The top Democrat in the US Senate, Harry Reid, has announced he will not seek re-election in 2016.
Mr Reid, 75, has served in the Senate for almost 30 years, including as Senate majority and minority leader.
The Nevada senator said in video statement his retirement was not related to a recent accident nor to the Democrats losing control of the Senate.
"I want to be able to go out at the top of my game," Mr Reid told the New York Times.
Among those likely to replace Mr Reid in the leadership position are Illinois Senator Dick Durbin and New York Senator Chuck Schumer.
The Nevada senator, a former amateur boxer, first came to Congress as a Representative in 1983. He was elected Senate majority leader when Democrats took control of the chamber in 2007.
Mr Reid injured his eye and ribs when he fell in an exercise accident on 1 January. The senator said recuperating had given him to time "ponder and to think" about his political future.
He told the New York Times he had been contemplating possible retirement for months.
In his statement, he vowed to keep fighting for his party for his remaining 22 months in office.
"We have to make sure that the Democrats take control of the Senate again," Mr Reid said, adding he felt it was "inappropriate for me to soak up all those resources" when it could be used on other elections.
Mr Reid had a tough re-election fight in 2010 and could have faced another in 2016.
He also issued a warning to Republicans, telling Senate Majority Leader Mitch McConnell "don't be too elated" because there are still 22 months to go.