Though the Nebraskan often voted with
He becomes the seventh member of the Democratic caucus to announce retirement plans.
Nelson was ranked as the most conservative Democrat in the Senate in annual vote ratings by the National Journal.
He served two terms as Nebraska's governor before being elected to the Senate in 2000. He was reelected in 2006, a strong year for
Several Republicans had already lined up to challenge him next fall, including the state's attorney general and treasurer.
Democrats floated former Sen. Bob Kerrey as a possible candidate to stand in Nelson's place. Nelson replaced Kerrey in 2001.