Democrats will be defending a slim Senate majority in 2024. Here’s a look at the states where both parties are fighting for control of the chamber.

However, the class of senators who were elected and reelected in 2018 — a Democratic wave year that saw several vulnerable red-state members of the party win and lose — will face a challenging map in 2024.

Much of the party’s performance will likely be tied to President Joe Biden, who is seeking reelection to a second term.

Former President Donald Trump, who will once again be the Republican presidential nominee this year, remains unpopular among moderates and suburban voters who often decide close Senate elections.

A major development that boosts the GOP: Sen. Joe Manchin of West Virginia in November 2023 announced that he wouldn’t seek reelection in 2024, a major blow to Democrats as he was not only the lone statewide officeholder from the party but the only Democrat who could conceivably have run a competitive race given its conservative lean. The GOP is now in the driver’s seat in the Mountain State, which backed Trump over Biden by 39 points in 2020.

In 2024, 34 seats will be up for grabs, including 20 currently held by Democrats, 11 held by Republicans, and three currently held by Independents.

Here are the key states that both parties are set to target:

Arizona

Sinema’s decision to become an Independent gave Democrats jitters while they were still rejoicing Georgia Sen. Raphael Warnock’s runoff victory in December 2022, but since then, her decision hasn’t impaired the party’s ability to move legislation and approve judicial nominations.

However, after more than a year of speculation, Sinema announced in March 2024 that she wouldn’t seek reelection to a second term, ending fears from some Democrats that her candidacy might aid the GOP in a potential three-way race.

Rep. Ruben Gallego launched his campaign for the Democratic Senate nomination in January 2023 and remains the frontrunner to win the party’s primary in July.

Democrats have made major inroads in Arizona in recent years, and the party is aiming to bolster their political ascent in the state by electing Gallego.

On the Republican side, former television journalist Kari Lake and Pinal County Sheriff Mark Lamb are the leading candidates.

Lake, the 2022 gubernatorial nominee who lost to now-Gov. Katie Hobbs, jumped into the Senate race in October 2023 with the endorsement of Trump. While Lake ran a hard-charging conservative campaign two years ago, she has recalibrated her strategy and has sought to expand the GOP tent this year in anticipation of a competitive general election race in the purple state.

Meanwhile, Lamb, a conservative who has pushed for stronger security measures at the US-Mexico border, was the first major Republican to enter the race.

Blake Masters, the 2022 Republican Senate nominee who lost to Democratic Sen. Mark Kelly, is running for the House seat being vacated by GOP Rep. Debbie Lesko after the 2024 elections.

Abe Hamadeh, who was narrowly defeated in the 2022 race for state attorney general, had been mentioned as a potential contender but in October 2023 threw his support behind Lake. He is also running to succeed Lesko in the House.

Maryland

Former Republican Gov. Larry Hogan will face off against Prince George’s County Executive Angela Alsobrooks for the seat being vacated by veteran Democratic Sen. Ben Cardin.

Maryland is one of the most Democratic states in the country. The party has hefty majorities in the state legislature and dominates the state’s congressional delegation. In the 2020 election, Biden won the state by 33 points. And Gov. Wes Moore easily won the state’s open gubernatorial race in 2022.

Hogan’s candidacy presents a unique challenge for Democrats. The former two-term governor, one of the most prominent Republican critics of Trump, left office with high marks from Maryland voters. During his two gubernatorial runs, he won over many moderate Democrats and Independents en route to his victories in a state where Republicans are vastly outnumbered.

And Republicans have the financial means to compete in Maryland as they seek to replicate the coalition that gave Hogan eight years in Annapolis.

But Alsobrooks, a former prosecutor, gained substantial name recognition as she campaigned across the state and won a tough Democratic primary against Rep. David Trone, who spent over $60 million of his own money in his unsuccessful bid to secure the party’s nomination.

Alsobrooks also benefits from representing the state’s second-most populous county and in the primary she performed strongly in Trone’s suburban Washington backyard and in the Baltimore area.

If elected, Alsobrooks would be Maryland’s first Black US senator.

Michigan

Democratic Sen. Debbie Stabenow, a popular lawmaker now in her fourth term, announced in January 2023 that she would not run for reelection in 2024.

Stabenow, the chair of the Senate Agriculture Committee, was most recently reelected in 2018 by 6.5% against now-GOP Rep. John James. (James, the Republican Senate nominee in both 2018 and 2020, is running for reelection to his House seat anchored in suburban Detroit this fall.)

Republicans would very much like to flip this seat, but Michigan Democrats had a banner year in November 2022 — sweeping the top statewide offices and retaking control of the full legislature. Biden is set to compete hard in the state this year, but he continues to face significant intraparty pushback over the conflict in Gaza, an issue that will play heavily in the presidential race and the Senate contest given the state’s sizable Arab-American population.

Rep. Elissa Slotkin, a moderate Democrat who represents a Lansing-area swing district that stretches to rural and suburban areas northwest of Detroit, announced in February 2023 that she would enter the Senate race.

Slotkin, a former CIA analyst and the acting assistant defense secretary for international security affairs in the administration of President Barack Obama, is the most prominent elected official to seek the Democratic nomination.

The congresswoman, who was first elected in 2018, said in her announcement video that she would focus on bolstering the middle class “in the state that invented the middle class” if voters send her to the Senate.

Gov. Gretchen Whitmer, Lt. Gov. Garlin Gilchrist II, Michigan Attorney General Dana Nessel, Michigan Secretary of State Jocelyn Benson, Reps. Debbie Dingell and Haley Stevens, and state Sen. Mallory McMorrow all ruled out Senate campaigns.

The actor Hill Harper, best known for his roles on “CSI: NY” and “The Good Doctor,” jumped into the Democratic primary in July 2023.

Businessman Nasser Beydoun is also seeking the Democratic nomination.

A number of Republicans are currently in the race, including former Rep. Mike Rogers, Sandy Pensler, a businessman, and Sherry O’Donnell, a physician and former congressional candidate.

Former Rep. Justin Amash, who voted to impeach Trump in 2019, is also running for the GOP nomination.

Rogers, a former chairman of the House Intelligence Committee who served in Congress for 14 years, offers Michigan Republicans their most formidable candidate to date. But his appeal in a Trump-dominated GOP is untested on a statewide level.

Former Rep. Peter Meijer, one of 10 House Republicans who voted to impeach Trump for his role on January 6, 2021, entered the race in November 2023 but withdrew in April 2024.

Meijer served for one term in Congress and was ousted in a 2022 GOP primary by Trump-backed challenger John Gibbs. Gibbs eventually lost the general election race to now-Democratic Rep. Hillary Scholten.

Montana

Democratic Sen. Jon Tester is a political survivor, having first won in conservative-leaning Montana in 2006 before winning tough reelection contests in 2012 and 2018.

And Tester hopes to keep the streak going, announcing in February 2023 that he’d seek a fourth Senate term in 2024.

Despite the GOP lean of Montana, Tester has built a solid political brand over the years and has been able to appeal to many of the state’s Independents and Republicans in past elections. GOP leaders have long coveted this seat, though.

Former Navy SEAL Tim Sheehy, a favorite of Republican leaders in Washington, jumped into the GOP primary in June 2023 and has earned the endorsement of Trump.

Former Montana Secretary of State Brad Johnson is also a candidate in the GOP primary.

Rep. Matt Rosendale, a staunch conservative who lost to Tester in 2018, announced in February 2024 that he’d also run for the seat. The decision was poised to set up a competitive — and potentially bruising — primary with Sheehy. But less than a week after entering the race, Rosendale dropped his bid, citing the headwinds he’d likely face after Trump backed Sheehy.

After Rosendale pivoted to running for reelection to the House, he subsequently withdrew from that race as well.

Nevada

Democratic Sen. Jacky Rosen is running for a second term in office in one of the most competitive battleground states in the country. In 2018, Rosen, then a first-term congresswoman, ousted then-GOP Sen. Dean Heller by 5 points.

In 2024, Rosen will be running for reelection when Nevada, a perennial swing state, will be a top target for both Biden and Trump.

Senate candidates on the Republican side include former US ambassador to Iceland Jeffrey Gunter; Army veteran and businessman Sam Brown; former state lawmaker Jim Marchant; attorney Ronda Kennedy; retired Army Lt. Col. Bill Conrad; retired Air Force Lt. Col. Tony Grady; and real estate broker Stephanie Phillips.

Ohio

Sherrod Brown, who was also elected to the Senate in 2006, is running for a fourth term. He has maintained a strong populist connection with his constituents despite the continued reddening of Ohio, which only 20 years ago was widely seen as the nation’s premier swing state.

Republicans view the Ohio seat as one of their biggest targets, but Brown has proven to be an effective candidate adept at winning over Independents and even a slice of conservative-leaning voters.

In March 2024, businessman Bernie Moreno defeated state Sen. Matt Dolan and Ohio Secretary of State Frank LaRose in a hotly-contested GOP primary.

Moreno and Dolan previously ran for Senate in 2022 but fell short in that year’s Republican primary to now-Sen. JD Vance.

The general election matchup between Brown and Moreno, which could very well determine the Senate majority, is now set to be one of the most expensive races in the country.

Pennsylvania

Democratic Sen. Bob Casey Jr., who was first elected to the upper chamber in 2006, is seeking a fourth term in 2024.

David McCormick, a businessman who narrowly lost the 2022 Republican Senate primary to Dr. Mehmet Oz, will be the party’s Senate nominee in the fall. McCormick, who grew up in the Pittsburgh area, is viewed as a candidate who can potentially bolster the GOP among Independents and suburban voters.

Texas

The Lone Star State has been firmly in the Republican column since the 1990s.

Democrats have sought to run more competitive Senate and gubernatorial races in recent years but have largely fallen short by sizable margins, with the notable exception being the close 2018 senatorial contest between then-Democratic Rep. Beto O’Rourke and Republican Sen. Ted Cruz. That year, O’Rourke lost to Cruz by 2.6 points, a result that gave Democrats hope that they could once again win the state in the near future.

Cruz has long been a major foil for Democrats; the conservative lawmaker also ran for president in 2016 before his defeat in the GOP primary to Trump.

In March 2024, Rep. Colin Allred, a former NFL player and civil rights attorney, easily won the Democratic primary over candidates that included state Sen. Roland Gutierrez, state Rep. Carl Sherman, and former Nueces County district attorney Mark Gonzalez.

Republicans point to the conservative tilt of the state in projecting confidence in the race, but they are also cognizant of Cruz’s narrow 2018 victory.

Trump is favored to carry Texas in 2024, which would likely boost Cruz, but Allred was first elected to office by appealing to moderates and flipping a GOP-held district in the Dallas area. The congressman could potentially build on O’Rourke’s success by making further inroads in suburbs across the state, especially if abortion remains as potent an issue in 2024 as it was in the 2022 and 2023 elections.

Wisconsin

Sen. Tammy Baldwin announced in April 2023 that she’d seek a third term in the upper chamber.

While Wisconsin in recent years has been one of the most politically polarized states in the country, Baldwin was able to win over many rural and exurban voters during her 2012 and 2018 campaigns — while also racking up large margins in the Democratic-heavy population centers of Milwaukee and Madison.

Several of the most prominent Republicans who were thought of as potential candidates — including former Gov. Scott Walker, Reps. Bryan Steil and Tom Tiffany, and former Rep. Mike Gallagher — declined to enter the race.

The GOP candidates currently in the race include Eric Hovde, a businessman, and Rejani Raveendran, the chair of the University of Wisconsin-Stevens Point College Republicans.