Republicans face an uphill road to keeping their Senate majority in the November election.

They must fight to protect 24 GOP-held seats, while the Democrats have to worry about only 10.

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Here are the top 10 seats likely to flip, nine of which are held by Republicans.

1. Illinois

Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (R-Ill.) is running in a state that President Obama won by 17 and 25 percentage points in 2012 and 2008, respectively. The political makeup of the state makes him the most vulnerable incumbent, according to strategists in both parties.

Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE, the front-runner for the Democratic nominee, grew up in Illinois and won the state's Democratic primary with more than 50 percent of the vote.

While GOP-allied outside groups are spending heavily in various Senate battleground states, they’re not doing so in Illinois, a troubling sign for Kirk.

His opponent, Rep. Tammy Duckworth (Ill.), was the Democrats’ top recruit, although she has run into some turbulence of her own.

Duckworth has been accused by two former employees of the state’s Department of Veterans Affairs of retaliating against them while she headed the agency. A trial date has been set for Aug. 15.

2. Wisconsin

Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Wis.) is viewed as the second most vulnerable incumbent.

He faces a strong opponent in former Sen. Russ Feingold (D-Wis.), whom he defeated in 2010, a Republican wave year that was fueled by a national backlash against ObamaCare.

A WPR/St. Norbert poll from late April showed Feingold leading by 10 points.

Obama won the state by 7 and 14 percentage points in 2012 and 2008, respectively.

Unlike Kirk, Johnson has done little to tack to the center or position himself as a moderate. Some Senate GOP strategists privately wonder if he began preparing for this race early enough.

But Johnson has backing from outside groups. The U.S. Chamber of Commerce is on the air in Wisconsin, according to a strategist familiar with advertising buys.

3. Florida

Senate Republicans have stepped up their efforts to persuade Sen. Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (R-Fla.) to change his mind and run for reelection, a sign they’re not confident in their ability to otherwise hold his seat.

McConnell says he is “doing everything” to get Rubio to run.

He might be able to clear the field. Otherwise, four Republican candidates will battle for the nomination up until the Aug. 30 primary: Lieutenant Gov. Carlos Lopez-Cantera, Rep. Ron DeSantis, Rep. David Jolly and multi-millionaire homebuilder Carlos Beruff.

A Quinnipiac poll from early May showed the likely Democratic nominee, Rep. Patrick Murphy, leading DeSantis, Jolly and Lopez-Cantera by margins ranging from 1 to 3 percentage points.

Obama carried the state in 2008 and 2012, where Democrats have a 262,000-person advantage in voter registration, according to the Department of State’s division of elections.

A Mason-Dixon poll released last week showed Hillary Clinton leading Trump by 3 points in the state.

Democrats have their own primary to worry about. Rep. Alan Grayson Alan Mark GraysonFlorida's Darren Soto fends off Dem challenge from Alan Grayson Live results: Arizona and Florida hold primaries The Hill's Morning Report: Frustration mounts as Republicans blow up tax message MORE has embraced Democratic presidential candidate Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE and is trying to outflank Murphy from the left.

But Senate Minority Leader Harry Reid Harry Mason ReidGraham signals support for confirming a Supreme Court nominee this year Trump signals he will move to replace Ginsburg 'without delay' Senate Republicans signal openness to working with Biden MORE (D-Nev.) has thrown his weight behind Murphy, who reported $5.6 million cash on hand at the end of March.

4. New Hampshire

New Hampshire hasn’t voted for a Republican presidential candidate since 2000, when George W. Bush carried the state, and Sen. Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE (R) faces a highly touted Democratic challenger in sitting Gov. Maggie Hassan.

Ayotte is a formidable candidate herself, though, and considered a rising star in the Senate. She has differentiated herself from her party by supporting Obama’s effort to curb carbon emissions from power plants, which affects New Hampshire disproportionately because of prevailing wind patterns.

Outside groups are spending heavily in the state. The Senate Majority PAC, a Democratic group dedicated to flipping the upper chamber, now has a $1 million television campaign focused on the Boston media market.

The U.S. Chamber is on the air helping Ayotte.

Hassan is trying to use Trump to her advantage by insisting her opponent will “need to be held accountable for his statements.”

Republicans are highlighting the number of days Hassan has spent out of the state fundraising for her campaign.

5. Pennsylvania

Pennsylvania Sen. Pat Toomey (R) has tried to portray himself as a moderate to voters by championing legislation with Democratic Sen. Joe Manchin Joseph (Joe) ManchinMomentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' Barrett seen as a front-runner for Trump Supreme Court pick MORE (W.Va.) in the last Congress to expand background checks for firearms.

Toomey presents himself as a soft-spoken, reasonable conservative, drawing a contrast with fire-breathing Tea-Party candidates who turn off most voters in Pennsylvania.

But he will have to overcome a lot of history to keep his seat. Obama won Pennsylvania by 5 and 10 points in 2012 and 2008, respectively. A Republican presidential candidate hasn’t won the state since 1988.

There are 4 million registered Democrats in the state, compared to 3.1 million registered Republicans, according to Pennsylvania’s Department of State.

Toomey, however, is seen as having a weaker opponent than Ayotte in New Hampshire or Sen. Rob Portman Robert (Rob) Jones PortmanRomney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery House passes B bill to boost Postal Service MORE (R) in Ohio.

The Democratic Senatorial Campaign Committee and allied outside groups spent $4.86 million to help Katie McGinty win the Democratic primary over former Rep. Joe Sestak.

McGinty is now dealing with a controversy over her claim that she was the first member of her family to go to college, even though records show her older brother graduated well before she matriculated.

6. Ohio

Portman has a significant cash advantage over his Democratic rival, former Gov. Ted Strickland (D), and strategists rate him a better candidate overall.

Republicans say Strickland takes a sometimes lackadaisical approach to the campaign and predict Portman will outwork him.

Portman reported $13.4 million in cash on hand at the end of March, compared to Strickland’s $2.7 million.

Portman served as U.S. trade ambassador under George W. Bush, a potential liability in a state hit hard by globalization and the loss of domestic manufacturing.

He has tried to inoculate himself from attacks by opposing the Trans-Pacific Partnership, an accord with 11 other nations representing the largest trade deal in U.S. history.

Portman won a coup last week when the United Mine Workers of America endorsed him. It was an embarrassment for Strickland, who represented Ohio’s coal country in Congress.

Portman is considered slightly safer than Ayotte and Toomey because Ohio has voted Republican more frequently in presidential elections. George W. Bush carried it twice before Obama won it in 2008 and 2012.

A Quinnipiac poll released May 10 showed Trump with a 4-point lead over Clinton in Ohio.

Both sides will spend heavily in this presidential battleground state. Senate Majority PAC has spent $2.6 million to date and has reserved $9.5 million for the fall.

Portman this week launched three new television ads with a $15 million media buy.

Freedom Partners Fund, a conservative advocacy group, is also on the air in Ohio, according to a strategist who tracks media buys.

7. Nevada

The seat held by retiring Senate Democratic Leader Harry Reid is the only Democratic one in the top 10 most likely to flip.

Republicans have a strong candidate in Rep. Joe Heck, whose district covers southern Las Vegas and Henderson — a swing portion of the state.

Obama won Heck’s 3rd Congressional District, which has a large Hispanic population, by 9 points in 2008 and tied Mitt Romney there in 2012.

The Republican thinking is that if Heck wins his own district and the 2nd Congressional District in northern Nevada, where Republicans have a historic advantage, he’ll be in good shape to capture Reid’s seat.

But Trump and his poor numbers with Hispanic voters is likely to be more of a liability for Heck than for most other Senate GOP candidates because of Nevada’s heavy concentration of Hispanic voters.

“It’s certainly going to be toxic. It’s only the level of toxicity that’s going to be the question,” said longtime Nevada political commentator Jon Ralston about Trump’s impact on Heck.

Reid’s fabled Democratic machine, which he built over his long career, will be an asset to Democratic candidate Catherine Cortez Masto. She is running to be the first Hispanic woman elected to the Senate, and her campaign message will resonate with Clinton’s quest to be the first female commander in chief.

8. North Carolina

Former state Rep. Deborah Ross (D) emerged from relative obscurity in North Carolina, but she’s trying to turn that to her advantage in what is widely perceived to be the year of the outsider candidate.

She surprised observers by out-raising Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (R), who has served in Congress since 1995, in the first three months of the year. She collected $1.3 million to his $1.1 million.

But GOP strategists say Ross hasn’t been battle tested, and her numbers are likely to come down once the campaign heats up in earnest.

North Carolina is trending Democratic and that’s worrisome for Burr. Obama won the state in 2008 although Romney carried it narrowly in 2012.

The surprise emergence into a national political issue of North Carolina’s bathroom law — which restricts transgendered people’s choice of bathroom — has roiled the race.

Burr initially said the state proposal did not discriminate, but more recently he has criticized it as “too expansive” and called for it to be rolled back.

The difficulty in assessing how the issue plays politically reflects the fast-evolving politics of the state.

Trump has a four-point lead over Clinton, according to two recent polls, but she is expected to target the state in the fall.

North Carolina is 21 percent African-American and almost 9 percent Hispanic, and Democrats will try to use Trump’s sometimes heated rhetoric to push minority voters to back Clinton.

9. Arizona

Trump and his low approval rating among Hispanic voters hurts Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE more than any other Senate Republican candidate.

Twenty-two percent of the state’s eligible voters are Hispanic, and the number is likely to be higher come Election Day. Democrats say Hispanics are registering in greater numbers to vote against Trump, and given that 31 percent of the state’s population is Hispanic, there’s lots of room to grow the rolls.

McCain has a tough opponent in Rep. Ann Kirkpatrick Ann KirkpatrickArizona Rep. Tom O'Halleran wins Democratic primary Arizona Rep. Ann Kirkpatrick wins Democratic primary Cook shifts 20 House districts toward Democrats MORE (D), whose congressional district covers a huge rural swath in the northeast. She reported $1.3 million in cash on hand at the end of March, compared to McCain, who reported $5.5 million.

McCain knows he’s in a tough fight, but he faces a tricky spot because publicly rejecting Trump could hurt him among Republican voters, who supported him overwhelmingly in the presidential primary. At the same time, he needs to be mindful of the growing Hispanic electorate.

McCain is helped by a public name identification of close to 100 percent after serving nearly five terms in the upper chamber. His allies say his political persona is well established and predict he won’t have difficulty standing apart from Trump.

Bill Clinton William (Bill) Jefferson ClintonDolly Parton remembers Ginsburg: 'Her voice was soft but her message rang loud' Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Calls grow for Biden to expand election map in final sprint MORE was the last Democratic presidential candidate to carry the state, in 1996.

10. Missouri

Democrats are bullish on their recruit, Jason Kander, Missouri’s secretary of state. While he is viewed as a talented candidate, he is running in a very Republican state.

But Senate Democratic candidates have won in Missouri before, and the conditions in 2012 are promising for a repeat.

Kander outraised Sen. Roy Blunt Roy Dean BluntCDC tells Congress it urgently needs billion for vaccine distribution On The Money: Trump undercuts GOP, calls for bigger COVID-19 relief package | Communities of color hit hardest financially by COVID-19 | Businesses, states pass on Trump payroll tax deferral Trump undercuts GOP, calls for bigger COVID-19 relief package MORE, the Republican incumbent, in the first quarter of this year, $1.3 million to $1.25 million. Blunt has served in Congress since 1997, when he was elected to the House, and in the Senate since 2010.

He is not as entrenched an incumbent as McCain, but Missouri is 80 percent white and has a small Hispanic population of less than 4 percent. Trump’s incendiary rhetoric is not likely to have as much of an effect here as in other states.

Blunt does have to contend with the anti-establishment, anti-Washington winds that have roiled the presidential primaries. He is a member of the Senate Republican leadership and served as majority leader in the House.

He has developed a track record as a pragmatic dealmaker in Congress, something that could help win over voters frustrated with Washington gridlock. He recently put together a deal with Sen. Patty Murray Patricia (Patty) Lynn MurrayTrump health officials grilled over reports of politics in COVID-19 response CDC director pushes back on Caputo claim of 'resistance unit' at agency The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (D-Wash.) to provide $1.1 billion in emergency funds to fight the Zika virus.

Democrats are going to hit Blunt on his ties to lobbyists. His wife is a lobbyist for Kraft Foods, and three of his kids have worked in the industry.

“His whole family is lobbyists,” said a Democratic strategist.

Missouri has voted consistently for Republican presidential candidates in recent years, but the Clintons have a history of success here. Bill Clinton won the state in 1992 and 1996.

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