Sen. Tom HarkinTom HarkinGrassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream Do candidates care about our health or just how much it costs? MORE (D-Iowa) will retire from the Senate at the end of 2014, the five-term senator said Saturday.

Harkin said his decision not to seek a sixth term was based, in part, because he wanted to give a new generation of Iowa Democrats the chance to run for higher office.

"It's just time to step aside," Harkin, 73, told the Associated Press in an interview.

Harkin told the Des Moines Register on Saturday that he had mixed feelings about the decision. 

“To walk away from this position and this power is not an easy thing,” he said. “But I think it’s the right thing. And I have two years left. I’m not passing the torch sitting down — it’s a running relay. I intend to be very active over the next two years.”

Harkin said in a statement Saturday that two primary factors drove his decision to retire: He's served in the House and Senate for 40 years and would be 75 at the end of his current term, and he had made a promise to his wife that "we’re going to live together in a way we’ve often talked about — before it gets too late."

He told the Register he made the decision with his wife Ruth on Wednesday. "I'm going to learn to dance," he said.

Harkin added in his statement that his retirement from the Senate did not mean would be disappearing from the public eye. "I don’t by any means plan to retire completely from public life at the end of this Congress," he said.

Harkin’s retirement makes Iowa a ripe target for Republicans as they try to flip at least six seats to win back the Senate.

While Harkin was likely in for a contested race if he had run for reelection, he had $2.7 million in the bank already, according to Federal Election Commission records. He had a February fundraiser planned at $1,500-a-plate with pop superstar Lady Gaga, according to the Register.

Rep. Steve King (R-Iowa) did not rule out a 2014 Senate run in November, regardless of whether Harkin would have run.

"I have a consistent record. I think people know me," he said, arguing his views wouldn't be "an impediment at all."

Ten-term Rep. Tom Latham (R-Iowa) is the other Iowa Republican in the House. A primary between King and Latham could follow the tea party vs. establishment narrative that has played out over the past two cycles in Republican primaries.

Latham didn't give any hint of his intentions in a statement from his spokesman on Harkin's retirement.

"Congressman Latham respects Senator Harkin's decision," Latham spokesman James Carstensen said. "He looks forward to continuing to working with him and the rest of the Iowa delegation for the best interests of the people of Iowa over the next two years."

On the Democratic side, possible candidates include Rep. Bruce BraleyBruce BraleyTrump: Ernst wanted 'more seasoning' before entertaining VP offer Criminal sentencing bill tests McConnell-Grassley relationship Trump's VP list shrinks MORE, who has also talked about running for governor, Agriculture Secretary Tom VilsackThomas J. VilsackUSDA: Farm-to-school programs help schools serve healthier meals OVERNIGHT MONEY: House poised to pass debt-ceiling bill MORE and his wife, Christie Vilsack, who lost to King in a 2012 run for Congress.

Obama campaign strategist David Axelrod predicted on Twitter Saturday that the race could wind up between Braley and King in a battle of House members. 

“Harkin's exit could set up defining battle between 2 House members: Bruce Braley, a charismatic D; and Steve King, a hero of the far Right,” Axelrod tweeted.

President Obama said he and first lady Michelle ObamaMichelle ObamaWeek ahead: Net neutrality supporters rally on rule's second anniversary 5 ways politics could steal the show at Oscars Why cutting back ‘free’ school lunches would be a favor to families MORE joined Iowans in thanking Harkin for his service.

“By the time Senator Harkin finishes his fifth term, he will have represented the people of Iowa in the United States Congress for an incredible 40 years. He has served in the U.S. Senate longer than any Democrat in Iowa's history,” Obama said in a statement.

“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 added.

"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," Sen. Michael BennetMichael BennetA guide to the committees: Senate Senate advances Trump's Commerce pick Senate Dems move to nix Trump's deportation order MORE (D-Colo), chair of the Democratic Senatorial Campaign Committee, said in a statement. "Iowa has a strong record of electing great Democrats, and I'm confident that we will elect a new Democratic senator next November."

Harkin, who was one of the few Democrats to vote against the "fiscal-cliff" deal, passed on the chance to chair the powerful Senate Appropriations Committee in November. He opted instead to stay at the head of the Health, Education, Labor and Pensions Committee.

He is the second Senate Democrat to opt to retire rather than run for another term, as Sen. Jay RockefellerJay RockefellerObama to preserve torture report in presidential papers Lobbying world Overnight Tech: Senators place holds on FCC commissioner MORE (D-W.Va.) announced earlier this month that he would not run in 2014.

Republican Sen. Saxby ChamblissSaxby ChamblissWyden hammers CIA chief over Senate spying Cruz is a liability Inside Paul Ryan’s brain trust MORE (Ga.) also said Friday that he would retire at the end of his term.

--This report was last updated at 4:53 p.m.