Despite Democrats holding a narrow 51-49 majority in the U.S. Senate, the prospect of maintaining or expanding it in the upcoming year is becoming more challenging for President Joe Biden’s party.

As reported by Newsweek, the Senate election map appears even more ominous following the announcement from West Virginia Moderate Democratic Senator Joe Manchin that he won’t seek reelection. This decision comes despite expectations of a tough race against popular GOP Governor Jim Justice in the Mountaineer State.

“The senator’s decision adds further pressure to Democrats, who are likely to struggle to retain control of the Senate next year as they are confronted with a difficult election,” Newsweek noted further, adding:

Six Democratic senators are facing reelection in states that former President Donald Trump won at least once in the last two presidential elections—including the crucial swing states of Michigan, Pennsylvania, and Wisconsin.

Trump remains the frontrunner for the Republican Party’s 2024 presidential nomination, while President Joe Biden is once again seeking the Democratic nomination. There are no Republican senators facing reelection in states that Trump lost in either of the last two presidential elections.

In 2016, when Donald Trump defeated two-time presidential candidate Hillary Clinton, all 34 Senate races were won by the party that also secured the presidency in each respective state. Similarly, in 2020, when Joe Biden emerged victorious, his party prevailed in 34 out of 35 states in the Senate races.

“Democrats have multiple pathways to protect and strengthen our Senate majority and are in a strong position to achieve this goal,” David Bergstein, Democratic Senatorial Campaign Committee (DSCC) communications director, told Newsweek earlier this week.

“In addition to defending our battle-tested incumbents, we’ve already expanded the battleground map to Texas and Florida, where formidable Democratic candidates are outraising unpopular Republican incumbents, and the DSCC is making investments to lay the groundwork for our campaigns’ victories,” Bergstein added.

In Arizona, Senator Kyrsten Sinema, who switched from the Democratic Party to become an independent in 2022, is expected to face both Republican and Democratic challengers if she chooses to run for reelection. While she has filed the necessary paperwork, an official bid announcement is pending. Arizona voted for Trump in 2016 and Biden in 2020.

Also, Michigan will see Democrats striving to retain a seat left vacant by the retiring Democratic Senator Debbie Stabenow. Stabenow secured reelection in 2018 with 52.3 percent of the vote.

In traditionally red Montana, Democratic Senator Jon Tester is seeking reelection, and Republicans see a favorable opportunity to flip his seat. Tester narrowly won reelection in 2018 with just over 50 percent of the vote.

“Once considered a swing state and a bellwether for presidential elections, in recent years Ohio has been solidly Republican,” Newsweek reported. “However, Democratic Senator Sherrod Brown is seeking a fourth term in 2024 after he was reelected in 2018 with 53.4 percent of the vote in the increasingly red state.”

In Pennsylvania, Democratic Senator Bob Casey is pursuing a fourth term after securing reelection in 2018 with 55.7 percent of the vote. The presence of Trump on the ballot is expected to pose a greater challenge for him. The state’s junior senator is Republican J.D. Vance.