"I will be proud to be a sponsor of the important effort to enable patriotic gay Americans to defend our national security and our founding values of freedom and opportunity," he said in a statement Monday in which he noted his longtime opposition to the policy.
"To exclude one group of Americans from serving in the armed forces is contrary to our fundamental principles as outlined in the Declaration of Independence and weakens our defenses by denying our military the service of a large group of Americans who can help our cause," said Lieberman, who has angered liberals with his resistance to Democratic orthodoxy on issues like health care reform.
The New York Daily News, which first broke the news, interviewed the Connecticut senator about his opposition to the policy, which was crafted under President Bill Clinton in 1993 following a divisive battle over gays in the military.
"My own experience as a member of the Armed Services Committee, visiting our troops on bases here in this country and abroad, particularly in war zones, the most remarkable quality you'll find is unit cohesion," the former Democrat said. "What matters is not the gender of the other person in your unit or the color or the religion or in this case the sexual orientation. It's whether that person is a good soldier you can depend on."
President Obama has repeatedly vowed to end the policy, which has meant the discharge of thousands of members of the military since implementation. He has been criticized by gay rights group for not moving more quickly on gay rights issues, including the "Don't Ask, Don't Tell" repeal.
Momentum seems to be building for repeal, thanks in large part to comments earlier this month before Congress by Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, who said he personally believes the policy is wrong.
But the military's timeline has been a source of concern for advocates of repeal. Mullen and Defense Secretary Robert Gates have announced a working group to study integration of openly gay service members, which has a deadline of the end of 2010. The working group, which will poll service members for their attitudes about serving with openly gay peers, makes repeal this year unlikely.
On NBC's "Meet the Press" Sunday, Gen. David H. Petraeus defended the timeline, saying the process is necessary to determine the best "policies that could be used to implement a change if it does come to that."
Lieberman, who calls repeal "an extension, the next step of the civil rights movement," says limiting the pool of possible service members by banning those who are openly gay amounts to "diminishing military effectiveness."
Republicans have resisted a repeal of the policy, which they say has been effective, and argue that this is not the time to make a change. With polls showing that a majority of Americans support repeal, it is not clear whether Senate Republicans would use the threat of filibuster to force Democrats to garner 60 votes to overturn the legislation.