Sen. Tom Harkin (D-Iowa) will not seek reelection in 2014, he announced Saturday.

“It’s just time to step aside,” Harkin, 73, told the Associated Press, noting that he would be 81 by the end of another term. Multiple Democratic aides have confirmed the decision, and the Democratic Senatorial Campaign Committee has issued a statement.

Harkin's decision makes him the third senator up for reelection this cycle to announce his retirement. Sens. Jay Rockefeller (D-W.Va.) and Saxby Chambliss (R-Ga.) have both announced they will not seek another term.

Harkin is in his fifth term and has become one of the most outspoken liberals in the Senate in recent years. Over the past few weeks, he was one of few on the left who resisted the "fiscal cliff" deal and had pushed for reform of the Senate's filibuster rules. Senate leaders this week announced a filibuster package that some liberals criticized as inadequate.

In a statement, President Obama said that Harkin “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.”

Harkin also ran for president in 1992, starting as an early front-runner but eventually fading.

His retirement will give Republicans a much better chance at winning his seat. Iowa remains a swing state at the national level, but Harkin won with relative ease in recent years.

"Today's announcement by Senator Harkin immediately vaults Iowa into the top tier of competitive Senate races next year," said Rob Collins, executive director of the National Republican Senatorial Committee.

Rep. Bruce Braley (D-Iowa) is expected to be a front-runner on the Democratic side. He has also talked about running for governor in 2014, but seeking Harkin's seat should offer him a much better opportunity than challenging Gov. Terry Branstad (R).

Other potential Democratic candidates include former Iowa governor and current Agriculture Secretary Tom Vilsack, as well as his wife, Christie Vilsack, who ran unsuccessfully for Congress in 2012.

On the GOP side, Rep. Steve King (R-Iowa) has expressed interest in running for Senate, though he may be a little too conservative for the party leadership's taste. Rep. Tom Latham (R-Iowa) could also be in the mix.