Republicans immediately began claiming victory.
“We like our odds in West Virginia,” Senator Steve Daines, Republican of Montana and the chairman of the National Republican Senatorial Committee, crowed in a statement.
David Bergstein, a spokesman for the Democratic Senatorial Campaign Committee, argued that the party remained in a strong position to hold the chamber, suggesting its candidates could pick off some Republican-held seats.
“Democrats have multiple pathways to protect and strengthen our Senate majority and are in a strong position to achieve this goal,” Mr. Bergstein said. “In addition to defending our battle-tested incumbents, we’ve already expanded the battleground map to Texas and Florida, where formidable Democratic candidates are out-raising unpopular Republican incumbents.”
Given his status as a Democrat from a deeply Republican state, Mr. Manchin was a constant source of attention on Capitol Hill. He repeatedly frustrated his fellow Democrats by breaking with them on progressive legislation, often sinking it because of their slim majority. In recent years, he has enjoyed virtual veto power on his party’s agenda, given Democrats’ need to stay united in the face of Republican opposition in the nearly equally divided Senate.
Progressive groups noted a long list of grievances with the West Virginia Democrat.
“Joe Manchin watered down the Democratic economic agenda, made the cost of raising children higher and billionaire taxes lower, and now doesn’t even run for re-election,” said Adam Green, a founder of the Progressive Change Campaign Committee. “History, and West Virginians who are struggling, will not judge Joe Manchin well.”