Harkin, chairman of an influential Senate committee, announced his decision during an interview with The Associated Press, saying the move could surprise some.

The 73-year-old cited his age — he would be 81 at the end of a sixth term — as a factor in the decision, saying it was time to pass the torch he has held for nearly 30 years, freeing a new generation of Iowa Democrats to seek higher office.

"I just think it's time for me to step aside," Harkin told the AP.

Harkin, first elected in 1984, ranks seventh in seniority and fourth among majority Democrats. He is chairman of the Health, Education, Labor and Pensions Committee, and chairman of the largest appropriations subcommittee.

Harkin has long aligned with the Senate's more liberal members, and his signature legislative accomplishment is the 1990 Americans with Disabilities Act. He also served as a key salesman of President Barack Obama's health care overhaul to the wary left.

"I'm not saying that giving this up and walking away is easy. It's very tough," Harkin said at his rural Iowa home south of Des Moines. "But I'm not quitting today. I'm not passing the torch sitting down."

Harkin's news defied outward signals. Besides being beloved in his party, Harkin has $2.7 million in his campaign war chest, second most among members nearing the end of their terms, and was planning a fundraiser in Washington, D.C., next month featuring pop star Lady Gaga.

Obama released a statement saying Harkin will be missed and thanking the senator for his service. "During his tenure, he has fought passionately to improve quality of life for Americans with disabilities and their families, to reform our education system and ensure that every American has access to affordable health care," Obama said.

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."

He also nodded to his political longevity: "The effect of that cascades down and it opens a lot of doors of opportunity" for future candidates.

But by opening a door in Iowa, Harkin has created a potential headache for his party nationally.

Democrats likely would have had the edge in 2014 with the seat, considering Harkin's fundraising prowess and healthy approval. A poll by the Des Moines Register last fall showed a majority of Iowans approved of his job performance.

Democrats hold a 55-45 advantage in the Senate, requiring Republicans to gain six seats to win back the chamber. But Democrats have more seats to defend in 2014 — 20 compared with only 13 for Republicans. Historically, the president's party loses seats in the midterm elections after his re-election.

In GOP-leaning West Virginia, five-term Democratic Sen. Jay Rockefeller recently announced he would not seek re-election. And on Friday, Georgia Sen. Saxby Chambliss, a Republican, announced that he wouldn't seek a third term.

Democratic incumbents also face tough re-election races in Arkansas, Louisiana, Montana, North Carolina and Alaska — all carried by Republican Mitt Romney in November's presidential election.

Harkin's move opens a rare open Senate seat Iowa. Harkin, Iowa's junior senator, is outranked by Sen. Charles Grassley, who has held the state's other seat since 1980.

Attention will turn to U.S. Rep. Bruce Braley, a fourth-term Democrat from Waterloo who has long been mentioned as a possible Harkin successor. Braley, who was traveling in Iowa, did not immediately return requests by the AP for comments beyond an emailed statement calling Harkin a "mentor" and "progressive force" who leaves "a legacy few will ever match."

Harkin held open the possibility of endorsing a Democrat before the primary if the candidate "is a pragmatic progressive."

Although no Republicans have stepped forward, Harkin's news gives the GOP's private huddles new life.

"There are lots of conversations, but it's very early still," said Nick Ryan, an Iowa Republican campaign fundraiser.

U.S. Rep. Tom Latham of Clive is a seasoned Republican congressman, a veteran House Appropriations Committee member and a robust fundraiser who has won 10 consecutive terms. Aides to Latham declined to comment beyond a statement saying the congressman "respects Sen. Harkin's decision (and) looks forward to continuing to work with him."

Since November, Harkin has stepped up his role as one of the Senate's leading liberal populists.

He was a vocal opponent late last year of Obama's concession to lift the income threshold for higher taxes to avoid the so-called fiscal cliff. Harkin instead supported raising taxes on all earners making more than $250,000 a year.

He also endorsed Obama's call for banning assault rifles and larger ammunition magazines after the Connecticut school shooting.

Despite Harkin's strong political position, he has faced questions about his and his wife's role in developing a namesake policy institute at Iowa State University, Harkin's alma mater. The Harkins and their supporters have been pushing for the institute to house papers highlighting his signature achievements, including the ADA and shaping farm policy as the former chairman of the Agriculture Committee.

Harkin has avoided questions about fundraising for the institute after disclosure reports showed some of its largest donors are firms that have benefited from his policies.

Harkin dismissed that those questions had any bearing on his decision.