Why is this Democratic senator threatening the party’s best chance of retaking the Senate?

The most conservative Senate Democrat endorsed one of the most liberal Senate Republicans on Thursday.

Sen. Joe Manchin (D-WV) threw his weight behind Sen. Susan Collins (R-ME) as she heads for re-election in 2020.

"If she wanted me to, I would campaign for Susan Collins," Manchin said. "For America to lose somebody like Susan Collins would an absolute shame. I feel that strongly about her."

Democrats are in a fierce battle for the Senate in 2020. Already three seats behind, they'll need to flip at least four seats to win decisive control of the chamber — or at least flip three seats and win the White House, which would put a Democratic vice president in place to break any tie votes in the chamber.

Collins' seat in Maine is one of the best prospects Democrats have of making gains.

And if Democrats win the White House in 2020 without taking control of the Senate, the president's agenda will be essentially dead in its tracks. It would be a struggle to even get ordinary cabinet nominees approved, let alone any Supreme Court justices or crucial federal judges names, given how partisan routine appointments have become. And Democrats could forget about passing any major progressive legislation. If Collins were the 51st Republican senator in 2020, she would ensure that Mitch McConnell remains Senate majority leader, and McConnell would ensure that all Democratic priorities are thwarted.

So why is the West Virginia senator threatening his parties chances of reclaiming the chamber?

Manchin is by far the least liberal senator, which has allowed him to hold on to his seat in the deeply conservative West Virginia, where Trump won by 42 points in 2016. But it's hard to see why his endorsement of Collins is necessary, especially when he was just reelected. He may just have genuine affection for the Collins, who is similar to him ideologically, and little loyalty to his party, which is in many ways a liability in his campaigns.

For those bemoaning Manchin's endorsement, though, it may be comforting to consider that it may not be particularly helpful to Collins' reelection chances. It's not clear how much Maine voters will care about an out-of-state Democratic senator's point of view. If Collins' eventual Democratic opponent can make the case that she doesn't serve the state's interest, she may yet be defeated.