changing, Republican-held states such as North Carolina, Georgia and Arizona are truly ready to shift.

Current polling nationally and in some of the key states shows the president vulnerable when matched against several of the Democratic presidential candidates - though he overcame weak approval and favourability ratings to win the 2016 election.

Based on current attitudes, he will have to do so again to win re-election.

One obvious wild card is the identity of the Democratic nominee and how that shapes the election debate. Will that nominee be running on a platform that moderate voters see as too far left? Will that nominee be able to energize the party's woke base and still appeal to white working-class voters?

Regardless of who that person is, the 2020 election will put a focus on several demographic groups in particular.

First, white working-class voters who went strongly for Mr Trump and are an important part of the Republican base. Of particular concern for the president will be white women without college degrees.

Second, college-educated suburban voters, especially women, who have moved decisively into the Democrats' coalition and who powered the party's gains in 2018 House races.

Third, African-Americans, and particularly younger African-Americans, whose turnout levels will be critical to Democrats' fortunes.

Fourth, Hispanic voters, who will play a key role in Florida and some other states, especially in the West.

How strongly each of these groups supports Republican nominee Mr Trump or the Democratic nominee and the numbers by which they turn out to vote are variables that election modellers are analysing closely and tweaking regularly even at this early stage.

Marianne Williamson warns of Donald Trump's 'dark psychic force' in presidential debate

In 2016, Mr Trump lost the popular vote to Hillary Clinton by roughly 3 million people but won 304 electoral votes and the presidency. Based on current polling, his chances of winning the popular vote are at least as challenging as in 2016, leaving open the question of whether he can again produce an electoral college majority.

The electoral college combinations all begin with Florida and then move to the upper Midwest. One scenario not out of the realm of possibility would produce a 270-268 victory for Mr Trump, but only if he again secures one of Maine's four electoral votes.

If Mr Trump were to win Florida again, Democrats would need to recapture all three of the northern states - or find substitutes - to win the White House. If Democrats could win Florida, any one of the three in the upper Midwest would give them the White House, unless the US president can put something else in his column.

The three northern states were part of a group of 18 states and the District of Columbia that had voted for the Democratic nominee in every election between 1992 and 2012, what Democrats believed constituted a “blue wall” of resistance to repeated incursions by Republican nominees.

Mr Trump broke through that wall, threading the narrowest of paths to victory. His combined margin in those three states, however, was just 78,000 votes out of nearly 14 million cast.

Donald Trump's least presidential moments so far... Show all 30 1 /30 Donald Trump's least presidential moments so far... Donald Trump's least presidential moments so far... Threatening to shut down Twitter after being fact-checked After the president tweeted that voting by post would be "substantially fraudulent", Twitter attached a warning label to his tweet and referred readers to a site which explained how the claim was "unsubstantiated". Trump then said Twitter was "stifling free speech" and that he may have to shut it down, something which he would not have the power to do AFP/Getty Donald Trump's least presidential moments so far... Flippantly dismissing a serious allegation of sexual assault When author E Jean Carroll accused Trump of raping her, the president responded: “Number one, she’s not my type. Number two, it never happened. It never happened, OK?" AFP/Getty Donald Trump's least presidential moments so far... Insulting the Mayor of London as he landed in London Just before touching down at Stansted Airport for his state visit, Trump took time out to @ the London mayor Sadiq Khan on twitter. He said that Khan has done a "terrible job"as mayor and that he is a "stone cold loser" Reuters Donald Trump's least presidential moments so far... Taking plenty of "Executive Time" The president's official schedule sets aside the hours from 8 to 11am daily for "Executive Time". Further intermittent periods of "Executive Time" are scheduled throughout any given day, ranging from 15 minutes to 3 hours. His duties in these hours have not been officially disclosed, though Axios reports that he spends them watching TV, reading the newspapers and tweeting Getty Donald Trump's least presidential moments so far... Shutdown the government for over a month in an effort to secure funding for his wall With Mexico declining to pay for the wall, the president has faced difficulty in raising the required $5bn at home. Due to his demand that the money for the wall be included in the budget, and Congress's refusal, the government partially shut down on 22 December 2018. It remained shut for over a month, the longest period in history Getty Donald Trump's least presidential moments so far... Joking about the Nazi occupation of France to President Macron In this tweet from 13 November 2018, the president mocks Emmanuel Macron's suggestion of a "true, European army" by invoking the conflict between France and Germany in the world wars Donald Trump's least presidential moments so far... Railing against the Mueller investigation The president has repeatedly claimed that the Mueller investigation into Russian interference in the 2016 election, is a "rigged witch hunt" Reuters Donald Trump's least presidential moments so far... Contradicting a US intelligence report on Russian meddling in the presence of Vladimir Putin In the press conference that followed his landmark meeting with Russian president Vladimir Putin, Trump stated that he saw no reason why Russia would have meddled in the 2016 US election. This contradicted a 2017 report by the US Office of the Director of National Intelligence that found evidence of Russian interference in favour of Trump Getty Donald Trump's least presidential moments so far... Contradicting his contradiction of a US intelligence report on Russian meddling Following furious backlash in the US, the president claimed that he meant to say that he saw no reason why it would not have been Russia who meddled in the 2016 US election. As to why he would have intended to use such bizarre phrasing, he did not comment Reuters Donald Trump's least presidential moments so far... Colouring in the US flag wrong The president coloured in the US flag wrongly during a visit to a children's hospital in Columbus, Ohio. He added a blue stripe where in tradition, and statute, there have been only white and red stripes AFP/Getty Donald Trump's least presidential moments so far... Firing a Secretary of State over Twitter The president announced on Twitter that he was appointing Mike Pompeo as Secretary of State, much to the surprise of then Secretary of State Rex Tillerson Donald Trump's least presidential moments so far... Quoting a catchphrase from a reality TV show when discussing police brutality While addressing the issue of black athletes not standing for the national anthem in protest of police brutality, the president made reference to his catchphrase from reality TV show "The Apprentice": you're fired! Reuters Donald Trump's least presidential moments so far... Calling African nations "S***hole Countries" Ever one for diplomacy, the president reportedly referred to African nations as "s***hole countries". Asked to confirm this when meeting with Nigeria's President Buhari, Trump stated that there are "some countries that are in very bad shape". Reuters Donald Trump's least presidential moments so far... Defending Russian President Vladimir Putin Trump appeared to equate US foreign actions to those of Russian president Vladimir Putin, saying: “There are a lot of killers. You think our country’s so innocent?” Reuters Donald Trump's least presidential moments so far... Asking for people to 'pray' for Arnold Schwarzenegger At the National Prayer Breakfast, Trump couldn’t help but to ask for prayers for the ratings on Arnold Schwarzenegger’s show to be good. Schwarzenegger took over as host of “The Apprentice” — which buoyed Trump’s celebrity status years ago Getty Donald Trump's least presidential moments so far... Hanging up on Australian PM Malcolm Turnbull Early in his presidency, Trump reportedly hung up the phone on Australian prime minister Malcolm Turnbull after the foreign leader angered him over refugee plans. Mr Trump later said that it was the “worst call” he had had so far Getty Donald Trump's least presidential moments so far... The 'Muslim ban' Perhaps one of his most controversial policies while acting as president, Trump’s travel ban targeting predominantly Muslim countries has bought him a lot of criticism. The bans were immediately protested, and judges initially blocked their implementation. The Supreme Court later sided with the administration’s argument that the ban was developed out of concern for US security Getty Donald Trump's least presidential moments so far... Praising crowd size while touring Hurricane Harvey damage After Hurricane Harvey ravaged southeastern Texas, Trump paid the area a visit. While his response to the disaster in Houston was generally applauded, the president picked up some flack when he gave a speech outside Houston (he reportedly did not visit disaster zones), and praised the size of the crowds there AP Donald Trump's least presidential moments so far... 'Little Rocket Man' During his first-ever speech to the United Nations General Assembly, Trump tried out a new nickname for North Korea leader Kim Jong-un: Rocket Man. He later tweaked it to be “little Rocket Man” as the two feuded, and threatened each other with nuclear war. During that speech, he also threatened to totally annihilate North Korea Donald Trump's least presidential moments so far... Attacking Sadiq Khan following London Bridge terror attack After the attack on the London Bridge, Trump lashed out at London Mayor Sadiq Khan, criticising Khan for saying there was “no reason to be alarmed” after the attack. Trump was taking the comments out of context, as Khan was simply saying that the police had everything under control Getty Donald Trump's least presidential moments so far... Claiming presenter Mika Brezinkski was 'bleeding from the face' Never one not to mock his enemies, Trump mocked MSNBC’s “Morning Joe” co-host Mika Brzezinski, saying that she and co-host Joe Scarborough had approached him before his inauguration asking to “join” him. He noted that she was “bleeding badly from a face-lift” at the time, and that he said no MSNBC Donald Trump's least presidential moments so far... Claiming the blame for Charlottesville was on 'both sides' Trump refused to condemn far-right extremists involved in violence at 'the march for the right' protests in Charlottesville, even after the murder of counter protester Heather Heyer AP Donald Trump's least presidential moments so far... Retweeting cartoon of CNN being hit by a 'Trump train' Trump retweeted a cartoon showing a Trump-branded train running over a person whose body and head were replaced by a CNN avatar. He later deleted the retweet Donald Trump's least presidential moments so far... Tweeting about 'slamming' CNN Trump caught some flack when he tweeted a video showing him wrestling down an individual whose head had been replaced by a CNN avatar. Trump has singled CNN out in particular with his chants of “fake news” Donald Trump's least presidential moments so far... Firing head of the FBI, James Comey Trump’s firing of former FBI Director James Comey landed him with a federal investigation into Russia’s meddling in the 2016 election that has caused many a headache for the White House. The White House initially said that the decision was made after consultation from the Justice Department. Then Mr Trump himself said that he had decided to fire him in part because he wanted the Russia investigation Mr Comey was conducting to stop Getty Donald Trump's least presidential moments so far... Not realising being president would be 'hard' Just three months into his presidency, Trump admitted that being president is harder than he thought it would be. Though Trump insisted on the 2016 campaign trail that doing the job would be easy for him, he admitted in an interview that living in the White House is harder than running a business empire Reuters Donald Trump's least presidential moments so far... Accusing Obama of wiretapping him Trump accused former president Barack Obama of wire tapping him on twitter. The Justice Department later clarified: Obama had not, in fact, done so Reuters Donald Trump's least presidential moments so far... Claiming there had been 3 million 'illegal votes' Trump was never very happy about losing the popular vote to Hillary Clinton by 2.8 million ballots. So, he and White House voter-fraud commissioner Kris Kobach have claimed that anywhere between three and five million people voted illegally during the 2016 election. Conveniently, he says that all of those illegal votes went to Clinton. (There is no evidence to support that level of widespread voter fraud.) Donald Trump's least presidential moments so far... Leaving Jews out of the Holocaust memorial statement Just days after taking office, Trump’s White House issued a statement on International Holocaust Remembrance Day, but didn’t mention “jews” or even the word “jewish” in the written statement Getty Donald Trump's least presidential moments so far... Anger over Inauguration crowd size Trump’s inauguration crowd was visibly, and noticeably, smaller than that of his predecessor, Barack Obama. But, he really wanted to have had the largest crowd on record. So, he praised it as the biggest crowd ever. Relatedly, Trump also claimed that it stopped raining in Washington at the moment he was inaugurated. It didn’t, the day was very dreary Reuters

Election analyst Ruy Teixeira said the three states were so closely decided that even small changes could shift them to the Democrats, from the demographic changes taking place since the 2016 election to a white voting bloc that is moving away from Republicans or even to modest increases in turnout in the minority community.

“There are a lot of knobs you could twist and you don't have to twist them very far to move them into the Democrats' column,” he said. For Mr Trump, that means there is an “absolute necessity” to maintain and likely to increase his margins among white, non-college voters next year, he added.

While many Democrats are optimistic that the gains in the 2018 midterms foreshadow success in 2020, a report earlier this year by the progressive firm Catalist noted: “It is not safe to assume that Democratic gains from 2016 to 2018 will hold.”

In past campaigns, when there were a dozen or more truly competitive battlegrounds, presidential nominees could chart multiple paths to 270 electoral votes and worked to keep as many of those options alive as long as possible.

Over many years, however, growing polarisation has created red and blue strongholds, with bigger and bigger and bigger victory margins. In 2016, with a narrow popular vote margin, more than two dozen states were decided by margins of 15 points or more. In 1988, when the popular vote margin was seven points, there were just 17.

The electoral map is never truly static for long. Before the Democrats' “blue wall” there was the so-called “Republican lock” on the electoral college. Years ago, California, Illinois and New Jersey were presidential battlegrounds. Today all are solidly Democratic. Missouri long was considered a bellwether state. Republican candidate Trump won it by almost 19 points.

One way of looking at how the electoral map has changed in recent years is to gauge which states are most likely to provide the final votes needed to hit the necessary 270. During the two elections won by Mr Obama, Virginia and Colorado were perched at that tipping point.

Today, because of Democratic gains among college-educated voters, both have moved significantly towards the Democrats.

One of the most significant changes in the 2020 map is the reduced role Ohio will play in the calculations of the campaigns. Underlying trends have moved Ohio towards the Republicans in statewide races, and Mr Trump's candidacy provided an extra boost.

Democrats say they will not ignore the state, although how much they invest there is questionable. A Quinnipiac survey in June showed former vice president Joe Biden leading Mr Trump in Ohio by eight points.

If the president is trailing in Ohio by that margin in the late fall of 2020, his re-election prospects probably would be doomed.

Boris Johnson: Trump election 'a moment of opportunity' for Britain

Iowa is another example of how Mr Trump's appeal to rural areas and to older, predominantly white voters has changed the electoral map. No state in the country saw as many counties that had voted for Democratic nominees in five or more elections consecutively switch allegiance and support Mr Trump.

He won the state by nine points in 2016, and strategists in both parties see it as less competitive than other places. But the president's tariffs have hit the Midwest farmers hard. If they are feeling pain next year, Mr Trump's hold on Iowa could be at risk.

Florida has been ground zero in presidential politics for two decades. George W Bush won it by 537 votes in the disputed election of 2000. Mr Obama carried it by a fraction of a percentage point in 2012. Mr Trump won it by slightly more than a percentage point in 2016. In 2018, the gubernatorial and Senate races there were decided by margins of less than half a percentage point.

With 29 electoral votes at stake, Florida is the biggest competitive prize on the map. Neither Mr Trump nor the Democratic nominee can afford not to invest the maximum there. However at this point, the US president appears to have a slight advantage.

Florida is also the most complex and costliest of the major battlegrounds, and for Mr Trump, it is the most important state in the country.

His team has divided the country into nine political regions. Only Florida constitutes its own region. The president's re-election campaign even opened with a rally in Orlando.

Both sides know what they need to do in Florida. Compared with some other states, Florida's electorate is not just narrowly divided but also relatively inelastic. Unlike some other states, where Mr Trump's candidacy produced different regional margins from past elections, voting patterns in Florida more closely followed those of 2012.

Unlike the northern states, Florida was a state where both Mr Trump and Ms Clinton got more votes than Mitt Romney and Mr Obama in 2012. For 2020, the issue will become who comes out to vote.

Trump to drop out of 2020 race within months, former aide Scaramucci claims

Trump to drop out of 2020 race within months, former aide Scaramucci claims

Of the northern states, Mr Trump won Michigan by the fewest number of votes, just 11,000. Between 2012 and 2016, the Democratic vote badly eroded, with Ms Clinton falling about 300,000 votes short of Mr Obama's total, including about 75,000 fewer in Wayne County, which is home to Detroit.

Bitterness lingers among Democrats in Michigan over the 2016 campaign and how it was played by Ms Clinton's team. The 2018 midterms since have given Democrats some cause for optimism. Governor Gretchen Whitmer recaptured the governor's mansion and the party picked up two congressional districts.

Democrats made gains among white, working-class voters who had broken to the winning candidate in 2016, but also added votes in areas where Ms Clinton had done better than Mr Obama.

Matt Grossmann, director of the Institute for Public Policy and Social Research at Michigan State University, said that poses a question about the strategy for 2020.

“Is the better hope of returning some traditional Democrats back into the fold or continuing to make gains among highly educated voters that were more traditionally Republican?” he said.

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The Democrats' formula for success once followed Interstate 75 north from Detroit through Pontiac and Flint to Saginaw. But declining populations have hurt the Democrats.

“With lower populations in Detroit and Flint, that is not enough of a winning coalition,” said Amy Chapman, a veteran Democratic strategist in the state.

Macomb County, birthplace of the Reagan Democrats, still draws attention but is not as important to Democrats as nearby Oakland County, a once-Republican stronghold that has moved significantly towards the Democrats.

Democrats also have opportunities in some outstate areas. Kent County, home to Grand Rapids, is one area of Democratic growth. Other prospects include higher-educated counties such as Grand Traverse along Lake Michigan and Midland in the centre of the state.

Richard Czuba, a Michigan-based independent pollster, said the biggest threat to Mr Trump is the prospect of increased Democratic turnout.

Mr Trump got about the normal number of votes that past Republican candidates have received; Ms Clinton was significantly below not just Mr Obama but other Democrats. Though both sides are highly motivated, Czuba said: “The only upside in an increased turnout is for the Democrats.”

Pennsylvania presents another challenge for the current president and for the Democrats. Ms Clinton was criticised for her campaign's failure to pay more attention to Michigan and Wisconsin, but that was not an issue in Pennsylvania, where she campaigned constantly and invested heavily.

Trump claims he won 'every single poll' against Hillary Clinton following 2016 TV debates

Trump claims he won 'every single poll' against Hillary Clinton following 2016 TV debates

The results reflect the difference. Ms Clinton held her own in Philadelphia and its suburbs, winning the Philadelphia media market by about 25 points, roughly the same as Mr Obama in 2012, according to analysis by Clinton strategists.

But Mr Trump swamped her in other parts of the state. In the Scranton market, Mr Obama lost by about four points, Ms Clinton by 25. In the Johnstown area, Mr Obama lost by 25 points, Ms Clinton by 37. In Democratic Erie, which Mr Obama won by five points, The Democratic nominee lost by 13.

Terry Madonna, director of the Centre for Politics and Public Affairs at Franklin & Marshall College in Lancaster, said he doesn't rule out another Trump victory in the state.

“But to do it he's got to win those rural and small towns with significant turnout and/or find a way not to lose the Philly 'burbs by a larger percentage than [against Ms Clinton],” he said.

David Urban, a political adviser to the president and his campaign, predicted that Trump voters will be out in force but added: “The president put a lot of time and effort into Pennsylvania [in 2016] and it paid off. This time in 2020 he'll do the same, but he's going to have to put a little more effort into Pennsylvania because Democrats who took him for granted in 2016 will not be taking him for granted in 2020.”

Hillary Clinton: anyone but Trump would have been indicted after Mueller report

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Former Democratic governor Ed Rendell said he would watch five counties as clues to 2020: Delaware and Bucks in the Philadelphia suburbs; Luzerne, home to Wilkes-Barre and long a Democratic county; Cambria, home to Johnstown; and Ms Clinton, home to Lock Haven. But he said he was bullish about 2020 because he expects voters who seemed to abandon Ms Clinton at the end will be back in force.

That leaves Wisconsin, where Democrats will hold their convention next summer, as potentially the most competitive of the three. Wisconsin is seen as more difficult for the Democrats than Michigan or Pennsylvania because it has a higher percentage of white voters overall, particularly of white non-college voters, and because the Democratic infrastructure was weakened during former Republican governor Scott Walker's eight years in office.

Mr Walker left the state deeply polarised, but his policies caused erosion in the strength of organised labour. The percentage of union workers in Wisconsin fell below the national average during the Walker years, depriving Democrats of some of the ground forces that they traditionally counted on - and can count on in a state like Michigan. As one Democratic strategist put it: “You show up in Wisconsin and you're on your own.”

Wisconsin has seen a notable shift in voting patterns over the past decade, changes that Mr Trump accelerated in 2016. As Craig Gilbert of the Milwaukee Journal Sentinel wrote shortly after the 2018 elections: “Most Wisconsin voters live in places that are trending in one political direction or the other. But the state persists as a partisan battleground because all those regional shifts over the past two decades have somehow cancelled each other out.

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Northwestern Wisconsin has become more deeply entrenched as Republican territory, while southeastern Wisconsin has become less friendly to the Republican party. The city of Milwaukee and Dane County, home to Madison and the University of Wisconsin, remain the biggest and most important vote producers for the Democrats. Counties in the southwest and along the Mississippi shifted to Mr Trump in 2016 but could move back to Democrats next year.

In 2016, Ms Clinton suffered a falloff in the Milwaukee media market that was about double that of the state as a whole. Much of that erosion was concentrated in African-American precincts in the city of Milwaukee. Combined with Mr Trump's big margins in the northwestern part of the state, that was enough to doom her chances.

Where Democrats see particular opportunities - and Republican see reasons to worry - are in three Republican counties in suburban Milwaukee: Waukesha, Ozaukee and Washington. Dubbed the “WOW counties,” all still favour Republicans, but Democrats have been gaining in their share of the vote.

“Those counties are still voting Republican, but as much as 16 points less on the margin now than they were,” said Charles Franklin, who conducts the Marquette University Law School poll.

Brian Reisinger, a Republican strategist, said the Democratic growth in those suburban areas in 2018 was “a real wake-up call for people to see there was a way for our traditional coalition to not be quite enough”.

Both sides approach Wisconsin nervously. Republicans see the dangers but are more unified behind the president than they were in 2016. Democrats elected a governor - Tony Evers - in 2018, but narrowly. Then they lost a state Supreme Court race in the spring that they expected to win.

“We underperformed,” said a veteran Democratic strategist who spoke on the condition of anonymity to be candid. “We have to get smarter about how we do our work.”

Independent Minds Events: get involved in the news agenda

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The Trump campaign and Democratic strategists point to states beyond the big four as opportunities for flipping. The president's political team has its sights on New Hampshire, which the president lost by less than half a point, as well as Minnesota, which he lost by only a point and a half.

Mr Trump's team also has said Nevada and New Mexico will be targets in 2020. Both present obstacles unless the current president can expand his electorates. Trump advisers also have mentioned Oregon as a possible target, though Democrats take that less seriously.

Trump advisers say that, as Democrats focus on picking a nominee, they have an opportunity to begin to build organisations in these kinds of states and to identify sporadic voters who are attracted to the president.

“We have the luxury of having the resources to protect the states President Trump won in 2016 and expand into states we think we can add to his column in 2020,” said Tim Murtaugh, the Trump campaign's communications director.

Cornell Belcher, who was a pollster for Mr Obama's campaigns, said that, because of the changing status of some traditional battlegrounds such as Ohio and Iowa, it is more vital than ever for Democrats to compete hard elsewhere.

“Democrats have to expand the playing field and not put all our eggs in the basket of the traditional battleground states,” he said.

Donald Trump phones into Fox and Friends ahead of Midterm elections

David Bergstein, the Democratic National Committee's battleground states communication director, said the party is doing that even in the absence of a nominee.

“[We] are laying the groundwork now to ensure our eventual nominee has multiple pathways to 270 electoral votes,” he said.

Democrats see opportunities in North Carolina, despite losses there in 2012 and 2016, as well as Arizona and Georgia. Arizona, which Mr Trump carried by just 3.5 percentage points, might prove to be a more attractive target for Democrats than either of the two southern states.

Some strategists see Arizona, with 11 electoral votes, as a possible hedge against a loss in Wisconsin, which has 10 electoral votes. In-migration from other western states and a growing Latino population have changed the political makeup of the state.

Public Opinion Strategies charted the ideological movement of Arizona voters earlier this year and found that the state today is only marginally more conservative than the nation as a whole, a significant change since 2010.

“The Arizona as you knew it is gone,” said Republican pollster Bill McInturff. But he added that Mr Trump's strong support among older white men and potential appeal to some Hispanic men could help to offset some of the movement in the state.

One state not likely to figure prominently into expansion possibilities for the Democrats is Texas. However attractive it might be as a state in transition, Texas would require an enormous investment. Democrats will play there only if everything else is moving in their direction.