By Kay Henderson

DES MOINES (Reuters) - Senator 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 senator.

Democratic Party officials said Harkin's announcement, coming early in the current two-year election cycle, would provide ample time to recruit a strong candidate.

U.S. Representative Bruce Braley, a Democrat, is widely seen as a front-runner to replace Harkin. Agriculture Secretary Tom Vilsack, a former Iowa governor, and his wife Christine Vilsack, who ran unsuccessfully for Congress last year, are also viewed as potential candidates.

Among Republicans, Representatives Tom Latham and Steve King could be contenders.

In his remaining two years in office, Harkin said he will focus on implementing President Barack Obama's healthcare reforms, expanding job opportunities for people with disabilities and expanding access to education.

A party maverick, he angrily opposed the White House over the recent fiscal cliff compromise that Vice President Joe Biden negotiated with Senate Republican leader Mitch McConnell. Harkin said the deal that raised taxes only on the very rich will help the wealthy at the expense of the middle class.

Harkin was first elected to the House of Representatives in 1974 and the Senate in 1984.

"When the current Congress is over, I will have served in the United States House of Representatives and the U.S. Senate for a total of 40 years. After 40 years, I just feel it's somebody else's turn," he said.

(Additional reporting by Richard Cowan, David Morgan, Charles Abbott and Vicki Allen in Washington; Editing by Doina Chiacu and Vicki Allen)