Democrats will be defending a slim Senate majority in 2024. Here's an early look at the states that will likely be in play for both political parties.

• 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.

A lot of the party's performance will be tied to President Joe Biden, who has not yet announced if he is running for reelection, though it is expected that he will do so. But former President Donald Trump — whose political brand took a huge hit last month after he endorsed multiple candidates who lost their races — has already thrown his hat into the race.

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 some of the key states that both parties will likely 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, is the first big-name Republican to jump into the race. Other potential GOP candidates include 2022 gubernatorial nominee Kari Lake and 2022 Senate nominee Blake Masters, who lost to Gov. Katie Hobbs and Sen. Mark Kelly, respectively. Jim Lamon, a former solar energy executive who ran for the party's nomination last year, has also been mentioned as a potential candidate.

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 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 widely seen as the frontrunner for 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.

Other Democratic politicians who might enter the race include Reps. Debbie Dingell and Rashida Tlaib, and former Rep. Brenda Lawrence.

Gov. Gretchen Whitmer, Lt. Gov. Garlin Gilchrist II, Michigan Secretary of State Jocelyn Benson, Rep. Haley Stevens, and state Sen. Mallory McMorrow have all ruled out Senate campaigns.

Michigan Attorney General Dana Nessel during a January interview on MSNBC said she intended to remain in her current position.

The actor Hill Harper, best known for his roles on "CSI: NY" and "The Good Doctor," may enter the Democratic primary this spring, while businessman Nasser Beydoun has announced a Senate exploratory committee and listening tour.

Two Republicans are currently in the race: Nikki Snyder, a member of the Michigan State Board of Education, and Michael Hoover, a small business owner.

Other GOP candidates who could potentially compete in the contest include Rep. Lisa McClain, former Reps. Peter Meijer and Mike Rogers, state Sen. Ruth Johnson, and 2022 gubernatorial nominee Tudor Dixon.

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

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 Republican orientation of the state, Tester has a solid political brand and has been able to appeal to many of the state's Independents and Republicans in past elections. But GOP leaders in Washington continue to eye his seat.

Reps. Matt Rosendale and Ryan Zinke are potential candidates on the Republican side.

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 will be a top target for the eventual presidential nominees of both parties.

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 will target this seat, 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.

Ohio Republican Secretary of State Frank LaRose in a February interview with Spectrum News confirmed that he is considering a Senate bid.

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.

Potential Republicans who could enter the race include former 2022 Senate candidate David McCormick and 2022 gubernatorial nominee Doug Mastriano.

West Virginia

Sen. Joe Manchin has not yet announced his intentions for 2024, but he is likely the only Democrat who can hold the seat in such a challenging state for his party. West Virginia was Trump's second-best performing state in the 2020 election, so Manchin will have to win a lot of ticket-splitters to secure another term in the nation's capital. But he has done it before, winning in 2018 despite Republicans going all-out to defeat him.

GOP Rep. Alex Mooney has already announced that he will run for the seat.

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.