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Later in the night, Democrats watched Republican Ron Johnson, a businessman, knock off Mr. Feingold, a three-term incumbent Sen. Russ Feingold in Wisconsin.

Meanwhile, state Attorney General Richard M. Blumenthal, a Democrat, won the seat of retiring Sen. Christopher J. Dodd in Connecticut, and New Castle County Executive Chris Coons won the race for Vice President Joseph R. Biden Jr.’s former seat in Delaware — both of which had been seen as potential pick-ups for the GOP.

Some good news to come out of the initial results for Democrats came in West Virginia, where Gov. Joe Manchin III defeated Republican businessman John Raese, allowing Democrats to keep the Senate seat of the late Robert C. Byrd, the Democrat who had held the seat since 1959 until his death in June.

With three competitive races still undecided, the final breakdown of the new Senate remain unclear. But what was clear is that voters disapproved of the performance of both parties and of the White House.

“Voters in this election cycle are doing the same exact thing they did in 2006 and 2008, voting against the party in power,” Scott Rasmussen, a political pollster, told The Times before the initial results were announced.

Exit polls showed and Mr. Rasmussen predicted the results would reflect a fundamental rejection of both parties and a “rejection of a bipartisan political elite that’s lost touch with the people that they are supposed to serve.”

Mr. Bonjean agreed.

“Americans are giving Republicans a chance to change the direction of the country by creating more jobs and growing the economy,” he said. “If Republicans don’t succeed, Americans will send their message of change by throwing them out in 2012.”

That toxic political environment helped breathe life into the tea-party movement, a mishmash of disgruntled voters from various political stripes that was largely driven by conservatives who claimed their political philosophy was derived from the Constitution.

The tea party helped produce some of the more colorful candidates of the campaign season, including Mr. Paul in Kentucky, and Christine O’Donnell in Delaware.

Those candidates were seeing mixed results: Ms. O’Donnell was easily defeated by Mr. Coons, while Mr. Paul celebrated his victory last night at a party in Bowling Green, Ky.

“The American people are unhappy with what’s going on in Washington. But tonight there’s a tea party tidal wave, and we’re sending a message to them,” Mr. Paul told supporters.

Among other notable GOP winners: Sen. John McCain in Arizona, Kelly Ayotte in New Hampshire, Rob Portman in Ohio and Rep. Roy Blunt in Missouri.

The results will help set the political tone for the next two years, determining whether Mr. Obama’s health care package stays intact and what other issues will emerge as priorities in the 2012 presidential election.

The next Congress also will likely take up controversial subjects such as immigration reform and the war in Afghanistan.