Eighteen months out from Election Day 2024, Senate Republicans have plenty of reasons to be bullish about retaking the majority, but the same could be said last year, when Democrats actually gained one seat.

Still, CQ Roll Call’s first look at the 10 most vulnerable senators of 2024 is dominated by Democratic incumbents, some of whom have not made their intentions known.

Based on conversations with numerous campaign insiders and election analysts, the list has Democrats in six of the top seven spots. The only exception is Sen. Kyrsten Sinema of Arizona, who checks in at No. 2, behind West Virginia Democrat Joe Manchin III.

Sinema, who was last elected as a Democrat but declared herself an independent at the start of this Congress, is in the unusual position of facing challenges from both the left and the right if she decides to seek another term.

Utah GOP Sen. Mitt Romney is not likely to be vulnerable if he decides to run for another term and makes it through the Beehive State primary, but he checks in at No. 10 on the list because he could face a significant challenge from his right.

Others could still make a future version of the list, including Sen. Tim Kaine, D-Va., if dynamics shift or if any of the current top 10 opt for retirement instead of subjecting themselves to another campaign cycle and potential six years in the Senate. Manchin, for instance, has told reporters that he may not make his intentions known until the end of the year.

None of the races involving the 10 most vulnerable is likely to be the most expensive Senate contest in the country (though a Texas race between Republican incumbent Sen. Ted Cruz and Democratic Rep. Colin Allred could be quite costly). The honor will most likely go to the open seat race in California, where three top Democrats — Reps. Barbara Lee, Katie Porter and Adam B. Schiff — are running to succeed the retiring Dianne Feinstein, and where there’s likely to be only Democrats on the final ballot under the state’s top-two system.

Because this list only looks at vulnerable incumbents, it also does not include one other race that could figure prominently in GOP efforts to take the majority. That’s the seat Democratic Sen. Debbie Stabenow will be vacating in Michigan, a perennial presidential battleground.

Manchin has $9.7 million in his campaign account. He’s been a constant thorn in the side of President Joe Biden and the Democrats, a stance that may play well in ruby-red West Virginia. And on the Republican side, there’s a divisive ideological battle brewing between Rep. Alex X. Mooney and Gov. Jim Justice. Yet despite those factors, the Democratic senator is once again fighting for his political survival in a state that Donald Trump carried by nearly 40 percentage points. Manchin says he won’t formally announce whether he intends to run again until the end of the year, but the National Republican Senatorial Committee is already running ads blasting him.

Sinema has no shortage of announced and potential challengers, and it’s not clear how she would access the ballot, much less prevail in November 2024. The Arizona senator became an independent at the start of the current Congress while maintaining enough ties with Democrats to get her committee assignments. Rep. Ruben Gallego is running for the seat as a Democrat, and he has already picked up significant endorsements. Republicans could have a crowded primary, with Pima County Sheriff Mark Lamb already in the race and former gubernatorial candidate Kari Lake likely running. Lake and Lamb could prove unpalatable to some GOP voters, but it is unclear whether there’s enough of a middle ground for Sinema to prevail in a multicandidate race.

Tester is a blunt-talking third-generation farmer who has served in the Senate since 2007. He was the top fundraiser among vulnerable Democratic senators in the first quarter of 2023, taking in more than $5 million. Republicans have yet to field a candidate to face him. Both Republicans in Montana’s congressional delegation — Reps. Ryan Zinke and Matt Rosendale — along with Gov. Greg Gianforte are among the GOP’s possible recruits. Defeating Tester is one of the priorities of Republican Steve Daines, the other Big Sky Country senator and the head of the NRSC. The group is targeting Tester as a big-spending Biden ally.

Brown is a political survivor who has managed to hang on in a state that has grown increasingly inhospitable to Democrats. First elected to the Senate in 2006, Brown has nurtured a reputation as an economic populist who can connect with working class voters. Brown raised $3.6 million in the first quarter of 2023. Several Republicans are seeking to unseat him, including state Sen. Matt Dolan, whose family owns the Cleveland Guardians baseball team, and entrepreneur Bernie Moreno, who is running as a political outsider. Other potential GOP candidates include Secretary of State Frank LaRose and Rep. Warren Davidson.

Rosen is in a perennial battleground state that Biden carried narrowly in 2020, but she may be less vulnerable than that because of Nevada Republicans’ struggles to win federal races. Last year, former state attorney general Adam Paul Laxalt was a top GOP recruit and lost to incumbent Sen. Catherine Cortez Masto by about 8,000 votes, even as Republicans won the governor’s office. The only major declared challenger to Rosen so far is Jim Marchant, who previously lost races for Congress and secretary of state and has expressed the view that Biden was not the legitimate winner in 2020. Rosen had about $6 million in cash on hand at the end of March.

Casey hasn’t yet drawn any Republican challengers, but this battleground state will be competitive in the presidential election, and Republicans hope this will be the cycle they oust the three-term incumbent. Republicans have already signaled that they will attack Casey as a D.C. insider working to enrich his family. But the GOP could have a competitive primary ahead complicating their efforts. While the NRSC is recruiting Dave McCormick, who lost a GOP Senate primary last year, former gubernatorial nominee Doug Mastriano — who many Republicans consider incapable of winning a general election —  is also toying with a run. 

Wisconsin will be another presidential battlefield, but no Republicans have yet filed to run against Baldwin in her campaign to win a third term. Potential GOP candidates include Rep. Tom Tiffany, businessman Eric Hovde and former Milwaukee County Sheriff David Clarke, while Rep. Mike Gallagher hasn’t ruled out a run. A Democrat-backed candidate was easily elected to the state Supreme Court last month, but the state, which Biden won narrowly in 2020, is generally considered more divided than that election suggests. Baldwin had $3.9 million on hand at the end of March. She’s likely to focus on issues like the economy, health care and reproductive rights throughout the campaign. 

Cruz gained a national profile running for president in 2016 and raised $1.5 million in the first quarter of 2023. On Wednesday, the Texas Republican who is running for a third term gained an opponent: Rep. Colin Allred, a Dallas-area Democrat who  was an NFL linebacker and served in the Obama administration. In his introductory campaign video, Allred took aim at Cruz for traveling to Cancun during a winter freeze that devastated Texas in 2021. Allred is hoping to prove the Democrats’ theory that Texas is trending purple, but Cruz edged out his last Democratic opponent, then-Rep. Beto O’Rourke, in 2018 despite being significantly outspent. 

Florida is one of the Republican-held seats where Democrats could try to go on offense this cycle, but first they need to recruit a candidate who can raise enough to compete with Scott, who spent $64 million of his own money to flip the seat in 2018. Scott, who chaired the NRSC last cycle, is a favorite foil of Democrats, including Biden, because he proposed sunsetting all federal programs after five years, then made an exception for Medicare and Social Security. After Republican Gov. Ron DeSantis and Sen. Marco Rubio each won reelection by large margins last year, Scott’s race could be a barometer of how far to the right Florida has shifted. 

On paper, Romney would appear invincible in Utah. A seasoned politician who served as governor of Massachusetts and was the GOP’s presidential nominee in 2012, Romney won nearly 63 percent of the vote in 2018. But Romney’s comments criticizing Donald Trump and his two votes to impeach the former president have led to speculation that he could face a challenge from a fellow Republican. State Sen. Mike Kennedy, who lost to Romney in 2018, and state Attorney General Sean Reyes are among the potential candidates. Romney has filed paperwork with the Federal Election Commission but has not said whether he intends to run for reelection.