Democrats "don't agree on everything. We don't agree on Iraq," Mr. Clinton said, calling the conflict the "pink elephant in the living room."
But "the real issue is, whether you were for it or against it, what are we going to do now. And let me tell you something, no Democrat is responsible for the mistakes that have been made since the fall of Saddam Hussein that have brought us to this point."
In a 20-minute speech to a capacity crowd in an ornate theater, Mr. Clinton went easy on Ned Lamont, whose challenge gained traction when he accused Lieberman of being too close to President Bush on the war and other issues.
"He seems like a perfectly nice man. He's got every right to run and he's waged a vigorous campaign," the former president said.
By contrast, he lavished praise on Lieberman, a third-term lawmaker whose once formidable lead in the polls has vanished.
Mr. Clinton said Lieberman has long been a loyal Democratic vote on issues as diverse as organized labor and the environment.
Mr. Clinton was greeted with cheers louder than Lieberman received from the audience, and the words "Four More Years" were clearly audible in the crowd.
Lieberman wasn't nearly as deferential to Lamont as Mr. Clinton was. "My opponent is peddling what I would call a big lie, and that is I'm not a real Democrat," he said.
And he proudly recalled that Mr. Clinton first volunteered to help him in 1970, when he was running for the state legislature in his first campaign.
"I'm in a big fight here," he said more than once during the day, and the polls, the recent addition of campaign staff and the decision to seek help from Mr. Clinton were all evidence of that.
The former president wasn't the only nationally known Democrat campaigning for Lieberman as the lawmaker sought to rebuild support among Democrats who long supported him.
Sen. Barbara Boxer of California, an ardent critic of the war, praised Lieberman for his stand on other issues. "If you want to meet a leader on the environment, a leader on all the difficult choice issues, you got one here," she said at a campaign stop at a candy store.
The most recent public poll rated the race a tossup with the wealthy Lamont, whose Web site boasts that he will give Connecticut, "finally, a senator who will stand up to George Bush."