WASHINGTON—Sen. Joe Manchin (D., W.Va.) said Thursday that he won’t run for another term in the U.S. Senate, as he faced a tough re-election bid in a state that has turned more conservative during his tenure. 

The decision is expected to hurt Democrats’ effort to hold on to the Senate, which they currently control 51-49. West Virginia is seen as one of Republicans’ top pickup opportunities, along with Montana and Ohio, and Manchin was seen as the only Democrat who had a shot at holding on to the seat.