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CUMMING, Iowa — US Senator Tom Harkin, Democrat of Iowa, said Saturday that he will not seek a sixth term in 2014, a decision that eases some of the burden the national Republican Party faces in trying to retake the Senate.
Harkin, chairman of an influential Senate committee, said the move may surprise some. But 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 said.
Harkin, first elected to the Senate 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 Obama’s 2010 health care bill to the wary left.
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,’’ the president said
‘‘I’m not saying that giving this up and walking away is easy. It’s very tough,’’ Harkin said. ‘‘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 gala fund-raiser in Washington next month featuring pop star Lady Gaga.
Harkin said his health is good. 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.’’
But by opening a door in Iowa, Harkin has created a potential headache for Democrats .
Democrats likely would have had the edge in 2014 with the seat, considering Harkin’s fund-raising prowess and healthy approval ratings. A poll by the Des Moines Register taken last fall showed a majority of Iowans approved of his performance.
Democrats hold a 55-45 advantage in the Senate, requiring Republicans to gain six seats to win control of the chamber. But Democrats have more seats to defend in 2014 — 20 compared with 13 for Republicans. Historically, the president’s party loses seats in the midterm elections after his reelection.
In West Virginia, five-term Democratic Senator Jay Rockefeller recently said he would not seek reelection. And on Friday, Georgia Senator Saxby Chambliss, a Republican, said that he wouldn’t seek a third term.
Democratic incumbents also face tough reelection races in Arkansas, Louisiana, Montana, North Carolina, and Alaska — all states carried by Republican Mitt Romney in November’s presidential election.
Harkin’s move opens a rare open Senate seat in Iowa. Harkin, Iowa’s junior senator, is outranked by Republican Charles Grassley, who won Iowa’s other seat in 1980.
Attention will turn to US Representative Bruce Braley, a four-term Democrat from Waterloo, long mentioned as a possible Harkin successor.
Harkin held open the possibility of endorsing a Democrat before the party’s primary.
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 fund-raiser.
US Representative Tom Latham of Clive is a seasoned Republican representative,a veteran Appropriations Committee member, and a robust fund-raiser who has won 10 consecutive terms. He has not said whether he would run.
Since November, Harkin has stepped up his role as one of the Senate’s leading liberal populists. He was a vocal opponent last year of President Obama’s concession to lift the income threshold for higher taxes to avoid the so-called fiscal cliff. Harkin instead supported raising taxes on earners making more than $250,000 a year.
He also endorsed Obama’s call for banning assault rifles and larger ammunition magazines after a Connecticut school shooting last month.
Harkin has faced questions about his and his wife’s role in developing a namesake policy institute at Iowa State University. Reports have said some of its largest donors are from companies that have benefited from his policies.