"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."

link
http://cityroom.blogs.nytimes.com/2011/01/18/lieberman-will-not-run-for-re-election/