Sen. Tom Harkin, D-Iowa, turned Iowa politics upside down Saturday, announcing that he won’t seek another term in 2014.

The longtime force in the state said he came to the decision because he wanted to fulfill a promise to his wife, Ruth, and that after 40 years in Congress, it was time for him to give somebody else a turn.

Harkin, who is 73, has been in the Senate since 1985 and the U.S. Congress since 1975.

In an interview Saturday, he said his health is fine, even laughing as he noted he’d just undergone his latest colonoscopy. And he said he wasn’t worried about his re-election prospects, either. Harkin said he and his wife have just been talking about how they want to live their lives, what they want to do — and how much time they have left to do it.

“As we all know, life is very fleeting,” he said. If he served another term, he noted he’d be 81 by the time it was over.

He said he and his wife were ready for a new chapter in his life. “We want to do it before it’s too late,” he said.

Harkin said he’d been thinking about retiring since the last election, but it was just a few days ago that he firmly made his choice. “Really, the decision was just irrevocably made this week,” he said Saturday.

Harkin’s decision sets up what might well be a scramble, in both parties, for his seat.

On the Democratic side, Rep. Bruce Braley, D-Iowa, has long been rumored to be interested in the job. But there also are other Iowa Democrats who could jump into a primary race, too.

In a statement Saturday, Braley called Harkin’s decision a “huge loss” for the people of Iowa but gave no indication whether he’d run for the seat.

On the Republican side, Rep. Steve King, R-Iowa, has been rumored as a possible candidate for the seat, but as with the Democrats, there also are other GOP possibilities. Rep. Tom Latham, R-Iowa, surely will be talked about as a possible candidate. Former Iowa GOP Chairman Matt Strawn also has been talked about as a possibility.

Harkin said he wasn’t playing favorites in choosing a Democratic successor, but he suggested there wouldn’t be a divisive primary. “I’m just thinking just from talking with people, sometime relatively soon ... by this summer maybe ... Democrats in Iowa will probably be gravitating toward a certain individual,” he said.

He didn’t say who that person might be.

Harkin said he’d like to see a nominee who can raise money, organize a campaign — and who is a “pragmatic progressive.”

President Barack Obama issued a statement Saturday that praised Harkin, noting “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.”

Rep. Dave Loebsack, D-Iowa, called Harkin “my Iowa political hero.” He said Saturday that his “knowledge, compassion and fight cannot be replaced.”

Fellow Iowa Sen. Chuck Grassley tweeted out his thanks to Harkin for his service shortly after receiving a phone call from Harkin.

“Senator Harkin and I have rarely agreed on the issues or voted the same, but he and I have gotten along for a long time with open lines of communication, and I appreciate his public service and zeal for Iowa interests,” the Republican senator said in a statement.

Harkin’s decision will strip the party of its most successful officeholder and top-of-the-ticket presence at a time when it also must find someone to run for governor in 2014, perhaps against another of Iowa’s most successful politicians, Gov. Terry Branstad.

Harkin, the son of a coal miner father and a mother who was a Slovenian immigrant, won a seat in Congress on his second try, in the aftermath of Watergate. Ever since, he’s been a leading liberal voice.

Over the years, perhaps his greatest legislative achievement was the 1990 Americans with Disabilities Act, a groundbreaking law that outlawed discrimination against people with disabilities and is credited with opening more opportunities for them.

It was a fight that Harkin often talked about in light of his own experience. His brother, Frank, was deaf.

Harkin also was chairman of the Senate Agriculture Committee through the writing of two farm bills, in 2002 and 2007, and he had a role in writing key parts of the Affordable Care Act.

Harkin ran for president, in 1992, but it was a short-lived campaign. As expected, he won the Iowa caucuses, which because of his candidacy, weren’t really a factor in the presidential campaign.

It was one of Harkin’s rare political losses. A fiery campaigner who could raise the roof on a union hall or virtually any other setting, the Cumming, Iowa, native made it a habit of dispatching Republican congressmen who rose up to challenge him.

He defeated U.S. Reps. Tom Tauke in 1990, Jim Ross Lightfoot in 1996 and Greg Ganske in 2002.

In the intervening years, Harkin has steadily gained seniority and now chairs one of the most influential committees in the Senate — the Health, Education, Labor and Pensions Committee.

It’s a panel with a wide portfolio from which Harkin says his last two years will be spent aggressively seeking change in the areas of education, health care, improving retirement security for seniors and employment prospects for individuals with disabilities.

In fact, the committee has such a sizeable influence — and fits so well with issues that motivate Harkin — that his decision to step down was a shock to many in the state Saturday.

Sue Dvorsky, the outgoing Iowa Democratic Party chairwoman, told Radio Iowa the news was “earthquaky.”

Harkin met with members of the state party’s central committee in Des Moines on Saturday, breaking the news as they were preparing to choose their new party chairman.

“There were a lot of tears today,” said Susan Frembgen, the chairwoman of the Scott County Democratic Party and a member of the central committee. “It’s a sad day for Democrats in Iowa.”

In the interview, Harkin said leaving wasn’t easy, noting the seniority he’d built up. “It’s tough,” he said. “I’d be lying to you if I said it was easy to walk away from this.”

He said his decision was made Wednesday, before the Senate decided on a filibuster reform that wasn’t as far-ranging as Harkin had proposed. But, unlike others who have left Congress, Harkin said he wasn’t going to complain about the partisanship or difficulties in getting things done.

“I’m not going to bash the Senate,” he said. “I love the Senate.”