Republicans scored the first vital U.S. election wins on Tuesday after a long and bitter campaign that could sweep Democrats from power in Congress and slam the brakes on President Obama's legislative agenda.
Anxiety over the stumbling economy and discontent with Obama propelled Republicans to the threshold of huge gains that could give them a majority in the House of Representatives and perhaps even the Senate.
Republicans picked up their first Senate seat from Democrats in Indiana and held a Senate seat in Kentucky when Republican Rand Paul became the first conservative Tea Party candidate to win a Senate race.
Stocks futures were higher in reaction to the early Republican gains. (Click here for the latest futures quotes)
The first polls closed in six states, although it could be hours before many winners are determined. Voting will end in other U.S. states over the next six hours.
Opinion polls and independent analysts project Republicans will win at least 50 House seats, far more than the 39 they need to take control and topple Democratic House Speaker Nancy Pelosi from power.
More than 90 Democratic seats are in danger, according to the nonpartisan Cook Political Report. Dozens of races are considered too close to call.
Republicans are also expected to make big gains in the Senate, but it will be more difficult for them to pick up the 10 seats they need for a majority.
They would need to prevail in seven of eight tight races across the country.
Republican control of even one chamber of Congress would likely spark legislative gridlock, weakening Obama's hand in fights over the extension of soon-to-expire income-tax cuts and the passage of comprehensive energy or immigration bills.
Republicans also led for a pair of House seats in Democratic hands, and projected confidence they would succeed in winning a majority and installing Rep. John Boehner of Ohio as speaker.
All 435 seats in the House were on the ballot, plus 37 in the Senate in an election shadowed by recession power and stirred by a rebellion of tea party conservatives.
An additional 37 governors' races gave Republicans ample opportunity for further gains halfway through President Obama's term.
Former Sen. Dan Coats easily dispatched Rep. Brad Ellsworth in Indiana to win back the seat he voluntarily gave up a dozen years ago.
In Kentucky, where Paul was making his first run for political office, he prevailed over Democrat Jack Conway.
The victory preserves a Republican Senate seat Democrats had hoped to win and staunch expected losses elsewhere in the congressional elections. He will replace retiring Republican Senator Jim Bunning.
Paul, a practicing ophthalmologist and the son of libertarian Republican Representative Ron Paul, had received early backing by the Tea Party movement, which has pushed an agenda for lower taxes and limited government.
The Indiana seat marked a gain for Republicans, and the Kentucky race was the first indication that voters were ready to embrace at least some of the Republican candidates whom Democrats had pilloried for months as being too extreme to win elections.
Rob Portman won the Ohio Senate race, keeping Republicans in control of the seat that Sen. George Voinovich is vacating. Portman spent 12 years in the U.S. House starting in 1993.
He later was budget director and then U.S. trade representative under President George W. Bush. Portman defeated Democratic Lt. Gov. Lee Fisher, who struggled the keep the contest close.
Easily winning re-election as expected were Sens. Jim DeMint, R-S.C., and Patrick Leahy, D-Vt.
Spurred by Obama's handling of the struggling economy, Republicans are heavy favorites to capture at least the 39 Democratic seats they need to gain a majority in the U.S. House of Representatives.
They could come close to picking up the 10 Democratic seats they need for Senate control.
Stocks got a boost Wednesday from investors' expectations of strong Republican gains in Congress.
The Dow Jones Industrial Average rose 64.10 points, or 0.6 percent, to close at 11,188.72 after eking out a slim gainin a rocky trading session Monday.
The S&P 500 Index rose 9.19 points, or 0.8 percent, to close at 1,193.57. The Nasdaq rose 28.68 points, or 1.1 percent, to close at 2,533.52, its highest level since June 2008.
Shares in sectors such as healthcare, deemed likely to benefit from the rebalancing of power, advanced.
The outcome also will have a major impact on the financial sector, under new scrutiny this year as a result of regulatory reform legislation.
Major companies affected by this legislation include Goldman Sachs , Morgan Stanley, Citigroup , Bank of America and JPMorgan Chase .