The US presidential election is on a knife-edge, with polls suggesting Hillary Clinton and Donald Trump are close. As Americans get ready to cast their votes, both campaigns are working flat out to give their candidates the edge in the key battleground states that will decide the election.

Mrs Clinton has led for much of the campaign but was rocked two weeks before polling day by news the FBI was once again investigating emails she had sent and received using a private email server. That announcement saw polls narrow significantly and gave her rival, Mr Trump, renewed momentum as the race entered the finishing straight.

The final polls suggest Mrs Clinton remains ahead and her campaign was boosted by the FBI’s announcement late on Sunday that it had found no evidence of wrongdoing in her email usage.

But could Donald Trump still win? It would be an upset - but polls suggest it is distinctly possible. To do so, however, the businessman must have an exceptionally good night and take states that have been leaning towards Mrs Clinton for many months.

Here are the 15 key states that will determine whether Donald Trump or Hillary Clinton ultimately becomes the 44th President of the United States.

Arizona

A long-standing stronghold for the Republicans, and Donald Trump will rightly be confident of winning Arizona’s 11 votes.

However, the state has an increasingly large number of Hispanic voters and that could help Hillary Clinton. Both she and her running mate, Tim Kaine, have campaigned there in recent days, suggesting Democrats may believe the contest in the state is closer than polls suggestion.

It would take an exceptionally good night for the Democrats to take a state they have won just once (in 1996) since 1952.

Colorado

The Democrats have a well-established lead in Colorado but Mr Trump appears to be rapidly gaining ground. The most recent poll suggests he is now just two points behind his rival and could be in with a change of grabbing the state’s nine electoral votes.

The Republican nominee has made 12 campaign stops in the state, suggesting his campaign data shows he is in with a chance of winning it. That would be a big upset in a state that President Obama won by a five-point margin in 2010, and would be an indicator of a disastrous night for Hillary Clinton.

Like elsewhere, Democrats are relying heavily on a strong turnout among Hispanic voters in Colorado if they are to retain the state.

Only termed a swing state in the past few elections, Colorado has been shifting to the left rapidly. This year, for the first time in decades, Democratic and unaffiliated voters outnumber Republicans - the growing Latino population is one reason for the shift.

Florida

Florida is the key marginal and the state most likely to determine the election. It is almost impossible for Donald Trump to win the White House without winning the Sunshine State.

Its 29 electoral college votes make it a key target for any presidential hopeful. It was George Bush’s tiny lead in the state that nudged him to victory over Al Gore in 2000, while President Obama held it by just 0.9 per cent in 2012.

Florida is currently on a knife-edge, with polls suggesting Mr Trump and Mrs Clinton are neck-and-neck on 45 per cent. The Democrat is ahead among early voters, while her rival has a big lead with people who plan to vote on the day – meaning a high turnout could deliver a hugely significant win for Mr Trump.

Both candidates have spent millions of dollars and many hours campaigning in Florida, knowing it is the state that, more than any other, could determine which of them becomes President.

The high number of Hispanic voters in Florida favours Mrs Clinton, who does particularly well among this demographic. But Democrats will be concerned by reports of low turnout amongst black voters – another element of their core support.

Georgia

Despite being a long-standing southern heartland for the GOP, the state has become unexpectedly competitive after one poll showed Mrs Clinton trailing Mr Trump by just one point.

An average of other recent polls suggests the Republicans remain on course to retain the state, however, and any sign of Mrs Clinton emerging from nowhere to steal the its 16 electoral votes would constitute a huge upset.

Georgia has not backed a Democrat presidential candidate since 1992 and supported the Republicans by a five-point margin in 2008 and almost eight points in 2012.

That the Green Party candidate, Jill Stein, is not on the ballot in Georgia could help Mrs Clinton if left-leaning Green supporters instead back the Democrat. Early voting patterns in the state suggest high turnout in the most Democrat counties – another promising sign for the Clinton campaign.

Iowa

Another swing state but one that looks likely to be captured this time by Mr Trump. The latest poll suggests he has a 7-point lead and 37 of the last 39 polls show him being ahead.

The state is 90 per cent white and therefore promising territory for the Republicans. Mr Trump has targeted the state heavily in recent days as he seeks to sure up its six electoral votes.

The Clinton campaign has admitted Iowa and Ohio are the swing states it is most likely to lose but they will not be overly concerned at the prospect. While Mr Trump has to win Iowa and all the other battlegrounds if he is to enter the White House, Hillary Clinton has a number of possible routes to reaching the all-important 270 electoral votes.

Maine

With only two electoral votes, Maine is significantly more interesting for what it says about the state of the nationwide race than for its actual impact on the Electoral College.

It is another traditionally safe Democrat state, in which Clinton has held comfortable, double-digit leads for much of the campaign. But, as elsewhere, polls have tightened since the FBI announced it was investigating newly uncovered emails relating to Mrs Clinton.

Her lead has been cut to just four per cent, recent data suggests, while Five Thirty Eight, the website run by renowned US election analyst Nate Silver, now gives Trump a one in four chance of winning the state – something that would have been unthinkable earlier in the race.

Michigan

Michigan has not voted Republican since 1988 and is considered part of the Democrat’s “firewall” of safe states that spans the north-east of the US. Two polls last week showed Mrs Clinton leading by seven points, but there are indications the race has narrowed in recent days.

Both campaigns have poured resources into the Great Lakes State – Democrats in order to seal what should be 16 safe electoral college votes, and Republicans to try to compensate for the loss battleground states, like Pennsylvania and Nevada, that they looks set to lose.

Ms Clinton has already been dealt one blow in Michigan, with the state backing Bernie Sanders in the Democratic primary

The Trump campaign has been focusing its efforts on securing the support of voters in many of Michigan’s poorer, rural areas.

Even if Mr Trump wins every battleground, he will still need to win a traditionally Democrat-leaning state. It is Michigan, along with Minnesota, where his best hope may lie.

Nevada

Nevada is one of several battleground states currently on an electoral knife-edge. Polls have swung back and forth and the latest suggests a dead heat between Mr Trump and Mrs Clinton.

The state is likely to provide a strong hint as to who is on course for the White House, having backed the eventual winner in all but one presidential contest in the 10 election cysles since 1972.

Democrat officials are confident a high turnout among the state’s Latino and Asian-American communities will be enough to secure a welcome return on the party’s sizable investment its Nevada field campaign.

Early voting data suggests Mrs Clinton has a clear lead in the state but strong Republican turnout on polling day could push it into the red column.

New Hampshire

Another bellwether state. Polls in recent weeks suggest the candidates are neck-and-neck in the race for New Hampshire’s four electoral votes, although the two most recent surveys showed Mrs Clinton in the lead.

The Democrats have a strong ground operation in the Granite State but its voters are generally late in deciding how to vote, and New Hampshire remains too close to call.

That Mrs Clinton lost out heavily to Bernie Sanders in the New Hampshire Democratic primary will be a cause for concern amongst her team, but they will be reassured by the fact that Barack Obama carried the state comfortably in 2008 and 2012.

If Donald Trump fails to win states like New Hampshire, he would need a shock victory in traditional Democrat heartlands like Michigan or Minnesota if he is to emerge victorious.

That both campaigns are fighting hard in New Hampshire - a state with just four electoral college votes – shows just how close they are expecting the overall race to be.

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North Carolina

Along with Fthe third key battleground state that Donald Trump has to win if he is to have any real chance of overall victory. The Republican has trailed in seven of the last ten polls but the race has tightened during the last week of the campaign and some polls now show him edging ahead, while others suggest Mrs Clinton maintains a tiny lead.

Barack Obama won the state in 2008 but it has otherwise voted Republican in every presidential election since 1980.

Black voters make up 23 per cent of North Carolina’s electorate, and turnout among this group could determine the eventual result of the state. If turnout is high, the Clinton campaign will be confident of victory.

With 15 votes, victory in North Carolina would put Mr Trump on 253 electoral votes if he also wins Florida and Ohio and holds what should be safe Republican seats elsewhere – that’s just 17 short of the 270 needed to win the presidency.

Ohio

Long considered a bellwether state because it has mirrored the national outcome in every election since 1960. That bodes well for Donald Trump, who polls suggest has a one per cent lead in the Ohio. It is, along with Florida, the most marginal state and frequently switches between the two main parties.

This Rust Belt state’s demographics play to Donald Trump’s strengths, with a population that is about 80 percent white — and heavy with working- and middle-class Americans who are anxious about the economy.

Five Thirty Eight suggests Mr Trump currently has a 64 per cent chance of taking Ohio’s 18 electoral votes.

If the Republican does indeed take Ohio and Florida, his road to the White House suddenly becomes a lot clearer, although he would still need to win at least one traditionally Democrat state.

Pennsylvania

Historically Pennsylvania has been an easy Democrat win and that looks set to continue this time around: Donald Trump has trailed in 39 consecutive polls in the state.

Pennsylvania switching to the Republicans would be a huge shock and an electoral game-changer, heralding an awful night for Hillary Clinton and instantly opening up several possible paths to a Trump presidency.

But if the Republicans have a particularly good night, its 20 electoral votes could be enough to carry Mr Trump over the finish line.

Knowing the importance of holding onto the Keystone State, Mrs Clinton will deliver her final big rally of the campaign there on Monday alongside husband Bill, President Obama and Michelle Obama.

Ms Clinton has concentrated on urban areas Philadelphia and Pittsburgh while Mr Trump has looked to energise Republican areas of the state.

Utah

Utah is a red state that has not chosen a Democrat for president since 1964 - and is not likely to do so on Tuesday. But it could be the only state to hand a loss to both Clinton and Trump.

Evan McMullin, a Utah-born Mormon, is running for president as an independent. He claims to be competitive in 34 states, either on the ballot or as a write-in candidate. Some polls have shown him tied with Mr Trump here, with Mrs Clinton not far behind.

Virginia

Virginia is another state that the Democrats will expect to win comfortably. It voted Republican in every election from 1968 to 2004 but swung to the Democrats in 2008 and was held relatively comfortable by them in 2012.

Hillary Clinton appears to be ahead in the state but polls have narrowed significantly, and her lead has reportedly been cut from double figures to between three and five points.

That should still be enough to secure another comfortable Democrat victory, but Clinton officials have been sufficiently concerned about Trump closing the gap to pile resources into the state.

It would take a significant swing towards the Republicans for Trump to capture Virginia and would be a sure sign that he is on course for the White House.

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Wisconsin

Wisconsin has voted Democrat in the last seven presidential elections and Mrs Clinton is on course to make it eight. She has not visited the state since April, a clear sign of Democrat confidence that it is already in the bag.

Despite this, Wisconsin remains an ambitious target for Republicans. The Trump campaign will know that more than a third of the electorate in the state consists of non-college-educated white voters – the demographic typically most receptive to their candidate’s populist message. Some Republican strategists believe a high turnout amongst this group could tip the balance in their favour if Democrat supporters do not turn out in the numbers expected.

And with some key battleground states appearing to slip away from the Trump campaign, the Republican nominee has made a last-minute dash across Democrat strongholds in an attempt to wrest at least one them into his column.