Democrats will be defending a slim Senate majority in 2024. Here's a look at the states that will likely be in play as both parties fight for control of the chamber.

• The 2024 Senate elections — which will run concurrent with the presidential race — are approaching.
• Democrats will have to defend several vulnerable incumbents in swing and GOP-heavy states.

In the 2022 midterm elections, Democrats defied political expectations by holding on to their Senate majority, with every incumbent securing reelection and then-Pennsylvania Lt. Gov. John Fetterman flipping the open seat being vacated by retiring Republican Sen. Pat Toomey.

The upper chamber in January then shifted from its previous 50-50 split — with Senate control in 2021 and 2022 resting on Vice President Kamala Harris' tiebreaking abilities — to a 51-49 majority led by Majority Leader Chuck Schumer of New York. And despite Arizona Sen. Kyrsten Sinema switching from the Democratic Party to register as an Independent late last year, she continues to retain her committee assignments through her former party, and the lawmaker herself downplayed any notion that she would change her voting habits.

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 is currently the Republican presidential frontrunner, but he 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 party switch gave Democrats jitters while they were still rejoicing Georgia Sen. Raphael Warnock's runoff victory last December, but her decision so far hasn't impaired the party's ability to move legislation and approve judicial nominations. However, Sinema's election itself is another matter; she has not yet announced whether she will run for reelection next year, but the Wall Street Journal reported that she is mapping out a potential campaign. If Sinema runs again, she'll set herself up for an epic clash with Rep. Ruben Gallego, who announced his campaign for the Democratic nomination in January 2023 and remains the most prominent candidate for the party.

Both Sinema and Gallego could very well end up in a three-way race with a Republican candidate, potentially splitting Democratic-leaning voters and affording the GOP a much-desired win. Gallego could also rally support among base Democrats — who have become increasingly disenchanted with Sinema over her opposition changing filibuster rules to pass key voting-rights legislation — along with Independents who have soured on the incumbent lawmaker.

Those scenarios would not only complicate Sinema's standing in the Senate, but force Democrats to decide whether they will back her candidacy or get behind Gallego. Democrats have made major inroads in Arizona in recent years, and the party would like to keep the seat out of Republican hands.

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

Kari 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.

Blake Masters, the 2022 Republican Senate nominee who lost to 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 last year's 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.

Michigan

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

Stabenow, the chair of the Senate Agriculture Committee, was most recently reelected in 2018 by 6.5% against now-GOP Rep. John James.

Republicans would very much like to flip this seat, but Michigan Democrats had a banner year last November — sweeping the top statewide offices and retaking control of the full legislature — and Biden is expected to compete hard in the state if he runs for reelection.

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 have 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, while businessman Nasser Beydoun, former state Rep. Leslie Love, Michigan State Board of Education president Pamela Pugh, and attorney Zack Burns are also angling for the party nod.

A number of Republicans are currently in the race, including former Reps. Mike Rogers and Peter Meijer; former Detroit police chief James Craig; Nikki Snyder, a member of the Michigan State Board of Education; Ezra Scott, a former Berrien County Commissioner; Michael Hoover, a small business owner; and Alexandria Taylor, an attorney.

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.

Meijer, one of 10 House Republicans who voted to impeach Trump for his role on January 6, 2021, 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.

James in February 2023 said that he would seek reelection to his House seat anchored in suburban Detroit.

And Rep. Lisa McClain told The Detroit News in July 2023 that she wouldn't run for the Senate.

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 that he'll 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 quickly earned the endorsement of Rep. Ryan Zinke, a former Interior secretary under Trump.

But Rep. Matt Rosendale, a Trump-aligned conservative who lost to Tester in 2018, is inching toward another Senate run, according to Politico.

Former Montana Secretary of State Brad Johnson has also entered the GOP primary.

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.

Next year, Rosen will be running for reelection when Nevada will be a top target for the eventual presidential nominees of both parties.

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; businessman Barry Lindemann; 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 was once the nation's premier swing state.

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

GOP state Sen. Matt Dolan, who also ran for the US Senate in 2022 but fell short in the Republican primary to now-Sen. JD Vance, is seeking the party's nomination. Businessman Bernie Moreno, another candidate who ran last year, is also in the GOP race.

In July, Ohio Secretary of State Frank LaRose announced that he would also seek the GOP nomination.

Pennsylvania

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

Casey, who has won all three of his prior Senate races with relative ease, will likely benefit from running in a presidential year when turnout in the Democratic strongholds of Philadelphia and Pittsburgh is poised to be very high.

However, Casey has also generally done well in many of the state's working-class towns and cities, and he could post an electoral performance similar to Fetterman, who dominated in the vote-rich Philadelphia suburbs last year.

David McCormick, who narrowly lost the 2022 Republican Senate primary to Dr. Mehmet Oz, jumped into the 2024 race in September.

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.

The leading Democrats in the Senate race include Rep. Colin Allred, a former NFL player and civil rights attorney, and state Sen. Roland Gutierrez.

Allred has been the strongest fundraiser on the Democratic side by far, with his campaign reporting $7.9 million in cash in hand at the end of the third quarter of 2023, according to FEC filings. Gutierrez reported $380,000 in cash in hand at the end of the same period.

Republicans have pointed to the conservative tilt of the state when discussing the Senate contest, but they are also cognizant of Cruz's narrow 2018 win and the potential strength of an Allred candidacy should the congressman win the Democratic nomination.

Trump is favored to carry Texas in 2024 should he win the Republican presidential nomination, 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 build on O'Rourke's success and make further inroads in suburbs across the state, especially if abortion remains as potent an issue next year as it was in the 2023 elections.

Wisconsin

Sen. Tammy Baldwin announced in April that she will 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.

Republicans will almost certainly try to field a strong candidate, but Baldwin has been able to navigate the sharp ideological divides in her state in a way that has bedeviled members of both parties.

GOP Reps. Mike Gallagher, Bryan Steil, and Tom Tiffany have all indicated that they would not enter the race.