Factbox-Joe Manchin's exit makes it harder for Democrats to defend US Senate majority

WASHINGTON (Reuters) - U.S. Senator Joe Manchin's decision not to seek reelection will make it even harder for his fellow Democrats to defend their 51-49 majority in the 2024 elections, when they will be defending 23 seats, while Republicans are defending only 11.

Here are some of what are expected to be the most challenging races for Democrats as they try to keep their grip on the legislative chamber whose power includes confirming nominees to the Supreme Court:

WEST VIRGINIA

With Manchin's retirement, popular Republican Governor Jim Justice aims to replace the Democrat in a state that has become solidly Republican. Republican former President Donald Trump beat Democratic President Joe Biden here by 39 points in 2020. Justice, a former Democrat, must first win the Republican Party primary. One of his opponents is right-wing U.S. Representative Alex Mooney, who worked to overturn Biden's election in 2020.

OHIO

Liberal Democratic Senator Sherrod Brown is seeking a fourth six-year term in a state, like West Virginia, that increasingly is Republican. Trump scored an 8-point victory over Biden in 2020. There are at least four Republicans seeking their party's nomination, including State Senator Matt Dolan.

MONTANA

Third-term incumbent Senator John Tester, a farmer with an easy demeanor, is used to close elections and 2024 might be no different. Trump won the state by 16 points in 2020 and Tester is the only Democrat in the state's congressional delegation. Among those seeking the Republican nomination is Tim Sheehy, a former Navy SEAL.

PENNSYLVANIA

Senator Bob Casey was first elected in 2006 and is the likely Democratic nominee next year. Biden barely won the state in 2020. Among Casey's possible opponents is Republican David McCormick, who ran unsuccessfully for Senate in 2022 but did not advance past the Republican primary.

ARIZONA

Independent Senator Kyrsten Sinema, who was elected to the Senate in 2018 as a Democrat, has not yet said whether she will seek a second term. She would face a potentially tough three-way race that is expected to include Democratic Representative Ruben Gallego. The Republican challenger could be Kari Lake, a close ally of Trump who lost a 2022 governor's race. Sinema is one of three independents who caucus with Democrats and are included in their 51-49 majority.

MICHIGAN

Senator Debbie Stabenow is not seeking re-election in a state that often displays Republican sentiments. Biden captured Michigan by just three points over Trump and the open Senate seat has attracted several aspirants from Democratic and Republican benches. Among those are Democratic U.S. Representative Elissa Slotkin and Republican Peter Meijer, a former member of the U.S. House who voted to impeach Trump for his actions preceding the Jan. 6, 2021, Capitol attack.

(Reporting by Richard Cowan; Editing by Scott Malone and Daniel Wallis)