WASHINGTON—The senator Republicans love and liberals love to hate is at it again.

Connecticut Sen. Joseph I. Lieberman, who no longer considers him-self a Democrat but is nevertheless holding tight to a coveted chairmanship, thoroughly rattled the Obama administration and his colleagues in the last few days when he threatened to join Republicans in blocking a health care overhaul over the issues of government-run insurance and an expansion of Medicare.

“Say it ain’t so, Joe,” Vice President Biden said Tuesday on MSNBC’s “Morning Joe.” “I think Joe’s judgment is wrong.”

Don’t count him as a “no” vote just yet, Lieberman indicated Tuesday.

As long as any government plan or Medicare expansion stays out of the bill, “then I’m going to be in a position where I can say what I’ve wanted to say all along: that I’m ready to vote for health care reform,” the senator told reporters.

Lieberman isn’t the typical Democrat the party machinery is primed to appease as President Obama and Majority Leader Harry Reid, D-Nev., cobble together the 60 votes required to pass a health care bill.

No, this is Lieberman—Democratic presidential candidate Al Gore’s running mate in 2000 but now independent in every sense of the word. Lieberman lost his party’s primary in 2006 in part because of his pro-Iraq War votes and won re-election as an independent.

Then he committed what some Democrats still consider the ultimate apostasy: He endorsed Republican Sen. John McCain of Arizona with a rousing speech at the 2008 Republican National Convention and campaigned heartily for his longtime friend.

Back in the Senate with McCain— after Obama moved into the White House—Lieberman later apologized to his Democratic colleagues. Seething but needing Lieberman to get to the 60-vote majority required to break filibusters, they allowed him to keep the chairmanship of the Homeland Security Committee.

From Obama on down, they vowed to step past the unpleasantness and look to the future.

But there proved to be yet another chapter in Lieberman’s tortured relationship with Democrats.

By Tuesday, he was telling Obama and his colleagues that the debate has been tough on him. Outside with reporters, he blamed the tussle on Democrats trying to cram too much— namely, any kind of government health insurance—into the overhaul.

“What happens then is you run the risk of losing everything,” Lieberman told reporters.

That’s exactly what Democrats feared was happening Sunday, when they heard Lieberman call the latest iteration of their bill—a plan to allow people ages 55 to 64 to buy Medicare benefits—too costly.

There, potentially, went Reid’s 60th vote, a development that unnerved Democrats at both ends of Pennsylvania Avenue and must have come in some measure as a surprise to Reid. Only two months earlier, the majority leader had called Lieberman “the least of Harry Reid’s problems.”

By Monday, Lieberman headed Reid’s list of senators who must be accommodated. Democrats circulated a videotaped newspaper interview from September showing Lieberman saying he supports the idea of a Medicare expansion. Lieberman said in a statement later that the details of the bill had changed and so had his opinion.

The White House got involved, and by nightfall, the Medicare provision was out. And Lieberman sounded like he was back in.

During a private meeting with Senate Democrats on Tuesday at the White House complex, Lieberman said, Obama greeted him warmly.

“He said, ‘Hello, how are you, brother?’ It was a good conversation,” recalled the senator, who in an interview with CNN signaled that he could switch parties again. “It’s unlikely I’d run as a Republican, but I wouldn’t foreclose any possibility. I think the more likely thing is that I’d run as an independent because that’s what I am,” he said.

Congressional Democrats, both furious and wary of re-offending their mercurial colleague, began speaking of Lieberman only when asked, mostly in the bland terms.

What was Lieberman thinking? “I have no idea,” Sen. Debbie Stabenow, D-Mich., said with a polite grin. “Our focus is on 60 votes.”

Does he have too much power over the shape of the overhaul?

“That’s a Senate matter,” House Speaker Nancy Pelosi, D-Calif., demurred.

“You’re too Lieberman-focused,” Sen. Jay Rockefeller, D-W. Va., told reporters afterward.

Lieberman himself said later that he is prepared to live with his opposition to publicly funded insurance— convinced, he said, that it would increase taxes.

Inside Obama’s closed-door meeting at the White House complex Tuesday, Lieberman stood.

“I said, ‘I haven’t really had a lot of fun the last couple of weeks,’ ” the besieged senator told reporters.

“And then [Ohio Democratic Sen.] Sherrod Brown got up and said, ‘I haven’t been having fun, either,’ ”Lieberman recalled. “And the president said, ‘Why don’t we all begin to have some fun? Let’s pass the bill.’ ”