Updated at 5:44 p.m. ET

Sen. Joe Lieberman, a former Democrat who angered the party by supporting Republican John McCain's presidential bid, reportedly will not run for re-election next year.

Unnamed Democratic sources in Connecticut told The Huffington Post that Lieberman will not seek a fifth term. The Washington Post also reported that Lieberman plans to retire, citing two Democratic sources familiar to the decision.

Lieberman, who became an independent in 2006 after losing a Democratic primary, is set to formally announce his plans at 12:30 p.m. ET Wednesday at the Marriott hotel in Stamford, Conn.

Erika Masonhall, a Lieberman spokeswoman, told Politico earlier in the day that the senator "made a decision about his future over the holidays." She declined further comment.

If Lieberman is not a candidate for re-election, that would make three Senate retirements in about a week. Sen. Kent Conrad, D-N.D., said today he is not going to run for re-election and Sen. Kay Bailey Hutchison, R-Texas, made her announcement last week.

Democrat Susan Bysiewicz, a former secretary of State in Connecticut, announced this morning that she intends to run for Lieberman's seat. She told the Hartford Courant she was heartened by the encouragement from Democrats across the state.

Bysiewicz criticized Lieberman as she announced her Senate campaign, saying Connecticut needs "a senator who is 100% focused on helping our state."

Lieberman, 68, is in his fourth term in the Senate. His seat is one of 23 that Democrats have to defend in the 2012 elections.

In 2000, Lieberman became the first person of Jewish faith on a major national ticket when Democrat Al Gore selected him to be his running mate.

The senator, long known for working across the aisle with Republicans, became an independent after being defeated by Ned Lamont in the 2006 Democratic Senate primary. The race became a referendum on the war in Iraq and Lamont was steadfastly anti-war.

Lieberman was an active supporter of his friend McCain's presidential campaign in 2008, and spoke out on the Arizona senator's behalf during the Republican National Convention that year.

Last year, Lieberman worked with Republican Susan Collins to push the Senate in overturning the "don't ask, don't tell" policy that banned gay men and women from serving openly in the military.

Republican Linda McMahon, who ran unsuccessfully for Connecticut's other Senate seat last year, has indicated she is open to running again in 2012. McMahon, former CEO of World Wrestling Entertainment, spent more than $40 million of her own money. She was defeated by Democrat Richard Blumenthal.

Brian Walsh, communications director of the National Republican Senatorial Committee, vowed to mount an aggressive campaign for Lieberman's seat.

"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," Walsh said in a statement.

In other 2012 campaign news, veteran Republican Sen. Richard Lugar told reporters that he intends to run for a seventh term in Indiana despite the Tea Party movement's attempt to find a challenger to run against him. Lugar made his announcement at a breakfast for reporters organized by the Christian Science Monitor.