In a major blow to US President Barack Obama and the Democrats, the Republicans have won Obama's old Senate seat in Illinois.
Republican Mark Kirk defeated Illinois Treasurer Alexi Giannoulias in one of the nation's ugliest and most closely watched Senate races.
The Illinois seat was held by Obama before his move to the White House.
Obama and top administration officials had campaigned for Giannoulias in a bid to keep the seat in Democratic hands.
According to media sources, a loss also would weaken Democrats' hold on the Senate by depriving them of a seat in what is supposed to be a Democratic state.
Republicans are heading for a massive mid-term election triumph in the US.
Earlier, opinion polls logged just hours before the polling commenced show Republicans set to win the House of Representatives and eat hugely into the Democratic majority in the Senate, dividing power in Washington and forging a polarised prelude to Obama's 2012 re-election bid.
Republicans held a six point edge 49 per cent to 46 per cent over their traditional Democrat rivals, according to the final NBC News/Wall Street Journal survey.
The voters appeared demoralised by a faltering economy and appeared to be in no mood to respond to last minute wooing by President Barack Obama and other Democrat heavyweights like Bill Clinton, the survey said.
Republicans, electrified by the ultra-conservative Tea Party movement have vowed to overturn Obama's sweeping health reforms and have promised a budget crunch and tax cuts, which they claim will reduce the deficit and ignite growth.The election for all 435 House seats in the Lower House and 37 in the Senate comes as the euphoria stirred by Obama in 2008 seems an age away.