Opposition Republicans have delivered a stinging blow to President Barack Obama, capturing control of the House of Representatives.
They made big gains also in the Senate but fell short of a majority.
The Republican gains usher in an era of divided government for the United States. Obama will have to deal with a more conservative Congress, which will include members of the anti-establishment tea party movement.
The results reflected Americans' frustrations with the weak US economy and disillusionment with Mr Obama, who was swept into office two years ago on a message of hope and change.
Republican John Boehner, destined to replace Nancy Pelosi as House speaker, called the results "a repudiation of Washington, a repudiation of big government and a repudiation of politicians who refuse to listen to the people."
House Republicans captured at least 220 seats and were leading in 20 other races. They needed 218 seats for control of the House. Republicans have had a net gain of 53 seats and were leading for another 13 Democratic-held seats. If current trends hold, Republicans could record their largest gains in the House in more than 70 years.
In the Senate, Republicans won at least six Senate seats now held by Democrats. Among the victories was Mr Obama's old seat in Illinois, captured by a congressman, Rep Mark Kirk.
But Democrats won the biggest single race, with Senate Majority Leader Harry Reid defeating Sharron Angle, a favourite of the tea party movement. Democrats also won seats targeted by Republicans in California and West Virginia.
Republicans needed to pick up 10 seats to take control of the 100-seat Senate.
The Republican gains will complicate Obama's ability to enact his proposals during the next two years and possibly force him to fight off attacks on health care legislation and other bills already signed into law. And although international affairs had little role in the campaign, Mr Obama's global agenda also would be affected in areas such as arms control and climate change.