Joe Lieberman, the former US vice-presidential candidate who infuriated Democrats by backing Republican John McCain for president in 2008, is to retire.
Word of the four-term senator’s decision came hours after North Dakota senator Kent Conrad also announced his retirement, leaving Democrats with two seats to defend in a difficult political environment.
Mr Lieberman, 68, will officially announce his decision at an event in Stamford, Connecticut.
The senator nearly won the vice presidency on the Democratic ticket with running mate Al Gore in 2000. He was defeated the last time he ran for the Democratic Senate nomination in Connecticut in 2006, but won a new term running as an independent in a three-way race.
There had been speculation about whether Mr Lieberman, who usually votes with Democrats, would run in 2012 as a Democrat, Republican or independent.
But Democratic officials said Mr Lieberman would not run for re-election in 2012.
Top Democrats like Senator Christopher Dodd and President Barack Obama who had supported Mr Lieberman in the 2006 primary instead backed Democratic nominee Ned Lamont in the autumn election. Mr Lieberman was disappointed that some old friends were not loyal to him.
In the years since, he aligned himself with Democrats in the Senate, who permitted him to chair a committee in return. But in 2008 he supported Mr McCain, the Republican presidential candidate, who put the Connecticut politician on his list of potential vice presidential running mates.
Mr Lieberman’s decision to speak at the 2008 Republican presidential nominating convention angered Democrats and the speech he gave contrasting Mr Obama to Mr McCain infuriated them even more.
“In the Senate, during the three and a half years that Senator Obama has been a member, he has not reached across party lines to accomplish anything significant, nor has he been willing to take on powerful interest groups in the Democratic Party to get something done,” Mr Lieberman said at the time.