Without Sen. Joe Lieberman, Democrats could not have taken control of the Senate.
When Lieberman sought re-election two years ago, his support for the war in Iraq cost him his party's nomination. He went on to win as a self-styled "independent Democrat," but still caucused with Senate Democrats, giving them a bare majority of 51. Democratic leaders, in turn, made Lieberman chairman of the Homeland Security Committee.
But now Lieberman is backing Republican John McCain for president, and that has sparked an online drive to have him stripped of that chairmanship.
A Web site suggestively named LiebermanMustGo.com features a video montage of Lieberman uttering to news anchors what sound like heresies to many Democrats.
"I think the Democratic Party, to its damage, has left that tradition of a strong foreign and defense policy," he says in one clip. "And that includes the leading Democratic presidential candidate."
The site also invites visitors to sign an online petition to have Lieberman stripped of his committee chairmanship after the November elections, when Democrats expect to pick up more Senate seats.
Robert Greenwald's production company, Brave New Films, made the video. He says the point is to pressure Senate Democrats with a petition that now has more than 48,000 signatures.
"Given that Lieberman is actively and aggressively attacking the candidate for presidency of his party, it seems pretty clear he should not be given any of the perks that come with being a member of the Democratic Party," Greenwald says.
But Lieberman says he honestly does not care about the drive to remove his chairmanship.
"That decision is up to my colleagues in the Democratic caucus next year," he says. "I'm going to do what I think is right now and not worry about what the political consequences may be for me."
Lieberman says he will not only continue to campaign for McCain, but he is also willing to sing McCain's praises at the GOP convention in St. Paul.
"I strongly support John McCain. I believe in him, his strength, his principle, his effectiveness, so if he thought that I could help him by speaking at the Republican convention, I would do that," he says. "And I would do it really not to go and attack anybody but to explain why I as a Democrat have crossed party lines to support John McCain and why I hope a lot of other Democrats and independents will do the same."
Such talk has not gone over well with Lieberman's Democratic Senate colleagues, including California's Dianne Feinstein.
"I very much regret what he's doing, candidly, but there isn't anything I can do about it, and that's really all I want to say," Feinstein says.
Senate Democrats are generally reluctant to criticize Lieberman publicly. Chris Dodd, who like Lieberman is from Connecticut, says the anti-Lieberman petition drive is misguided.
"I think they're heading in the wrong direction," Dodd says. "And I see Joe as an ally on most matters and a person who's been supportive of Democrats here on most issues."
Another defender is Maine Republican Susan Collins, to whom Lieberman gave $10,000 for her re-election.
"My hope is that he will not be punished for doing what he thinks is right," Collins says.
Might Lieberman switch sides to hand Republicans the Senate majority? Lieberman says that thought hasn't really crossed his mind. He has no complaints, he says, about Senate Majority Leader Harry Reid.
"Sen. Reid has been easy to work with," Lieberman says. "I have a lot of good friends in the caucus, and I don't have any intention of doing that before the end of this session of Congress."
What's still unanswered is what Lieberman might do in the next session of Congress, when Democrats expect to return with a bigger majority. Though Reid defends Lieberman, he too is vague about the future.
"Anytime we have a problem here, with the exception of Iraq, Joe Lieberman's with us," Reid says. "So I wish people would leave him alone."
When asked about next year, he says, "We'll get to next year when we get there."
In the meantime, Lieberman seems assured his apostasy won't cost him his chairmanship.