(CNN) - Sen. Tom Harkin, a Democrat who has represented Iowa in the U.S. Senate since 1984, will not seek re-election in 2014.
"I have the privilege to be able to make this decision on my own terms, which not everyone can, and I'm deeply grateful to the people of Iowa that I do have that opportunity. I've been extremely fortunate," he said in a written statement, adding he does not "by any means plan to retire completely from public life at the end of this Congress."
Before serving five terms in the Senate, Harkin represented the state in the U.S. House and served in the Navy. The 73-year old would be 81 years old at the end of a sixth term.
He told the Des Moines Register he has "mixed feelings" about this decision, but said in his statement, "After 40 years, I just feel it's somebody else's turn."
He is chairman of the Senate Health, Education, Labor and Pensions Committee, with a broad swath of jurisdiction. He played an important role in support of President Barack Obama's health reform law.
Democratic Senatorial Campaign Committee Chairman Michael Bennet issued a statement saying he has "seen firsthand Tom Harkin's strong commitment to the people of Iowa" and appreciated "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."
President Barack Obama wrote in a statement, "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. 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. Senator Harkin will be missed, and Michelle and I join Iowans in thanking him for his long-standing service and wish him and his wife, Ruth, all the best in the future."
Senate Democratic Leader Harry Reid released a statement describing his retiring colleague as "a pillar of the Senate."
"Senator Harkin has been a champion for improving Americans' access to affordable health care throughout his entire career, and Americans will continue to benefit from his commitment to preventive care," Reid said. “Senator Harkin's work on behalf of people with disabilities has changed millions of lives and advanced the cause of equality for which he has fought tirelessly for decades. And his work combating child labor and human rights abuses will endure for years to come."
Democratic National Committee Chairwoman Debbie Wasserman Schultz posted to Twitter that Harkin's "decision not to seek re-election is a loss for us all."
Republicans see the retirement as an opportunity to pick up a new seat in the Senate.
"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. "With Democrats already on defense in a number of key states such as West Virginia, Louisiana, South Dakota and others, there is no question that this is very troubling news for Harry Reid and his liberal friends in Washington."
At this point, Democrats will be defending 21 of the 35 Senate seats up for election in 2014. They currently have an advantage with 55 seats in the Senate (including two independents who caucus with them) to Republicans' 45 seats.
Harkin's announcement comes after Sens. John D. Rockefeller IV, D-West Virginia, and Saxby Chambliss, R-Georgia, each announced this month they will not seek re-election in 2014.
Harkin won his 2008 election with 63% of the vote.
Iowa's other senator is Republican Chuck Grassley, whose sixth term expires in 2016. Grassley is 79 years old. A prolific user of Twitter, he posted on Saturday afternoon, "Thank you to Senator Tom Harkin for his service. I wish him well."
In his second bid for the U.S. House – in 1974, his first successful run - Harkin worked a variety of jobs as part of his campaign to communicate he understood people's concerns.
– CNN's Mark Preston, Dana Bash, Paul Steinhauser, Peter Hamby, Kevin Bohn, Kevin Liptak and Gregory Wallace contributed to this report