Sen. Joseph Lieberman, the Connecticut independent who votes with Democrats, will announce Wednesday that he is not running for re-election in 2012, a Lieberman aide told Fox News.
"At this stage in his life, it's time for a new season and new chapter," the aide said.
Lieberman, who was Democratic presidential nominee Al Gore's running mate in 2000, intends to remain in public life and fight for causes in which he's been involved, including national security, the environment and homeland security, the aide said.
Lieberman became an independent in 2006 when he lost the Democratic primary to Ned Lamont. But defying calls from Democratic leaders to call it quits, he launched a successful independent bid in the general election.
Lieberman would go on to irk Democratic leaders when he endorsed John McCain in the 2008 presidential election and speak at the Republican National Convention. But he has continued to caucus with Democrats, giving them crucial votes on key issues such as health care overhaul.
Lieberman's announcement will come on the same week that Sen. Kent Conrad, D-N.D., said he would not run for re-election in 2012, a development that could boost Republican chances of capturing the Senate.
Democrats will be defending 23 Senate seats in the 2012 election -- two of them held by Democratic-leaning independents -- while Republicans are only defending 10.
"With yet a second member of the Senate Democrat caucus preparing for retirement within a 24-hour period, all of us are left to wonder how many more Democrats may follow in their footsteps," Brian Walsh, a spokesman for the National Republican Senatorial Committee said in a statement.
"While next year's election is still a long way away, one thing is increasingly clear, which is that Connecticut Democrats are facing the prospect of a very contentious primary within their ranks," Walsh said.
"Even without a serious primary in the 2010 cycle, national Democrats were still forced to spend roughly $2 million to save a candidate who had initially enjoyed a 30-point lead in the polls," he said, referring to Dick Blumenthal, who would go on to easily defeat Republican nominee Linda McMahon, 53 to 45 percent, in a race to succeed retiring Sen. Christopher Dodd.
"So we are confident that in the months ahead, several well-qualified Republicans will look closely at this race and this will again be a competitive battleground next year," he said.