Senators Joe Manchin and Kyrsten Sinema are both up for reelection in 2024. They're also both heading to Switzerland to represent America at the World Economic Forum.
Interestingly, both of these senators have yet to announce whether they're going to seek reelection in 2024. They're also people who previously ran as Democrats but have been getting further and further away from America's liberal party.
Sinema already renounced her Democratic Party membership, becoming an independent in 2022. Manchin hasn't officially left the Democratic Party yet, but there have been plenty of rumors indicating he could soon.
That's a problem for Democrats, because these two seats are VERY important to their majority in the Senate. So important, in fact, that other liberals could actually be contemplating spending money to make sure that these two get reelected, despite them recently denouncing the party.
Manchin's seat is regarded as possibly the hardest in the country to keep, as West Virginia went red for Trump by 29% in 2020.