DES MOINES, Iowa - U.S. Sen. Tom Harkin, an Iowa Democrat and one of the most liberal senators, said on Saturday he will not seek re-election in 2014, saying he felt that after 40 years in Congress, it will be "somebody else's turn."

Harkin, 73, who has focused much of his long congressional career on farm policy and legislation aiding people with disabilities, is the third senator facing re-election in 2014 who has announced his retirement, following Democrat Jay Rockefeller of West Virginia and Republican Saxby Chambliss of Georgia.

"I don't by any means plan to retire completely from public life at the end of this Congress," Harkin said in a statement. "But I am going to make way for someone new in this Senate seat. I think that is right not just for me, but for Iowa, as well."

Iowa, site of the early presidential caucuses, is considered a political swing state and Harkin's departure could put the Democratic Party's long-time hold on the seat at risk. Republican Charles Grassley is Iowa's other U.S. senator.