It's somebody else's turn. It's time for me to step aside ... . I think that's not only good for our party, it's good for our state and for our nation," Harkin said in an interview with Reuters. He said he had no health problems but had promised his wife that he would quit before it was too late to enjoy other things in life. Iowa, site of the country's first presidential nominating contest, is considered a political swing state. In remarks to the Iowa Democratic Party central committee after his announcement, Harkin said he would stay politically active. President Barack Obama, a fellow Democrat, praised Harkin for his decades of public service. "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," Obama said in a statement. Senate Majority Leader Harry Reid, a Nevada Democrat, in a statement described Harkin as "a passionate progressive, whose deeply held principles have provided a guiding light to Democrats for decades." Among Democrats, U.S. Representative Bruce Braley is widely seen as a front-runner. U.S. Agriculture Secretary Tom Vilsack, a former Iowa governor, and his wife, Christine Vilsack, who ran unsuccessfully for Congress last year, are also viewed as potential candidates. Among Republicans, U.S. Representatives Tom Latham, a moderate, and Steve King, a conservative, are mentioned as possible candidates, which could produce a divisive Republican primary. Obama won Iowa in the November election. But the state has a Republican governor, and a divided legislature and congressional delegation. The party needs to pick up six seats in the mid-term elections next year to get a majority in the 100-member Senate. One of the last of the Senate's old-guard liberals, Harkin angrily opposed the White House over the recent fiscal cliff compromise that Vice President Joe Biden negotiated with Senate Republican leader Mitch McConnell of Kentucky. First elected to the House of Representatives in 1974 and to the Senate in 1984, Harkin said someone younger needs to take his place. I've been there 40 years. I'm 73. By the time I run (for re-election), I'd be 75," he said. Additional reporting by Richard Cowan, David Morgan, Charles Abbott and Vicki Allen in Washington; Editing by Greg McCune and Xavier Briand