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.
In Cleveland, Republican Party chairman Ken Mehlman seized on the results in the Connecticut primary to assail the Democrats on national security and called Lieberman's defeat a "shame."
"Joe Lieberman believed in a strong national defense, and for that, he was purged from his party. It is a sobering moment," Ken Mehlman said.
Lieberman's loss made him only the fourth incumbent senator to lose a primary since 1980, and came just six years after he was the Democrats' choice for vice president.
Connecticut's results posed questions that went far beyond state lines.
Critics targeted Lieberman for his strong support for the Iraq war and for his close ties to President Bush. They played and replayed video of the kiss President Bush planted on Lieberman's cheek after the 2005 State of the Union address.
A CBS News/New York Times exit poll found that 78 percent of Democratic primary voters disapprove of the war in Iraq. Among opponents of the war, 60 percent voted for Lamont; among supporters of the war, 78 percent voted for Lieberman.
It's a sentiment that anti-war bloggers tapped into. They waged a successful cyber-campaign against their three-term senator, Regan reports.
"And they're using every tool at their disposal to try to push the nation toward a direction that is going to be better for people who are outside of Washington," Eli Pariser of MoveOn.org said.
Vote totals showed roughly 16,000 more ballots cast for the Democratic Senate primary than the party primary for governor.
"I think if you are running now as a status quo sort of establishment candidate, this sends a very dangerous signal to you, which is that voters are looking for something," Amy Walter, Cook Political Report senior editor, told The Early Show. "There is some change in the air. We're not just seeing it in primaries. We're clearly seeing this in national polls. Voters are saying 'We don't like the direction of the country.'"
Lieberman has had poll results on his side when it comes to a general election. A mid-July Quinnipiac University poll found that while Lieberman trailed Lamont among Democrats, he came out well ahead of both Lamont and Schlesinger among registered Connecticut voters of all affiliations.
In the run-up to the primary, 14,000 new Connecticut voters registered as Democrats, while another 14,000 state voters switched their registration from unaffiliated to Democrat to vote in the primary.
Wednesday morning, Lieberman campaign representative Dan Papermaster handed over two boxes of petitions to officially get Lieberman's name on the ballot, then left the Secretary of the State's office without commenting.
All the signatures turned in must be validated by the town clerks in the towns where they were submitted. Those that are not will go back to the town clerks, who will have two weeks to verify them.
Lieberman's petition to run as an independent raised eyebrows even before he filed: the CBS News / New York Times exit poll found that 61 percent of Democratic primary voters are opposed to Lieberman running as an independent. Even among those who voted for Lieberman in the primary, one in five said they did not favor his running as an independent.
Former Sen. Lowell Wiecker, a Lamont supporter who was unseated by Lieberman 18 years ago, said he believes a Lieberman defeat can embolden the Democrats for this fall's midterm elections, Regan reports.
"It sends a signal to the Democratic Party that you aren't going to win anything unless you stand up and make a clear case for getting out of Iraq," said Weicker, a legend in the state of Connecticut for his often independent views, and who was governor of the state as an independent.