A nationwide poll by Gallup found 55 per cent of likely voters planned to vote for a Republican, compared to 40 per cent Democrat and 5 per cent undecided.

Gallup said the size of the lead suggested that "regardless of turnout, the Republicans will win more than the 40 seats needed to give them the majority in the House." Historically, leads as small as just two percentage points have been enough to deliver parties a majority of the 435 House seats, the pollster said.

FiveThirtyEight, a polling website that predicted the outcome of the 2008 election almost precisely, forecast the House would end up split 232-Republican and 203-Democrat.

This would mean a net loss of 52 for the Democrats, close to the 54 lost by the party in 1994, two years into Bill Clinton's presidency.

But Nate Silver, the site's founder, said that with momentum still gathering behind Republican candidates in some races, Democrat losses could be even worse.

The party could even face a "doomsday" scenario of losses approaching the record Republican loss of 75 in the 1948 midterms, he suggested.

Other polls were almost as grim for Mr Obama's party, which took control of both houses of Congress in 2006.

The final NBC News/Wall Street Journal poll found 49 per cent of likely voters wanted the return of Republican control, while 43 per cent wanted the Democrats to endure.

The latest CBS/New York Times poll also gave the Republicans a lead of six points, while CNN forecast a national lead of 10 points and Fox News 13 points.

FiveThirtyEight's forecasts suggested the Democrats would hold on to the Senate, but with their majority cut from 59-41 to 52-48.

Such a decline in influence in the upper chamber would sharply decrease Mr Obama's hand in Congress and reduce his ability to push through his agenda.

Polls in Nevada indicated that Harry Reid, the Democratic leader in the Senate, trailed by Sharron Angle, the Republican Tea Party favourite, by 2.7 per percentage points on average.