Lieberman's office did not offer any comment on his plans, saying only that he would make an announcement tomorrow in Stamford, Conn. "The Senator's remarks tomorrow will stand on their own," said a Lieberman spokeswoman.
Lieberman will be the second Democratic senator to make public his plans to leave the chamber in the past 24 hours. On Tuesday morning, North Dakota Sen. Kent Conrad said he would not seek a fifth term in 2012.
On the Republican side, Texas Sen. Kay Bailey Hutchison announced her retirement last week.
Lieberman's long-awaited decision means an end to a political career that took him from the 2000 Democratic vice presidential nominee and 2004 Democratic presidential candidate to a speaking role at the 2008 Republican National Convention in support of Sen. John McCain (R-Ariz.).
Lieberman's increased alienation from the Democratic Party -- primarily over his ardent support for the war in Iraq -- led to his defeat at the hands of cable television executive Ned Lamont in 2006. But Lieberman formed his own independent party and beat Lamont in a general election to win a fourth term.
His numbers among Democrats only declined since that time, however, making it next-to-impossible for Lieberman to run and win as a Democrat in 2012.
With Lieberman out, the focus will turn to a very tightly contested Democratic primary between Secretary of State Susan Bysiewicz and Rep. Chris Murphy. Bysiewicz announced her candidacy Tuesday while Murphy was expected to formally enter the race within days. Rep. Joe Courtney has also said he is considering a candidacy.
For Republicans, Linda McMahon, who lost an open seat race to Sen. Richard Blumenthal (D) in 2010, is considering another bid. Former Rep. Rob Simmons is also mentioned as is Tom Foley who was narrowly edged out in the gubernatorial race last fall.
Given Connecticut's Democratic lean, the party has to feel good about its chances now that Lieberman won't be splitting up the vote three ways and thereby lowering the win number for the eventual GOP nominee.
It's a rare bit of good news for Democrats who face daunting odds in 2012. They must defend 23 seats while Republicans have only 10 incumbents up for reelection in November 2012.