US President Barack Obama will wake up to a bleak political landscape after voters punished Democrats by delivering a divided Congress in America's midterm elections.

Republicans pushed Democrats decisively from power in the House of Representatives and strengthened their ranks in the Senate, a result that could herald legislative gridlock when the new Congress takes power in January.

In particular, Republican control of the House will weaken Obama in fights over the extension of income tax cuts set to expire at the end of the year and over efforts to pass comprehensive energy or immigration bills.

"The ability of this administration to get major new programs done was already limited. This just seals the deal," said Jaret Seiberg, policy analyst with the investment advisory firm Washington Research Group.

Television networks projected Republicans would pick up at least 60 House seats, far more than the 39 they needed for a majority. This will elevate conservative John Boehner to House speaker, put Republicans in charge of House committees and slam the brakes on Obama's agenda.

"Our new majority will be prepared to do things differently," Boehner told supporters late on Tuesday.

"It starts with cutting spending instead of increasing it, reducing the size of government instead of increasing it and reforming the way Congress works."

Obama made a late-night call to congratulate Boehner and discuss ways they could work together to create jobs and improve the economy, a Boehner aide said.

The president is due to hold a news conference at 1700 GMT to talk about the post-election landscape.

All 435 House seats, 37 of the 100 Senate seats and 37 of the 50 state governorships were at stake in Tuesday's voting.