Ben Nelson, the Senate's most conservative Democrat and one of the party's most endangered incumbents, is expected to announce Tuesday that he will not seek a third term in 2012, a Democratic source confirmed.
Though the Nebraskan often voted with Republicans, Nelson's decision would be a blow to Democratic hopes of retaining control of the Senate next year, when they are faced with defending 23 of the 33 seats up for reelection.
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 Democrats nationally.
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.
However, Kerrey has not been a presence in the state in years, having served for a decade as president of the New School in New York.Copyright © 2015, Los Angeles Times