Drew Angerer/The New York Times

Senator Joseph I. Lieberman will announce on Wednesday that he will not seek a fifth term, according to a person he told of his decision.

Mr. Lieberman, whose term is up in 2012, chose to retire rather than risk being defeated, said the person, who spoke to the senator on Tuesday.

“I don’t think he wanted to go out feet first,” the person said.

A longtime Democrat who lost a bitter primary battle to Ned Lamont in 2006, Mr. Lieberman won re-election as an independent that year, largely benefiting from a weak showing by the Republican candidate, who received less than 10 percent of the vote.

But Linda McMahon, the wealthy pro-wrestling tycoon who spent $50 million on an unsuccessful Senate race last year, has already signaled she may run again in 2012.

Aides to Mr. Lieberman were in Stamford late Tuesday making preparations for his announcement on Wednesday.

One aide, speaking on condition of anonymity ahead of the announcement, said Mr. Lieberman arrived at his decision around Thanksgiving but postponed revealing his decision until after the legislative effort that culminated in the repeal of the “don’t ask, don’t tell” ban on gays in the military – an effort in which Mr. Lieberman played a central role.

Mr. Lieberman’s decision comes as potential challengers start to emerge. On Tuesday, the state’s former Democratic secretary of state, Susan Bysiewicz, announced on her Facebook page that she intended to run for Senate next year.

A person close to Representative Christopher Murphy, a three-term Democrat from Connecticut, said he was considering a run for Mr. Lieberman’s Senate seat and would announce his intentions in the coming weeks.