The race for the Senate is heading into the homestretch as Republicans seek to maintain their grip on their slim majority.

Democrats must net five seats — or four and retain the White House — to regain control of the upper chamber. They are defending 10 seats, while Republicans face a more challenging path, needing to defend 24 seats.

ADVERTISEMENT

Here are the Senate seats most likely to flip:

1. Illinois — 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)

Kirk’s seat is most likely to turn blue this cycle. President Obama won the state by double digits in 2008 and 2012. He faces a formidable challenge from Democratic Rep. Tammy Duckworth.

Kirk has tried to capitalize on the workplace retaliation lawsuit against Duckworth when she served as head of the state’s Department of Veteran Affairs, but it has failed to gain steam, and the GOP senator, known for making gaffes, is catching flak for calling Obama a “drug dealer in chief” in regards to last month's $400 million payment to Iran.

2. Wisconsin — Ron Johnson Ronald (Ron) Harold JohnsonThis week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate MORE (R)

Johnson is also viewed as one of the most vulnerable senators in his rematch with former Democratic Sen. Russ Feingold. Republican groups have scaled back funding as Johnson trails in the polls.

Conflicting polls released Wednesday, however, are a bright spot for Johnson. While one poll has him down 13 points, another has him trailing by only a few points. Both surveys show GOP presidential nominee Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE closing the gap in Wisconsin, a sign the state is not entirely off the map.

3. New Hampshire — Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE (R)

Ayotte is expected to easily overcome her primary challenge in mid-September, but she faces an uphill battle against likely Democratic opponent Maggie Hassan. The New Hampshire governor holds a narrow lead over Ayotte in a state that Obama also carried in 2008 and 2012.

Ayotte has sought to distance herself from Trump, even as Hassan has tried to tie her to him.

Hassan, for her part, has campaigned with Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE but got tripped up when asked in an interview if she thinks Clinton is honest. The governor repeatedly dodged the question before clarifying in an interview the next day that she thinks she’s trustworthy.

4. Pennsylvania — Pat Toomey (R)

The presidential race appears to be trickling into Toomey’s reelection. Political observers in the state say he’s running a strong campaign, but his dip in the polls is largely thanks to the top of the ticket.

Toomey continues to withhold his support from Trump. But his opponent, Katie McGinty, a little-known former gubernatorial chief of staff, has been helped by Clinton’s consistent lead over Trump in the Keystone State. McGinty has maintained a lead since mid-July, though one survey has Toomey up 7 points.

Toomey has clinched endorsements from gun control groups praising his 2013 bipartisan bill on background checks and has a strong cash advantage, but strategists see the race largely hinging on the top of the ticket.

5. Indiana — Open seat (R)

Indiana makes its debut on the list after Democratic Sen. Evan Bayh’s last-minute bid for his old seat. It has prompted election handicappers to move the once safe GOP seat to a toss-up or Democratic-leaning.

Bayh has already made a splash with his $9 million war chest and has the benefit of name recognition, an issue that plagues his GOP opponent, Rep. Todd Young Todd Christopher YoungSenate GOP eyes early exit Why the US should rely more on strategy, not sanctions Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE. Polls show Bayh comfortably ahead in the race to replace retiring Sen. Dan Coats Daniel (Dan) Ray CoatsFBI chief says Russia is trying to interfere in election to undermine Biden The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Former Intel chief had 'deep suspicions' that Putin 'had something on Trump': book MORE.

But Bayh struggles with scrutiny over his residency. Republicans paint Bayh as a Washington lobbyist who has abandoned Indiana. In one interview, Bayh said he “sacrificed” moving to the wealthy D.C. neighborhood of Georgetown, and a CNN report found he’s classified as an “inactive” voter in Indiana.

Indiana would rank higher without a presidential race. Trump comfortably leads in Indiana, which has gone Democratic once in 52 years.

6. Nevada — Open seat (D)

Nevada is one of Republicans' only real pick-up opportunities. The race for Senate Minority Leader Harry Reid Harry Mason ReidOn The Trail: Battle over Ginsburg replacement threatens to break Senate Democrats fear Russia interference could spoil bid to retake Senate Graham signals support for confirming a Supreme Court nominee this year MORE’s seat has been relatively quiet with limited polling.

Reid has vowed to help keep the seat blue and elect former Nevada Attorney General Catherine Cortez Masto. But Rep. Joe Heck is a strong GOP recruit who has kept the race a true tossup with a razor-thin margin. He has yet to be dragged down by Trump even in a state with a large Latino population.

7. North Carolina — 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)

Once low on the list, Burr’s race has been moved up as Republicans grow concerned about his reelection and both presidential nominees shift their attention and resources to the Tar Heel State.

Former state Rep. Deborah Ross wasn’t Democrats’ top recruit for the seat, but she’s proven to be a strong fundraiser and has started to cut into Burr’s polling lead as Clinton also performs well in the state. Both candidates will have to overcome a name recognition hurdle in the final two months.

8. Ohio — Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE (R)

Portman is defying Trump’s down-ballot drag. While Clinton leads Trump by several points, the GOP senator tops former Ohio Gov. Ted Strickland (D) by as much as 15 points in the latest poll. His strong performance prompted groups from both parties to withdraw spending in the state and shift elsewhere.

It’s not just Portman’s polling advantage that has him well-positioned for November. He’s snagged four labor union endorsements that have backed Strickland in his previous races. And he’s kept up a cushy cash advantage as the former governor fails to make significant strides in his fundraising.

Portman and outside groups have inundated the airwaves with ads criticizing Strickland’s gubernatorial tenure, but the former governor recently hit back in his own ad that he led during the recession.

9. Florida — Marco Rubio Marco Antonio RubioMurky TikTok deal raises questions about China's role Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic MORE (R)

Both Rubio and Rep. Patrick Murphy (D) easily beat back primary challenges Tuesday and wasted no time attacking each other the following day.

Strategists expect the race to tighten by November, but as it stands, Rubio looks favored to hold onto his seat. He’s ahead in nearly every poll — and also outperforming Trump — and benefits from high name ID after his unsuccessful presidential run.

While Rubio breezed through his primary, Murphy has a few scars that will likely follow him into the general. Republicans continue to hammer him over reports he exaggerated parts of his resume and knock him for his father’s vast financial backing.

Honorable mentions:

Arizona — John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R): McCain cruised to victory in Tuesday’s primary. But the long-time senator faces his real test against Democratic 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. Democrats are tying McCain to Trump every chance they get in a state with a large Latino population. McCain has signaled he’s not taking anything for granted and came out of the primary swinging with a new ad that vows to be a check on Clinton.

Missouri — Roy Blunt Roy Dean BluntThis week: Supreme Court fight over Ginsburg's seat upends Congress's agenda McConnell locks down key GOP votes in Supreme Court fight Murkowski: Supreme Court nominee should not be taken up before election MORE (R): Blunt is getting a tough challenge from Missouri Secretary of State Jason Kander (D). Kander has proven to be a strong campaigner and fundraiser as one of Democrats’ top recruits. Blunt still leads in the polls, but at the top of the ticket, Clinton is polling close to Trump in the deep red state.

Colorado — Michael Bennet Michael Farrand BennetOVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency Next crisis, keep people working and give them raises MORE (D): Once viewed as one of the only ripe opportunities for Republicans, Bennet appears poised to sail to reelection. Republicans aren’t coming to the aid of Darryl Glenn, a county commissioner who trumpeted his conservative bona fides during the primary. But he’ll need to look beyond his base in a state that Obama carried twice and also has a large Latino population.