Democrats’ chances to regain control of the Senate are looking brighter as Hillary Clinton and Donald Trump take shape as the presumptive nominees of their parties.
Tuesday's contests set up the slates in Illinois, Ohio and North Carolina and signal tough general elections in these battleground states.
Sen. Mark Kirk (R-Ill.) tops the list as the most vulnerable incumbent, as he faces a high-profile Democratic challenger in a state that went to President Obama in both the 2008 and 2012 elections.
Many of the vulnerable Republicans have said they’ll back Trump if he’s the GOP nominee, and Democrats have already used their support to link them to the controversial front-runner.
The upheaval over President Obama’s Supreme Court nominee only further complicates Republicans' chances to hold on to their small majority.
Here's a list of the top 10 Senate seats most likely to flip this cycle:
Kirk easily emerged from Tuesday’s primary as the GOP nominee, but he faces the steepest climb going into the general election.
The GOP senator has positioned himself as a moderate in deep-blue Illinois but will run for reelection during a presidential year that typically favors Democrats.
Kirk has also been known for making a series of gaffes and will have to steer clear of garnering negative media attention and creating fodder for political ads used against him.
Looking toward the general election, he now faces Rep. Tammy Duckworth (D-Ill.), a formidable Democratic opponent who outpaced him in fundraising heading into Tuesday. There’s been minimal polling, but a survey from last July found the congresswoman leading by a few points.
Kirk has gone the furthest of any Senate Republican regarding Obama’s court nominee and broke with his party to call on his colleagues to “man up” and hold a vote.
Still, Democrats are not wasting any time tying him to Republican leadership that has refused to hold hearings or a vote for the president’s nominee.
Kirk’s critics are also pouncing on the opportunity to knock him over his support for Trump if he’s the party’s nominee.
Wisconsin’s Senate race will likely set up a rematch between Sen. Ron Johnson (R-Wis.) and former Sen. Russ Feingold (D-Wis.). In 2010, Johnson edged out Feingold out by 5 points.
The odds aren’t likely in Johnson’s favor this time around, though, and he faces a similar situation to Kirk: The GOP senator is running in a Democratic-leaning state that voted for Obama in 2008 and 2012.
Feingold maintains his double-digit lead over Johnson in a recent poll from last month and has a slight money edge based on end-of-the-year fundraising reports.
But Johnson’s campaign has painted the former senator as out-of-touch and spending more time outside the state as a college professor.
Johnson has said he’ll back whoever becomes the GOP presidential nominee, but he didn’t go as far as Kirk about the Supreme Court. The Wisconsin senator said he’s open to meeting with Obama’s pick but agrees with other Republicans that a hearing shouldn’t be scheduled.
3. New Hampshire
Sen. Kelly Ayotte (R-N.H.) faces tough odds in a liberal-leaning state during a presidential year in a race that’s expected to be one of the most expensive this cycle.
She will likely face popular New Hampshire Gov. Maggie Hassan in the general election. Ayotte has a significant cash advantage, but a recent poll shows her only narrowly leading Hassan.
The GOP senator has sought to make national security a wedge issue in the Senate race and hit Hassan over the release of Guantánamo Bay detainees and support for Obama’s Iran nuclear deal.
But Hassan was the only Democratic governor to support halting Syrian refugees from resettling in the United States, creating party backlash.
Campaign finance has also taken center stage in the race. Ayotte proposed a pledge to limit outside spending in the race, but Hassan increased the wager by including a $15 million spending cap.
Ayotte has also found herself caught up in the chaos surrounding Trump on the top of the ticket and the Supreme Court battle that will likely be used against her as the race progresses.
The seat vacated by former GOP presidential hopeful Marco Rubio is up for grabs as both parties are still figuring out who will emerge as their standard bearers.
Democrats have a slight upper hand, as the party’s establishment continues to rally behind Rep. Patrick Murphy (D-Fla.). His primary opponent, Rep. Alan Grayson (D-Fla.), is under fire over his hedge funds, and Senate Minority Leader Harry Reid (D-Nev.) has called on the liberal firebrand to drop out.
For Republicans, a crowded field of five has taken shape. The conservative Club for Growth backed Rep. Ron DeSantis (R-Fla.), while Florida Lt. Gov. Carlos López-Cantera got a huge boost from Miami billionaire Norman Braman, who will helm his campaign’s finance team.
Rep. David Jolly (R-Fla.) and businessmen Todd Wilcox and Carlos Beruff are also running.
A poll from earlier this month found both Democratic Senate candidates leading the top three GOP candidates in most head-to-head general election match-ups.
Nevada is one of the few Democratic-held seats on the list of competitive races this cycle.
Democrats were able to get a star recruit for the seat vacated by Reid. The Senate Minority Leader has endorsed former Nevada Attorney General Catherine Cortez Masto for his seat, and, if elected, she’d be the first Hispanic in the U.S. Senate.
But Republicans were also able to net a strong candidate, Rep. Joe Heck (R-Nev.), who gets high performance ratings in his swing district.
Both candidates will need to court Hispanic voters, and immigration is likely to play a pivotal role in this race.
Heck has said he’ll support whoever the GOP presidential nominee is, but he has tried to distance himself from Trump and has been critical of the GOP front-runner’s proposal to build a wall on the border with Mexico and to temporarily ban Muslims from entering the United States.
But if Trump is at the top of the ticket, this could turn out Hispanic voters in droves and will likely be an advantage for Cortez Masto.
The fierce general election between Sen. Rob Portman (R-Ohio) and former Ohio Gov. Ted Strickland in the Buckeye State ramped up quickly following Tuesday’s primaries.
Not even 24 hours later, a super-PAC that spearheads the conservative donor network helmed by billionaire industrialists Charles and David Koch launched a $2 million TV and digital ad buy targeting Strickland.
The former governor’s campaign hit back and released digital ads labeling Portman as “the ultimate Washington insider.”
Portman still faces a tough reelection bid, but he’s in good shape financially and easily coasted to the GOP nomination.
Strickland, who has been criticized for lackluster fundraising, spent some time and resources against his primary challenger and had to defend his past gun record.
According to a Public Policy Polling survey from last week, the race was a dead heat.
Sen. Pat Toomey (R-Pa.) has a significant cash advantage going into the spring and is in good shape in his competitive swing seat.
Democrats face a messy primary. Former gubernatorial chief of staff Katie McGinty has the party’s establishment support but falls short when it comes to fundraising compared to former Rep. Joe Sestak and runs behind Sestak in polls.
Sestak riled party leaders by running and successfully winning against then-Sen. Arlen Specter (D), ultimately losing to Toomey by several points in the 2010 election.Suburban Pittsburgh Mayor John Fetterman is also running in the Democratic primary.
Toomey still faces a tough battle in a more Democratic-leaning state, but a contested Democratic primary gives him time to prepare and raise money before a general election battle is in full force.
Sen. Michael Bennet (D-Colo.) still has a large target on his back by Republicans, but he is currently in good financial shape and has a head start in this race.Republicans had difficulty recruiting for this race and now have a crowded field of a dozen candidates.
State Rep. John Keyser, who served in both wars in Iraq and Afghanistan, appears to be one of the party's most promising candidates. According to The Denver Post, he met with the National Republican Senatorial Committee before announcing his bid early this year.
Even though Republicans are uncertain of who will be their standard bearer in the fall, they’re not wasting time waiting to target Bennet and have already attempted to tie him to the Democratic Party and Obama.
Bennet has already tried to distance himself from the president and expressed concerns over Obama’s plan to close the U.S. prison at Guantánamo Bay, saying he remains opposed to moving detainees into Colorado. But his critics continue to link the Democratic senator to the administration’s foreign policy.
9. North Carolina
Sen. Richard Burr (R-N.C.) and former Democratic state Rep. Deborah Ross both glided to their party’s nominations on Tuesday and now face a competitive fall election.
Democrats had a similar situation to Colorado Republicans. They had trouble recruiting high-profile candidates for this race — including former Sen. Kay Hagan (D-N.C.) — but the establishment quickly rallied around Ross after she entered the race.Burr still holds a slight advantage in a state that President Obama won by a razor-thin margin in the 2008 election and narrowly lost in 2012.
The GOP senator also has a significant cash advantage over Ross, but only held a single-digit lead over his Democratic opponent in a February poll. And he will also have to overcome a negative job-approval rating.
Democrats still face tough odds unseating Sen. Roy Blunt (R-Mo.) in the GOP-leaning state that went to the GOP nominee in both 2008 and 2012.
But Democratic Senate candidate Missouri Secretary of State Jason Kander has impressed those within his party and has received endorsements from high-profile Missouri lawmakers early in his campaign.National security has become a premier issue in this race.
Late last year, Kander's campaign released a lengthy report that knocked Blunt, who sits on national security committees, for voting against a bill to fund the Department of Homeland Security and missing hearings on the threat of the Islamic State in Iraq and Syria.
And Democrats have already blasted Blunt for hiring his son, who’s a lobbyist, to helm his reelection bid.
Blunt also has a significant cash advantage over Kander, with about $3 million more cash on hand.There’s been no recent polling in the state, but a survey from August showed Blunt ahead of Kander by only 5 points.