In a written statement issued in Washington, Sens. Harry Reid of Nevada and Chuck Schumer of New York, the party's leader and the head of its campaign committee, said they "fully support" Lamont.
House Minority Leader Nancy Pelosi, D-Calif., is also throwing her support behind Lamont.
"Voters in Connecticut and across the nation are seeking a new direction to make America safer, our economy stronger, and to broaden opportunity for all," Pelosi said in a statement. "I respect the decision of the voters and endorse the Democratic candidate, Ned Lamont, and look forward to working with him for a great congressional victory in Connecticut."
Connecticut Democratic leaders tossed aside their longtime friendships with Lieberman Wednesday, pledging to support primary winner Ned Lamont over Lieberman's independent campaign this fall.
"This is a difficult moment," said Sen. Christopher Dodd, D-Conn., who waited with Lieberman on Tuesday night as the returns came in. "He's made a decision to run as an independent. I regret that decision, but it was his decision to make."
Sen. Hillary Clinton, D-N.Y., is also making the switch, even though she, like her longtime ally Lieberman, supported President Bush's decision to invade Iraq, CBS News correspondent Trish Regan reports.
"I've already spoken to Ned Lamont. I've already offered him financial help as well as any other help that he needs," Clinton said.
Lieberman said he was not bothered by losing the support of his Democratic peers, noting he lost Tuesday's primary even with their support.
"I think it would be irresponsible and inconsistent with my principles if I were to just walk off the field," Lieberman said in an interview with The Associated Press.
"I'm not sure if Lieberman understands the fact that he is now on his own," explained Maurice Carroll, director at Quinnipiac University Polling Institute. "The biggest obstacle is he is not a democrat. And, also, he's tarnished. He lost. You're not supposed to lose in politics."
His campaign had collected more than 18,000 signatures, more than twice the 7,500 needed to secure a spot on the November ballot. If approved, that would set up a three-way race for the fall among Lamont; Lieberman, a supporter of the war, and Republican Alan Schlesinger.
Final primary returns showed Lamont defeating Lieberman 52 percent to 48 percent.
"I believe as the race closed here in Connecticut, (voters) got a better sense of the true position I have going forward on Iraq, and we closed strong,"
"The Democratic voters of Connecticut have spoken and chosen Ned Lamont as their nominee," said Reid and Schumer, who said they "fully support" Lamont's candidacy and congratulated him on the victory and a "race well run."
"I hope that over the course of the coming days, Joe's friends, neighbors and constituents will prevail upon him to reconsider and unite with Democrats across Connecticut who voted for change tonight," Lamont's campaign said in an e-mail sent out late Tuesday night.
Asked Wednesday if there was anyone who could call and get him to change his mind, Lieberman replied:
"Respectfully no. I'm committed to this campaign," he said in an interview on NBC's "Today" show.