Then, on Sunday, Lieberman seemed to reverse himself on that provision, notifying Senate Majority Leader Harry Reid of Nevada that he couldn't support the expansion in Medicare after all. That leaves Senate Democratic leaders, who can't afford to lose a single vote on health care, poised to strip that out as well, with the White House's acquiescence.
Some liberals say Lieberman is shifting his positions to infuriate liberals, in part out of revenge for the 2006 campaign that saw him lose the Democratic primary.
"He's petty and egotistical, and that's what's driving his actions," said Darcy Burner, executive director of the American Progressive Caucus Policy Foundation. "He's decided that his ego is far more important than doing what the American people need."
Marshall Wittmann, a Lieberman spokesman, dismissed such criticism as "just noise."
Although Lieberman supported a Medicare "buy-in" expansion as part of the Gore-Lieberman platform in 2000, that was before the current fiscal crisis exacerbated concerns over deficits, and before a workable bill emerged that would accomplish many of his other health care priorities, he said.
"The fundamental core of the [Senate] bill he agrees with," Wittmann said. "What he disagrees with, and what has become the contentious part, are what he views as the unnecessary add-ons, such as the public option and the Medicare buy-in."
The fight is in some ways an outgrowth of the dispute that led liberals to try to oust Lieberman from the Senate in 2006. If anything, they've grown angrier with Lieberman since then, particularly after his aggressive advocacy on behalf of Sen. John McCain's presidential campaign last year.
Lieberman was a solid liberal for much of his Senate career, up until the moment he distinguished himself by being among the first Democrats in Congress to speak out forcefully against President Bill Clinton's extramarital affair with Monica Lewinsky.
That episode helped seal a spot for Lieberman as Al Gore's running mate in 2000. And while his breaks with the left started to emerge on national security issues during President George W. Bush's first term, Lieberman remained a plausible Democratic presidential candidate in 2004.
But his outspoken support for Bush on national security issues quickly put him in the minority in his own party in Bush's second term. That fueled a primary race by businessman Ned Lamont, a newcomer to politics who went on to defeat Lieberman in the primary.
Lieberman wound up winning reelection in 2006 as an independent, with a vow to continue to caucus with Democrats. While Lieberman's splits with Democratic orthodoxy were mostly confined to national security at the time, they've widened since then to include items such as government spending and, now, health care legislation.
"He clearly lied to the voters of Connecticut," Burner of the American Progressive Caucus Policy Foundation said.
Lieberman spokesman Wittmann disputes such talk, although he acknowledges that his boss' outlook on serving in the Senate was changed by his experience in 2006.
"From his perspective, it was a very liberating experience," he said. "He was reelected in a unique way, as an independent. He's going to view things through an independent prism. Sometimes it upsets his Democratic colleagues, but that's what he believes his obligation is to his state and to his country."