The US Republicans have swept to power in both houses of Congress for the first time in eight years in midterm elections shaped by President Barack Obama's unpopularity.

Results continue to trickle in across the US, but the conservatives won seven seats from Democrats, one more than they needed to take control of the Senate.

West Virginia, Arkansas, South Dakota, Montana, Colorado, North Carolina and Iowa all fell to the conservatives.

The Democrats are still battling to hold on to Senate seats in Virginia and Alaska, while Louisiana's race will go to a run-off next month.

The Republicans also successfully defended Senate seats in Georgia, Kentucky and Kansas.

Kentucky Republican Mitch McConnell, who cruised to re-election, will become the new leader of the Senate. He told cheering supporters as he captured a sixth term: "It's time to go in a new direction! It's time to turn this country around!"

Republicans, who already control the House of Representatives, were widely expected to pad their majority in that chamber, too.

Overall on Tuesday, 36 of the 100 Senate seats, all 435 House districts and 36 of 50 governors' seats were up for grabs.

President Obama, who was shunned by many Democrats on the campaign trail because of his low approval rating, called Democratic and Republican congressional leaders to the White House on Friday to discuss the new political landscape.

Exit polls showed most of the Americans voting on Tuesday were unhappy or even angry with the Obama administration. There was also criticism of Republican congressional leaders.

The conservative takeover means President Obama has even less hope of passing bills for his remaining two years in office. He is also likely to face pressure to overhaul his White House team.

However, even with control of the Senate, the Republicans do not have the 60-vote super-majority needed to pass major legislation.

A difficult election had been expected for President Obama's allies: governing parties tend to lose seats in midterm votes and many Democrats were defending seats in states that lean Republican.