Sen. Tom Harkin, D-Iowa, was elected to the Senate in 1984. (Susan Walsh/AP)

Sen. Tom Harkin (D-Iowa), a five-term senator and one of his party’s most outspoken liberals, will not seek reelection in 2014, presenting Democrats and Republicans with key tests of their midterm electoral strategies.

Harkin’s surprise announcement Saturday makes him the third senator up for reelection this cycle to announce his retirement. Sens. John D. Rockefeller IV (D-W.Va.) and Saxby Chambliss (R-Ga.) have said recently that they will not seek another term.

The race to succeed Harkin likely will be one of the most competitive Senate contests next year and key to either party’s chances of controlling the chamber. Democrats hold a 55-seat majority but will be defending 20 of the 34 seats up for grabs, mostly in rural and western states; Republicans need to gain six seats to retake the majority.

Harkin’s departure “immediately vaults Iowa into the top tier of competitive Senate races next year,” said Rob Collins, executive director of the National Republican Senatorial Committee.

Acknowledging the challenge his party faces, Sen. Michael F. Bennet (D-Colo.), chairman of the Democratic Senatorial Campaign Committee, said in a statement, “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.”

Harkin, 73, said in an interview Saturday that the recent death of his friend, Sen. Daniel K. Inouye (D-Hawaii), was a factor in his decision to retire.

“It begins to bring home that life is fleeting,” he said of Inouye’s death. “I’ve had the privilege of 40 years in the House and Senate. I thank my Iowans for the privilege, but it’s somebody else’s turn.”

Harkin said he plans to remain active in policy debates and also spend more time with his wife, Ruth. He said he will finally learn how to dance and maybe participate in RAGBRAI, the Register’s Annual Great Bicycle Ride Across Iowa, a seven-day event that is one of the state’s quirkier traditions.

Harkin was first elected to the House in 1974 and won his Senate seat in 1984. He was a champion of the 1990 Americans with Disabilities Act, ran unsuccessfully for president in 1992 and was a key supporter of President Obama’s health-care law.

As for potential successors, both parties have several prospects.

Republicans, eager to avoid the missteps of the more outspoken conservative candidates nominated in recent years, may coalesce around Lt. Gov. Kim Reynolds or U.S. Rep. Tom Latham, who represents Des Moines and is a close friend of House Speaker John A. Boehner (R-Ohio). But U.S. Rep. Steve King, a staunchly conservative tea party favorite and frequent cable news guest, has openly discussed a Senate campaign.

U.S. Rep. Bruce Braley is expected to be a front-runner on the Democratic side but has also talked of running for governor in 2014. Other potential Democratic candidates include Agriculture Secretary Tom Vilsack, a former Iowa governor, and his wife, Christie Vilsack, who unsuccessfully challenged King in 2012.

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Aaron Blake and Sean Sullivan contributed to this report.