Republicans were projected to win control of the US House of Representatives in congressional elections Tuesday, dealing a major blow to President Barack Obama just two years into his term in office, dpa reported.
The conservative party also made inroads in the 100-member Senate, though early signs were that Obama's party might still be able to hold onto its majority in the upper legislative chamber.
The Republican takeover of the House of Representatives marks the first time the party will control one of the two US chambers of Congress since 2006.
The centre-right party appeared ready to gain well more than the 39 seats needed to control the 435-member lower chamber, according to early projections by CNN and Fox News.
All 435 seats in the House and 37 seats in the Senate were up for grabs Tuesday as well as 37 governorships and many state and local positions.
The Republican victories have come on the back of intense voter frustration at Obama's handling of the economy and an unemployment rate that still sits at 9.6 per cent. Initial exit polls found that the still-sluggish US economy was the top concern for 62 per cent of voters, according to pollster John Zogby.
Republicans, who would need to gain 10 seats to win control of the Senate, picked up two seats held by Democrats in Indiana and Arkansas, according to early projections.
The party also held Senate seats in Florida and Kentucky, two early victories for the grassroots Tea Party movement that has energized the conservative base.
"We've come to take our government back," said Kentucky's Rand Paul, a Tea Party favourite who defeated Democrat Jack Conway in the state's Senate race.
But Democrats did manage to hold on to some key Senate seats that could wind up leaving them in control of the upper chamber of Congress, even with a much smaller majority.
West Virginia's sitting Governor Joe Manchin was projected to defeat Republican business magnate John Reese in a closely-fought senate race.
Chris Coons was declared the senate winner in Delaware, defeating Tea Party-backed candidate Christine O'Donnell and holding the seat once held by Vice President Joe Biden. Democrats also held Connecticut, with Richard Blumenthal defeating Linda McMahon, the former manager of World Wrestling Entertainment.
In Florida, Republican Marco Rubio was projected to win over sitting Governor Charlie Crist, a former Republican who launched an independent campaign for the Senate seat.
In a last-minute bid to stem the losses, Obama made a round of radio interviews with such popular personalities like Ryan Seacrest, of American Idol fame.
"I want everyone to remember that you can't shape your future if you don't participate," Obama said on Seacrest's syndicated show. "You've got to get out there and vote."
With many neck-and-neck races across the country and ballots counted across six time zones, it will likely be a long night before final results are known. Some races could take days to sort out.
The Republican Party's takeover of Congress, together with the Tea Party's uncompromising stance on most political issues, has many predicting legislative gridlock during Obama's next two years in office leading up to the 2012 presidential elections.
Polls have shown major dissatisfaction with incumbents from both parties. "Dump incumbents" signs peppered the streets in and around Washington.