Not just the policy middle; the area he has carved out as his own since declaring himself an "Independent Democrat" and winning a new Senate term despite losing a primary race in his home state of Connecticut.
Not just the political middle; although he's right there, too, perhaps the most important arbiter of what can fly in President Obama's health care bill.
Lieberman is once again in the center of a raging debate about political loyalties and what it means to be a Democrat.
In just one manifestation of liberals' frustration, filmmaker Michael Moore today called on Connecticut voters to start a recall drive to oust Lieberman from office, "or we'll boycott your state."
Depending on one's point of view, Lieberman might be a turncoat who deserves to be booted from the Democratic caucus for good. Or he's a critical moderate voice who's protecting Democrats from far-left elements inside their own party.
On this, both sides agree: Lieberman himself couldn't be happier.
"Lieberman is one vote, wagging the other 59 votes in the Democratic caucus, and getting tons of news time and attention in the process," said Adam Green, a fierce Lieberman critic who is co-founder of the Progressive Change Campaign Committee. "He loves that."
Lieberman has been an ebullient presence of late on Capitol Hill. He typically emerges from long meetings over arcane legislative details with a broad smile on his face, eager to share his take with the waiting collection of microphones and cameras.
"I'm for health care reform, and if we get together, we're going to deliver a health care reform bill that will provide the ability to get health care insurance for 30 million people that don't have it now," Lieberman told reporters after one such meeting Monday night.
Lieberman the Moderator
Does that mean Democrats can count on his vote? "Well, I've gotta see how things go now," Lieberman said.
To Lieberman's friends and allies -- a diminishing group inside the Democratic Party -- it's Lieberman at his best. He's serving as a great moderator -- in both senses of the word -- and is sticking to his principles despite competing political pressures.
Some moderate Democrats argue that Lieberman's efforts will ultimately pull the Obama White House's top priority in a direction where it can receive the broadest possible support.
"It's an important service to the country, and I think it's also an important service to the Democratic Party," said Al From, the founder of the centrist Democratic Leadership Council, who has worked with Lieberman for decades.
"He's done the right thing. And I really do believe that centrist Democrats -- Lieberman, [Nebraska Sen.] Ben Nelson, and the others -- are, in a sense, Obama's best allies. They may not realize it," From said.
Many on the left aren't buying that. Lieberman has scrambled the politics of health care overhaul continually in this process.
His objections to including a public option were a main reason Democrats fashioned a compromise that would replace such a mechanism with a Medicare expansion, allowing people as young as 55 to buy into the system.
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.
Lieberman the Independent
"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.
Lieberman the Target
"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."
As for what comes next, many liberals are pushing for Reid to strip Lieberman of his seniority and his committee chairmanship, if he joins a GOP filibuster of the health care initiative. But Reid is unlikely to take any such move because Lieberman's vote is critical on so many matters not related to health care.
If that can't happen, some are arguing that Democrats just ignore Lieberman, and instead put their focus on passing a health care bill through the budget reconciliation process, where only 50 votes would be necessary. But that's not the preferred White House strategy, and arcane budget rules could limit what might be accomplished inside the budget process.
Lieberman's next political move is unclear. He's up for reelection in 2012 but it's not certain whether -- or how -- he would run for reelection.
Asked by ABC's Jonathan Karl in October whether he'd run as a Democrat in 2012, Lieberman said, "That's an open question."
Liberal activists say that if he seeks another term, they'll finish what they started in 2006.
"I kind of see him as dead man walking in Connecticut," said Green, whose Progressive Change Campaign Committee stands ready to support would-be challengers.
In the meantime, Lieberman benefits from the simple fact that Democrats need him, even if many of them don't like him. He might be among the few political figures who comes out of the grueling health care debate with a smile.
"Transformational reforms need the broadest support possible," said the Democratic Leadership Council's From, who served as a top aide in the Clinton White House. "He's doing the right thing. It's at a time that they need it -- they really do need it."