Republicans have made significant gains in mid-term elections, capturing a majority of seats they need to take control of the US Senate.
The Republicans padded their control of the House of Representatives by at least 12 seats, and in the big prize of Tuesday's midterm election they retook the Senate.
The victory puts the Republicans in position to shape if not dictate the congressional agenda, and their priorities are likely to focus on the economy.
The party picked up at least six Senate seats to guarantee at least 51 members of the 100-member chamber, TV networks said.
Republicans romped to victory against incumbent Democrats in Arkansas and Colorado, and snatched the seats of retiring opponents in Montana, South Dakota and West Virginia.
Top Republican Mitch McConnell was re-elected in Kentucky for a sixth time, fulfilling his ambition of becoming Senate majority leader.
The Republicans also retained two seats in South Carolina.
"A few short days ago some were speculating that ... Mitch McConnell would struggle to hold on to his Kentucky seat. As it turned out, his victory was one of the first called, in a night the Democrats will want to forget," said Al Jazeera's Mike Hanna, reporting from Washington.
The new Republican margin of control in the House with a cushion of at least 12 seats takes them close to achieving or surpassing their largest majority of the post-World War II era.
They will go from having 45 to at least 52 seats in the 100-member Senate.
The elections are seen as a referendum on the policies of President Barack Obama, and the win for the Republicans will likely complicate his final two years in office.
"This election matters as it will determine the final two years of President Obama's term and his ability to govern. With Republicans in control of the House of Representatives and poised to take control of the Senate, the legislative gridlock in Congress could continue as Republicans seek to block Obama's legislative agenda. This has already been one of the least productive Congress in decades and could continue if politicians and the Executive branch can't find common ground.
"This dysfunctional paralysis has led to voter dissatisfaction which may translate into low turnout at the polls. Republicans and Democrats have been working hard to counter that voter apathy by spending enormous amounts of money on television campaign ads. In fact, this is poised to be the most expensive election in US history with more than $3.67bn being spent for what may amount to just a handful of seats shifting to another party.
"So, with a race this close, the outcome may take some time. It will take time to count the ballots and there may even be a run-off election in Georgia or Louisiana. If that happens, the final outcome of the election may not be known until early January."
Millions headed to the polling stations on Tuesday to elect 36 senators, 36 governors and all 435 members of the House of Representatives.
While Obama's name is not on the ballot, the campaigns have been influenced by his low job-approval rating, partisan gridlock in Washington and a US economy that is widely seen as not growing enough to help many in the middle classes.
Republicans are expected to pick up seats in the Senate, but polls show eight to 10 races are still toss-ups and it is unclear whether the Grand Old Party can gain the six seats they need to control the 100-member chamber for the first time since the 2006 election.
Al Jazeera's Kimberly Halkett, reporting from Washington DC, said that there had been a lot of "dissatisfaction" among voters.
"There is a feeling, especially among young people, that their vote will not change anything, therefore there is no point in casting a ballot."
The battle for control of the Senate also could extend beyond Tuesday night.
Senate races with multiple candidates in Louisiana and Georgia, where the winner must get more than 50 percent of the vote, could be forced into run-offs in December and January, respectively.
Seizing the Senate would give Republicans complete control of both chambers of Congress, complicating Obama's last two years in office.
That would constitute the most dramatic political shift since Obama entered the White House in early 2009.
The White House has tried to play down the prospect of sharp changes in strategy by the president after the election.
White House spokesman Josh Earnest said that many of the contested Senate races where Democrats were in trouble were in states Obama lost to Republican Mitt Romney in the 2012 election.
"It would not be wise to draw as broad a conclusion about the outcome of this election as you would about a national presidential election simply by virtue of the map," he told reporters.
Obama's low public approval rating of about 40 percent made him a political liability in some states on the campaign trail, where his last election appearance was on Sunday in the city of Philadelphia, a Democratic stronghold.
Vice President Joe Biden promoted the president's economic agenda on radio in key states, telling one host that even if Republicans win the Senate, he and Obama will push for policies that address what he called an "overwhelming dislocation of wealth".
Democrats on the ballot did not so much defend the president as insist they were independent of him.
Early voting topped 18 million ballots in 32 states, and both parties seized on the number as evidence of their own strength.
Source: Al Jazeera and agencies