WASHINGTON Democratic Senator Ben Nelson said on Tuesday he will retire next year, dealing a significant blow to his party's hopes of keeping control of the Senate in the November 2012 election.
Nelson's decision not to seek re-election puts his seat in heavily Republican Nebraska up for grabs, boosting Republicans' hopes of wresting control of the chamber from Democrats.
Republicans already have a majority in the House of Representatives, where they have heavily resisted many of President Barack Obama's spending plans over concerns about a build-up of U.S. deficits and debt levels.
Nelson, 70, was already seen as vulnerable. The Democratic Senatorial Campaign Committee had spent more than $1 million to shore up his re-election campaign. It must now scramble to find a replacement candidate.
Announcing his retirement, Nelson said it was "time to move on" and urged those following him "to look for common ground and to work together in bipartisan ways to do what's best for the country, not just one political party."
Obama, in a statement from Hawaii, where he is vacationing with his family, also noted the senator's work to bridge partisan divides at a time when Washington has struggled to overcome political gridlock.
"Ben's commitment to working with both Democrats and Republicans across a broad range of issues is a trait far too often overlooked in today's politics," Obama said.
The Democrats' slender majority in the Senate is under threat in the November 2012 elections.
Republicans only have to defend 10 seats, while 23 Democratic senators are up for re-election, including vulnerable incumbents in Ohio, Missouri, Florida and Pennsylvania.
Democrats now hold 51 Senate seats. Republicans control 47, and there are two independents.
It is unclear who might replace Nelson as the Democratic candidate.
Former Senator Bob Kerrey, a popular figure in Nebraska, has been widely talked about as a potential replacement, but he has not said whether he would enter the race.
The Republican field is led by state Attorney General Jon Bruning.
(Additional reporting by Laura MacInnis in Honolulu; Editing by Peter Cooney)