By Tim Reid
(Reuters) - Democratic Senator Ben Nelson will retire next year, news reports said on Tuesday, dealing a significant blow to his party's hopes of keeping control of the U.S. Senate after the November 2012 elections.
Nelson's decision not to seek re-election makes his seat in heavily Republican Nebraska a strong contender to be a gain for Republicans next year, boosting their hopes of wresting control of the chamber from Democrats. Republicans already control the House of Representatives.
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.
The Democrats' slender majority in the Senate is under threat next year. Republicans only have to defend 10 seats, while 23 Democratic senators are up for re-election, including other 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.
(Reporting By Tim Reid in Washington; Editing by Peter Cooney)