By Lisa Hagen - 09-06-16 06:00 AM EDT
The race for the Senate is heading into the homestretch as Republicans seek to maintain their grip on their slim majority.
Democrats must net four seats 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.
Here are the Senate seats most likely to flip:
1. Illinois -- Mark Kirk (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 the $400 million payment to Iran.
2. Wisconsin -- Ron Johnson (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 closing the gap in Wisconsin, a sign the state is not entirely off the map.
3. New Hampshire -- Kelly Ayotte (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, on her part, has campaigned with Democratic nominee Hillary Clinton, but got tripped up when asked in an interview if shethinks 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 comes at the expense of 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 overTrump 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. Polls show Bayh comfortably ahead in the race to replace
retiring Sen. Dan Coats.
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'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 (D). But Rep. Joe Heck is a strong GOP recruit who has kept the race a true tossup with a razor-thin edge. He has yet to be dragged down by Trump even in a state with a large Latino population.
7. North Carolina -- Richard Burr (R)
Once low on the list, Burr's race has beenmoved 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 (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 (R)
Both Rubio and Rep. Patrick Murphy (D-Fla.) 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.
Arizona -- John McCain (R): McCain cruised to victory in Tuesday's primary. But the long-time senator faces his real test against Democratic Rep. Ann Kirkpatrick. Democrats are tying McCain to Trump every chance they get in a state with a large Hispanic 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 (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 (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.