The big mystery in respect of Senator Manchin is why he hasn’t become a Republican. His explanation over the weekend on why he “just can’t” vote for President Biden’s budget suggests that his quarrel goes past the particulars of the president’s program. “My Democratic colleagues in Washington,” he said, “are determined to dramatically reshape our society in a way that leaves our country even more vulnerable to the threats we face.”

Added he: “I cannot take that risk with a staggering debt of more than $29 trillion and inflation taxes that are real and harmful to every hard-working American at the gasoline pumps, grocery stores and utility bills with no end in sight.” What of those sentiments wouldn’t sound more logical, more convincing, being uttered by a Republican? So why in the world hasn’t the Mountain State mensch made the switch?

We understand that the question has been out there for some time. He is, after all, the only Democrat elected statewide in West Virginia, which since 2000 has been voting Republican in presidential elections. As recently as Friday, no less a figure than Senator McConnell told reporters: “I’ve suggested for years it would be a great idea, representing a deep-red state like West Virginia, for him to come over to our side.”

Mr. Manchin has turned his denials into works of art — if sometimes hilarious ones. Here he is answering Fox News in July: “If switching a party, or whether you have a ‘D’ by your name or an ‘R’ by your name, changes who you are as a person, then you’re in the wrong profession, and it’s all about you and not about the oath you take to the office, the oath to the Constitution, to protect and defend.”

In October, Mr. Manchin was captured on Twitter suggesting that talk of a party switch was “bullshit spelled with a B, U, L, L, capital ‘B’.” Yet on December 8, CNN quotes him as saying: “I’m caught between the two, but the bottom line is you have to be caucusing somewhere … If they asked me to leave, well, I’ll just have to say, ‘I guess I’ll have to abide by your wishes.’ … I don’t intend to leave. But I intend to be honest.”

“That’s not exactly a ringing endorsement of him always staying in the Democratic Party, right?” asks CNN editor-at-large Chris Cillizza. Strange things happen in politics, no doubt, but it’s hard to see where Mr. Manchin — or the Democrats — go after the weekend’s events. The ex-governor of West Virginia made his “I just can’t” announcement on Fox News. The White House, via spox Jen Psaki, promptly issued a shockingly harsh statement.

The statement — a diatribe of 700 words — seemed to question both Mr. Manchin’s honesty and integrity. His comments on Fox, Ms. Psaki said, “are at odds with his discussions this week with the President, with White House staff, and with his own public utterances.” She claimed that Mr. Manchin had pledged to negotiate “in good faith,” suggesting, en passant, that he had in the end failed to do so.

Is he going to hang around the Democratic Party for that? To us it looks like the long goodbye in slow motion. And reminds us of President Reagan’s famous formulation about how he didn’t leave the Democratic Party, the Democratic Party left him. It’s a phenomenon that has happened to a lot of erstwhile Democrats, including your editor. Though rarely has it happened in such a high-octane drama as the one in which Mr. Manchin is starring.