Word of Lieberman's decision came just hours after North Dakota Sen. Kent Conrad, a Democrat, announced he would retire. Lieberman is an independent who usually votes with Democrats. Republican Sen. Richard Lugar of Indiana on Tuesday said he plans to seek a seventh term next year despite a challenge from tea party groups.
Lieberman's seat could pose a pick-up opportunity for Democrats in a state where President Obama has been popular.
Lieberman plans to announce his decision midday Wednesday at an event in Stamford, Conn. Lieberman nearly won the vice presidency on the Democratic ticket with running mate Al Gore in 2000.
Democratic officials, who requested anonymity because they were not authorized to speak publicly, said Tuesday that the independent who usually votes with Democrats would not run for re-election in 2012.
Lieberman with McCain (Photo: AFP)
Lieberman, 68, 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.
Top Democrats like Sen. Christopher Dodd and President Barack Obama who had supported Lieberman in the 2006 primary instead backed Democratic nominee Ned Lamont in the fall general election. Lieberman was disappointed that some old friends weren't loyal to him.
In the years since, he aligned himself with Democrats in the Senate, who permitted him to chair a committee in return. Yet in 2008 he supported McCain, the Republican presidential candidate, who put the Connecticut lawmaker on his list of potential vice presidential running mates.
Lieberman's decision to speak at the 2008 GOP presidential nominating convention angered Democrats, and the speech he gave contrasting Obama to McCain angered them more.
"In the Senate, during the 3 1/2 years that Sen. 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," Lieberman said at the time.
Connecticut Democrats also have criticized Lieberman's strong support of the Iraq war, although they were pleased when he led the recent Senate fight to repeal the ban on gays serving openly in the military.
Lieberman's poll ratings in his home state had slipped in recent years, encouraging Democratic challengers and sparking speculation about the senator's retirement. Lieberman's colleague, Dodd, recently retired from the Senate.
Hours before Lieberman's plans became public, former Connecticut Secretary of the State Susan Bysiewicz said Tuesday she'll run in 2012 for Lieberman's seat.
Two Connecticut House Democrats, Chris Murphy and Joe Courtney, are also considering a run. Republican Linda McMahon is also seen as a potential challenger, despite losing her Senate bid last year against Democrat Richard Blumenthal.
There had been speculation about whether Lieberman would run in 2012 as a Democrat, Republican or independent.
After the 2008 election and at Obama's urging, Senate Democrats decided not to punish Lieberman for supporting the GOP ticket. They voted to let him keep his chairmanship of the Homeland Security and Governmental Affairs Committee. Obama was eager to strike a bipartisan tone for his presidency.
Two years ago, some state Democrats wanted to censure Lieberman for his actions. Ultimately, he was sent a stern letter. Since that time, he has had scant interaction with the party.
Lieberman told The Associated Press late last month that he was considering whether to seek another term in the Senate.
"It's a difficult decision for me because I really have loved my service here in the Senate, and I feel privileged to be here," he said. "I guess the question is — and I think you've always got to ask is — 'Now, after 22 years, 24 years after this term is over, do I want to do it again? Or, do I want to try something else?' That's the question you've got to answer."
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