Senate Democrats are hoping to take advantage of a favorable electoral map in 2016 to swing majority control back their way.
This cycle, Republicans are defending 24 seats compared to Democrats' 10, including seven in states President Obama carried twice.
Democrats don't have any red-state incumbents facing reelection either, and they could be buoyed by higher turnout from a presidential cycle.
Here is The Hill’s latest ranking of the top 10 most vulnerable senators. See our previous ranking here.
1. Sen. Mark KirkMark KirkGOP lawmaker: 'Republicans were wrong’ to block Garland VA secretary comes under fire for comparing wait times to Disneyland Juan Williams: Electoral map looks grim for Trump MORE (R-Ill.) (last ranking: 1)
The first-term senator has worked hard to bolster his bipartisan credentials, backing gay marriage, immigration reform, gun control and environmental protections.
Democrats are hoping they can avoid a contentious and expensive primary, and most believe Rep. Tammy Duckworth (D-Ill.) is their best option. The Iraq veteran, who lost her legs in the war, neutralizes Kirk’s military credentials and voter sympathy due to his ongoing recovery following a stroke.
Rep. Bill FosterBill FosterLawmakers celebrate Jackie Robinson Day Overnight Energy: Fight breaks out over Interior budget Overnight Finance: Senate wants House to go first on debt MORE (D-Ill.) may not back down from a primary though, and if he jumps in other Democrats could see an opening that could give Kirk a better shot at hanging on in the deep blue state. Either way, Republicans admit Kirk faces an uphill fight.
There’s debate in both parties about whether Johnson or Kirk is the most vulnerable senator, though the pair top everyone’s list.
Johnson is likely to face a rematch against former Sen. Russ Feingold (D-Wis.), who just left his State Department job and is strongly hinting at a run.
Unlike Kirk, Johnson has remained a hardline conservative through his term. That’s dangerous for a man running in a state the GOP hasn’t carried for president since 1984.
Polls show Johnson isn’t that well-known or well-liked, and those who’ve spoken to Feingold say he’s learned his lesson after refusing outside money in 2010 en route to a five-point loss and will now allow super-PAC help.
Republicans are looking to paint Feingold as a man who abandoned Wisconsin since he first went to Washington decades ago. Wisconsin Gov. Scott Walker (R) could also boost Johnson if he’s on the presidential ticket. But strategists in both parties say Johnson is an underdog.
3. Sen. Harry ReidHarry Reid McConnell sets up vote to begin debate on defense policy bill The Trail 2016: Dems struggle for unity Senate candidate taunts Sanders: Why don't you endorse Alan Grayson? MORE (D-Nev.) (last ranking: 3)
The Senate minority leader is unpopular back home, but the man who could likely crush him — Nevada Gov. Brian Sandoval (R) — appears unlikely to run.
Nevada has been trending Democratic due to its fast-growing Hispanic population, and the party tends to do much better there in presidential years.
Reid also won by a surprisingly comfortable margin in 2010 against a deeply flawed GOP nominee. He’s proven to be a scrappy campaigner, and it’s unclear whether Republicans will be able to avoid the type of nasty primary that has hurt them before.
4. Sen. Kelly AyotteKelly AyotteVA secretary comes under fire for comparing wait times to Disneyland Juan Williams: Electoral map looks grim for Trump Liberal super-PAC hits Johnson for supporting Trump MORE (R-N.H.) (last ranking: 5)
Democrats are increasingly optimistic they can woo New Hampshire Gov. Maggie Hassan (D) to challenge Ayotte, though they don’t expect a decision until the summer.
The GOP’s nervousness is evident, and Crossroads GPS has already spent $280,000 on ads against Hassan.
And while Republicans believe Ayotte will be tough to oust, especially if the presidential race remains close in the state, they concede a race against Hassan will likely be a nail-biter.
5. Sen. Pat Toomey (R-Pa.) (last ranking: 4)
Rep. Joe Sestak (D-Pa.) is back for a rematch after narrowly losing to Toomey in 2010 — but not all Democrats are happy about it.
Sestak had a rocky campaign rollout this week and remains at odds with some state and national Democrats he infuriated by primarying the party-switching Arlen Specter for Senate in 2010.
Toomey has worked hard to shed his image as an unyielding Tea Party candidate, working with Democrats on some budget issues and crafting bipartisan gun-control legislation with Sen. Joe ManchinJoe ManchinRed-state Dem hits back over coal, court attacks How Senate Democrats are trying to deal with Sanders Tough choice for vulnerable GOP senators: Embrace or reject Trump MORE (D-W.Va.).
Pennsylvania’s blue lean in presidential years means Toomey's likely to face a tough fight, but Sestak still needs to show he’s ready for a marquee matchup. Some Democrats are still looking for another candidate, and a nasty primary fight could knock this race further down the list.
6. Sen. Marco Rubio (R-Fla.) (last ranking: 8)
Rubio is moving hard towards a presidential run, and an open seat immediately becomes a toss-up. However, either he or former Florida Gov. Jeb Bush (R) atop the ticket would boost the GOP.
But first, both parties are facing potentially divisive and expensive primaries.
Democratic National Committee Chair Debbie Wasserman Schultz (D-Fla.) and Reps. Patrick Murphy (D-Fla.) and Alan GraysonAlan GraysonSenate candidate taunts Sanders: Why don't you endorse Alan Grayson? GOP rep to retire, opening 10th Florida seat The Trail 2016: Sanders who? MORE (D-Fla.) are all giving the race a hard look, with most strategists believing the moderate Murphy would be Democrats’ best general-election candidate.
Potential GOP candidates include Florida Attorney General Pam Bondi (R), Florida Chief Financial Officer Jeff Atwater (R), Florida Agriculture Commissioner Adam Putnam, former Florida House Speaker Will Weatherford (R) and Rep. Vern Buchanan (R-Fla.).
7. Sen. Rob PortmanRob PortmanJuan Williams: Electoral map looks grim for Trump McConnell touts 'Senate squad' in Wes Anderson-style video Liberal super-PAC hits Johnson for supporting Trump MORE (R-Ohio) (last ranking: 9)
Former Ohio Gov. Ted Strickland’s (D) decision to run has moved this race higher, but with the formidable Portman it’s still a tough task.
Republicans believe Strickland is weak because of Ohio’s dismal economy while he was governor, and question whether the 73-year-old is prepared for another statewide bid.
Democrats say Portman can be tarred as a Wall Street elitist and that if Hillary ClintonHillary Rodham ClintonSanders blasts Clinton at Calif. rally for rejecting debate Michelle Fields warns Clinton camp: 'Don’t treat Trump as serious candidate’ Sanders steps up his attacks in homestretch MORE carries the state, Strickland can hang on to her coattails. But they admit Ohio starts out as an uphill race.
8. Sen. Richard BurrRichard BurrGOP senators: Obama bathroom guidance is 'not appropriate' Dem slams GOP for skipping vote on 'back doors' in devices CIA watchdog ‘accidentally destroyed’ copy of ‘torture report’ MORE (R-N.C.) (last ranking: 6)
Burr’s $1 million fundraiser in late January knocked down concerns he might retire, helping him replenish his depleted campaign coffers.
Polls show he isn’t that well-known in North Carolina, a disconnected state with 15 separate media markets. But he’s worked hard to establish himself as an even-tempered lawmaker in the slightly GOP-leaning state.
Democratic recruitment will be crucial here. Former Sen. Kay HaganKay Hagan10 Senate seats most likely to flip in 2016 Senate Republicans are feeling the 'Trump effect' Washington's lobby firms riding high MORE (D-N.C.) is taking a break this spring at Harvard University’s Institute of Politics, and Democrats hope they can convince her to run again.
9. Sen. Michael BennetMichael BennetGOP ad calls Clinton 'a living history of scandal' Trump, GOP agree: ObamaCare helps us GOP hopefuls struggle with support of Trump MORE (D-Colo.) (last ranking: 7)
Bennet narrowly won in 2010 against a gaffe-prone Republican and is coming off a rough cycle as Democratic Senatorial Campaign Committee chairman that saw him lose his home-state colleague.
Colorado is a pure toss-up state but Democrats have done better there in presidential years. The big question is whether Republicans can land a strong recruit — and Rep. Mike Coffman (R-Colo.) tops their wish list.
Democrats pulled out a recruiting coup when Missouri Secretary of State Jason Kander (D) jumped in, and state and national Democrats quickly rallied around him.
Still, Missouri leans Republican and has trended that way in recent years, and Blunt is a strong fundraiser and hard worker. Kander faces an uphill battle.