Aware of the legislative battles ahead, Senate Democrats said they wanted to adhere to Mr. Obama’s call for reconciliation and leniency for Mr. Lieberman.

But a pragmatic dynamic was at work as well. Having added seven new senators to their side, Democrats want to avoid driving Mr. Lieberman into the Republican fold. Even though they remain short of the 60 needed to cut off filibusters, the Democrats are aiming to keep their majority as large as possible next year when, for the first time since 1994, they have control of Congress and the White House. Two other Senate seats, in Minnesota and Georgia, have yet to be decided.

“We have got some big issues here, and we need all hands on deck,” said Senator Christopher J. Dodd, Democrat of Connecticut, who had pushed to keep the retribution against his home-state colleague to a minimum.

Democrats voted 42 to 13 to let Mr. Lieberman stay at the helm of the Committee on Homeland Security and Governmental Affairs while removing him from the Environment and Public Works Committee, where he led a subcommittee. The formal resolution before the Democrats, considered in what was described as an emotional meeting in the Old Senate Chamber of the Capitol, also declared that the Democratic caucus “rejects and disapproves of Senator Lieberman’s statements against Senator Obama in the 2008 presidential campaign.”

Image Senator Joseph I. Lieberman, who supported Senator John McCain in the presidential race, was allowed to keep his committee chairmanship as Democrats sought to strengthen their majority. Credit... Stephen Crowley/The New York Times

Lawmakers who attended the session said that Mr. Lieberman openly discussed the political and personal hurt he had experienced when many of his colleagues campaigned against him after he lost a Democratic Senate primary in 2006 before winning re-election as an independent. After the vote, he expressed some remorse for his campaign comments but noted that the resolution did not chastise him directly for backing Mr. McCain, who returned to Capitol Hill on Tuesday to resume life as a senator.

Mr. Lieberman, who only eight years ago was the party’s nominee for vice president, said he could have made some statements “more clearly.” He added: “And there are some that I made that I wish I had not made at all. And obviously in the heat of campaigns, that happens to all of us. But I regret that. And now it’s time to move on.”