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.
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.