Joe Manchin was a unicorn. Now he's retiring.

A bleak Senate outlook for Democrats just got bleaker.

On Thursday, Democratic Sen. Joe Manchin announced that he would not run for reelection in 2024. That's significant because Manchin occupies the reddest Senate seat currently held by a Democrat in the entire country: West Virginia.

West Virginia voted for former President Donald Trump by a whopping 39 percentage points in 2020. Democrats were already going to have a hard time holding the seat with Manchin as their standard bearer; now, Republicans are virtually guaranteed to flip it, giving them a big head start in their race to recapture a Senate majority.

Thanks to his moderate views and willingness to buck the Democratic Party line, Manchin managed to win three Senate elections in West Virginia despite the state's strong Republican lean in presidential races. He was, by far, the strongest Senate candidate Democrats have put up anywhere in the country in the last three election cycles, according to political analysis website Split Ticket.

Drawing on the concept of "wins above replacement" in baseball, Split Ticket has quantified candidate quality for every Senate candidate in 2018, 2020 and 2022 by comparing the candidate's margin to how an average Democrat or Republican would have done in that state based solely on non-candidate-specific factors such as the state's partisanship and the national political environment. In his last election, Manchin performed 31 points better than a generic Democrat in West Virginia would have, according to Split Ticket's calculations. That's almost twice as strong as Democrats' next-strongest candidate.

But even Manchin might not have been able to win West Virginia in 2024. While the state continued to regularly elect Democrats in down-ballot races through the early 2010s, presidential partisanship has caught up with the party. When Manchin was first elected to the Senate in 2010, Democrats held every statewide office in West Virginia: two Senate seats, the governorship, attorney general, secretary of state, treasurer, auditor and commissioner of agriculture. Today, Manchin is the only one left.

The West Virginia Democrats who got swept away were no slouches, either. Two-term Secretary of State Natalie Tennant and six-term Treasurer John Perdue were electoral juggernauts in their own right before losing in 2016 and 2020, respectively. Manchin very well may have met the same fate. According to early polls of the 2024 race, Manchin had a small lead over one of his potential Republican challengers, Rep. Alex Mooney, but was significantly trailing another, popular Gov. Jim Justice.

Still, Manchin at least had a fighting chance at holding his Senate seat for Democrats in 2024. Democrats probably don't have any other candidate capable of swimming against the partisan tide of the state. As already alluded to, Democrats not named Manchin have gone 0-for-8 in statewide elections in West Virginia since 2018; the closest they came was Perdue's 13-point loss in 2020. In recognition of that, election handicappers such as the Cook Political Report and Inside Elections have already moved the race to "Solid Republican."

Democrats currently have just a 51-49 majority in the Senate,* so the loss of Manchin's seat will sting. Even if no other seats change hands, the chamber will be tied 50-50, which could give Republicans control of the chamber if they win the presidential race (because the vice president breaks ties in the Senate).

But Republicans have a good shot at picking up even more seats, too. Two other Democratic senators are up for reelection in states Trump carried in 2020 (and is likely to carry again in 2024): Montana Sen. Jon Tester and Ohio Sen. Sherrod Brown. Neither Montana nor Ohio is anywhere near as red as West Virginia, but both states are golden pick-up opportunities for Republicans. By contrast, Democrats are hoping to make gains on less hospitable turf. Their best pick-up opportunities are in states that Trump carried by 3 points (Florida) and 6 points (Texas).

Manchin's retirement could redound to Republicans' benefit in those races, too. By taking West Virginia off the list of competitive states, it frees up Republican outside groups to spend money on those other races. That's also true for Democratic outside groups, of course, but they probably weren't going to need to spend as much in West Virginia anyway. Manchin retires with more than $11 million in the bank; Mooney and Justice have less than $3 million combined.

Republicans were already in a good position to flip the Senate in 2024 before Manchin's announcement. But his retirement strengthens their odds even further.

*Including three independent senators who caucus with the Democrats.