2. Ben Nelson (D) of Nebraska
The Democrat from Nebraska announced Dec. 27 that he would not seek reelection in 2012. A centrist, he parted company with fellow Democrats on several key votes, such as DREAM Act legislation to give some children of illegal immigrants a path to US citizenship. He backed the Bush tax cuts, and helped negotiate their passage in the Senate. He sat on the fence a long time over health-care reform, but finally provided the vote Democrats needed to hand President Obama the major domestic policy victory of his tenure.
Senator Nelson was expected to face tough competition in 2012, but he had prevailed before in difficult contests. In announcing his decision to retire, Nelson said simply, "It's time to move on." He urged successors to "look for common ground and to work together in bipartisan ways to do whatâ€™s best for the country, not just one political party.â€ť
He previously served two terms as governor of Nebraska. He was elected to the Senate in 2000.