WASHINGTON — WASHINGTON - U.S. Sen. Tom Harkin, a Democrat from Iowa who championed landmark legislation banning discrimination against people with disabilities, said Saturday that he would retire and not seek re-election next year to a sixth term.

The announcement from Harkin sets the stage for one of the most competitive Senate races in the country in the 2014 midterm elections.

“It’s not easy to walk away, but life is fleeting,” Harkin, 73, said in an interview Saturday. “I’ve had the privilege to be here for 40 years. Too many people hang on to power for too long, and that’s not right.”

In a Washington career that began in 1974 when he was elected to the House, followed a decade later by his elevation to the Senate, Harkin has been a forceful voice of populism. He said that his biggest achievement was the passage of the Americans withDisabilities Act in 1990, a bipartisan measure he pushed for on behalf of his brother, Frank, who was deaf. He was also a leading proponent of overhauling the nation’s health-care system.

Harkin sought the Democratic presidential nomination in 1992.

The announcement from Harkin took some Democrats by surprise, particularly because he had not previously signaled his intentions and had a campaign account of nearly $3 million. His is the latest in a series of Senate retirements, and it forces Democrats to try to defend an open seat that would have otherwise beenmore challenging for Republicans.

“I appreciate that Senator Harkin has made this decision so early in the cycle, giving us ample time to recruit a strong Democratic candidate for this seat,” said Sen. Michael Bennet of Colorado, the chairman of the Democratic Senatorial Campaign Committee. “Iowa has a strong record of electing great Democrats and I’m confident that we will elect a new Democratic senator next November.”

Although members of his family have been diagnosed with cancer, Harkin said his health is good - and reported a recent positive colonoscopy.But he said “you never know,” and that he wanted to travel and spend his retirement with his wife, Ruth, “before it’s too late.”

Harkin ranks seventh in seniority and fourth among majority Democrats in the Senate. He is chairman of the Health, Education, Labor and Pensions Committee, and chairman of the largest appropriations subcommittee.

Rep. Bruce Braley, D-Iowa, is among the early prospects for Harkin’s seat.

Republican party officials said one early contender could be Rep. Steve King.

Rob Collins, the executive director of the National Republican Senatorial Committee, said Saturday that Harkin’s planned retirement “immediately vaults Iowa into the top tier of competitive Senate races next year.” Information for this article was contributed by Jeff Zeleny of The New York Times and by Thomas Beaumont of The Associated Press.

Front Section, Pages 5 on 01/27/2013