US Republicans have seized the House of Representatives in mid-term elections, dealing a severe blow to President Barack Obama's ability to pass laws.
On a tide of economic unease, they won at least 60 seats from the Democrats.
However, Mr Obama's Democrats narrowly retained control of the Senate, despite losing six seats, including some to candidates backed by the Tea Party.
The president phoned John Boehner, the likely new House speaker, to say he hoped to "find common ground" with him.
Speaking after his own re-election, Mr Boehner vowed to cut spending and reduce the size of government. He said voters had sent Mr Obama a message to "change course".
Compounding the misery for Mr Obama's camp, a Republican captured the president's old Senate seat in Illinois.
But in Nevada, one of the most dramatic contests of the night, the Democrats' leader in the Senate, Harry Reid, fought off Tea Party challenger Sharron Angle. US Senator John Kerry reacted with delight: "Harry Reid isn't just Dracula, he isn't just Lazarus, he's our leader and our whole caucus is thrilled that he's unbreakable and unbeatable."
While some of the Tea Party's most eccentric candidates lost their races, the conservative wing of the Republican Party is now a power in the land, BBC North America editor Mark Mardell reports.
This is hugely difficult for the president, our editor says. While there will be much talk of compromise and reaching deals, many Tea Party supporters' explicit aim is to block and undo Mr Obama's agenda.
Up for election were all 435 seats in the House (the lower chamber of Congress), 37 of the 100 seats in the Senate, governorships of 37 of the 50 states and all but four state legislatures.
With some counts still to be completed, projections suggested the Republicans had obtained a net gain of 60 seats in the House, more than the 54 they won in the landmark 1994 mid-terms, and the biggest exchange of seats since the Democrats won 75 in 1948.
Senate results from Washington State, Alaska and Colorado have still to be called, as well as the race for governor of Florida.
And among various local referendums, an attempt to legalise marijuana in California was defeated, while Oklahoma backed a measure banning judges from using Islamic law in rulings.
As well as Illinois, Republicans took Senate seats from Democrats in Pennsylvania, Wisconsin, Arkansas, North Dakota and Indiana.
In Arkansas, John Boozman defeated incumbent Blanche Lincoln in a historic reverse for the Democrats.
Rand Paul, a favourite with the anti-establishment Tea Party movement, held Kentucky for the Republicans.
"We've come to take our government back!" Mr Paul said in his victory speech, adding there was a "Tea Party tidal wave".
However, his fellow Senate victor in Florida, Marco Rubio, sounded a note of caution.
"We make a grave mistake if we believe tonight these results are somehow an embrace of the Republican Party," he said after defeating both independent Charlie Crist and Democrat Kendrick Meek.
"What they are is a second chance - a second chance for Republicans to be what they said they were going to be not so long ago."
Backed by former Alaska governor and 2008 Republican vice-presidential candidate Sarah Palin, the Tea Party movement had managed to replace establishment Republican candidates with their candidates in some areas.
However, prominent Tea Party figure Christine O'Donnell lost her Senate bid in Delaware.
Other good news for the Democrats came in from West Virginia, where Joe Manchin won a Senate seat for the party, and Connecticut, where Richard Blumenthal also won a seat, beating Linda McMahon, a former World Wrestling Entertainment chief executive.
Analysts note that Mr Manchin had distanced himself from some of President Obama's policies.
Ohio switches hands
An ABC exit poll suggested that 88% of Americans believed the national economy was in bad shape - nearly as many as said the same just before Barack Obama was elected.
According to ABC's findings, 73% described themselves as dissatisfied or even angry (26%) about the way the federal government was working, compared with 69% in 1994, when the Republicans seized the House.
In addition to the Congressional races, some states elected governors
- John Kasich defeated Ted Strickland in Ohio, thus handing the Republicans control of a state considered crucial to the 2012 presidential election
- In California, Democrat Jerry Brown defeated Republican Meg Whitman, the former CEO of eBay
- In New York, Democrat Andrew Cuomo won the office his father held in the 1980s and early 1990s
In a given state, the party that controls the state legislature and holds the governor's office has influence over the redrawing of the Congressional district map for the next 10 years.