Harkin launched an unsuccessful presidential bid in 1992, with a campaign that focused on traditional liberal politics. He has not pursued higher office since, but has enjoyed the position of leading Democrat in a state where the first presidential voting is held, hosting candidates at his annual "Steak Fry" fundraiser.
Harkin's exact post office plans are unclear, though he has said he expects to be involved with a namesake institute at Drake University. Friends and colleagues say he will likely remain a public figure.
"I think there's no chance he fades away," said longtime political adviser Jeff Link.
U.S. Rep. Bruce Braley, the Democrat running to succeed Harkin in the Senate, has vowed to continue in his liberal footsteps. "The goal is to continue to fight for policies that are going to give people a shot at the American dream and expand the middle class," Braley said.