McNulty made headlines recently while sitting in the Speaker's chair during a contested House vote. Republicans claimed McNulty and the Democrats "stole" the vote, but Democratic leaders denied the allegation. McNulty later publicly apologized for the incident, and a select committee has been created to look into the matter.
McNulty, who just turned 60 last month, was first elected to the House in 1988. He serves as the chairman of the Social Security subcommittee on the Ways and Means Committee.
McNulty would be the third House Democrat to retire this cycle. The other two, Reps. Tom Allen (D-Maine) and Mark Udall (D-Colo.), are running for the Senate.