WASHINGTON (Reuters) - Republicans scored the first key election wins on Tuesday after a long and bitter campaign that could sweep Democrats from power in Congress and slam the brakes on President Barack Obama's 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. They also held Senate seats in Kentucky, where Republican Rand Paul became the first conservative Tea Party candidate to win a Senate race, and in Ohio.
Polls have closed in nine states, although it could be hours before many winners are determined. Voting will end in other states over the next five 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.
The Republicans' expected gains drove stocks higher on Tuesday on hopes of a more business-friendly Congress.
Before the first polls closed, Pelosi cast doubt on the projections of big Republican wins on Tuesday and said Democrats would hold the House.
"The people have to speak, and this election will not be determined by the pundits," Pelosi told reporters.
All 435 House seats, 37 of the 100 Senate seats, and 37 of the 50 state governorships are at stake in Tuesday's voting.
Obama swept into office two years ago with hopes he could lead the United States out of a deep economic crisis, but persistent high unemployment and a gaping budget deficit turned many voters against him and his fellow Democrats.
Anger over government spending and economic weakness gave rise to the Tea Party, a loosely organized conservative movement that backs smaller government and lower taxes.
Television networks reported their exit polls showed voters were deeply worried about the economy, with eight in every 10 voters saying it was a chief concern, and unhappy with Obama. Four of every 10 voters said they supported the Tea Party.
"People are just fed up with the Obama administration -- that 'hope and change' thing," said retail manager Nadine Leder in Milwaukee, Wisconsin, who planned to vote for Republicans.
In a final push, Obama gave Election Day interviews to radio stations in several battleground states.
"Things have gotten better over the last two years," he told KPWR in Los Angeles. "We can only keep it up if I've got some friends and allies in Congress and statehouses."
Obama will hold a news conference at 1 p.m. EDT on Wednesday to talk about the post-election landscape.
Republican candidates have pushed an agenda of spending cuts and at least a partial repeal of Obama's healthcare and Wall Street reforms, but Obama could veto their efforts.
"We can expect very substantial Republican gains leading to super-gridlock for two years," said University of Virginia political science professor Larry Sabato.
Some research suggests that stocks languish when power is split in Washington, but they have risen in recent months on the promise of a more business-friendly Congress.
That trend continued on Tuesday, with the greatest gains in sectors expected to benefit from a shift in power, though many forecast a sell-off in the days ahead. Health insurer stocks rose, with an index of the sector gaining 2.7 percent as investors bet that Obama will have to battle to keep his healthcare reform in place.
Republicans, who lost control of Congress in 2006, have said this election is more about rejection of Democrats than an endorsement of their party.
Representative John Boehner, who would take over as House speaker if the Republicans win control of the chamber, has said he would govern more cautiously than previous speakers.
Voters on Tuesday also will weigh in on a variety of topics: in California, for example, they could approve a measure that would legalize possession of marijuana.
In perhaps the country's most high-profile race, Senate Democratic leader Harry Reid is embroiled in a neck-and-neck re-election fight with Tea Party favorite Sharron Angle in Nevada. The race could hinge on how many voters pick the "none of the above" option on the Nevada ballot.
Additional reporting by Andy Sullivan and David Morgan in Washington, Caroline Valetkevitch, Nick Zieminski and Rodrigo Campos in New York and John Rondy in Milwaukee; Editing by Jackie Frank