Sen. Tom Harkin, D-Iowa, a five-term veteran of the Senate, told the Associated Press in an interview that he will not seek a sixth term in 2014, saying, "It's just time to step aside."
Harkin, 73, would be 81 years old by the end of a sixth term, a factor that weighed on his decision to not mount another reelection bid.
He also said the move would enable a new generation of Iowa Democrats to seek higher office.
Harkin's decision comes as a surprise, given his $2.7 million campaign war chest and a fundraiser scheduled for next month. No Republicans had stepped forward to challenge Harkin, though the prospect of an open seat may entice more candidates to join the fray.
Harkin's retirement follows the lead of Sens. Saxby Chambliss, R-Ga., and Jay Rockefeller, D-W.V., who have said they will not seek reelection in 2014.