Republicans wrested back control of the Senate on Tuesday by adding at least seven seats to their ranks, riding discontent and resentment about President Obama and his policies and consolidating Republican power on Capitol Hill, reports The New York Times.

And it was all down to what American voters perceive as Obama’s poor handling of the economy with its adjunct frustrations and despair.

Republicans picked up the sixth seat they needed with a win in North Carolina, as Thom Tillis, the State House speaker, defeated the incumbent, Kay Hagan, a Democrat, according to projections by The Associated Press. In Iowa, the Republican Joni Ernst, a state senator, defeated Representative Bruce Braley to win the seat vacated by longtime Senator Tom Harkin, who is retiring at the end of this year.

Voters in Arkansas and Colorado also ousted the Democratic incumbents Mark Pryor and Mark Udall, and elected Republicans in West Virginia, Montana and South Dakota.

With several contests still too close to call late Tuesday, unofficial results showed Republicans emerging as winners in enough states to claim victory. The party’s leaders in Washington will now control both chambers in Congress as Mr. Obama struggles to fashion an agenda in the remaining two years in his term.

Six years ago, Mr. Obama swept into office, carrying Democrats with him and using majorities in both chambers to push through health care and economic stimulus legislation. But two years later, a Tea Party revolt gave Republicans control of the House and made Representative John A. Boehner of Ohio the House speaker. Mr. Obama returned the favour in 2012, winning re-election and claiming a new mandate for his agenda.

On Tuesday, Republicans completed a congressional takeover that will most likely elevate Senator Mitch McConnell of Kentucky to become the chamber’s majority leader, deposing Senator Harry Reid, Democrat of Nevada.

The outcome is a blow to Democrats, who struggled in vain to defend incumbent senators in deeply conservative states where anger and frustration at Mr. Obama made him unpopular. The president was largely unwelcome in almost all of the Senate contests, where Democrats sought to distance themselves from Washington and the president’s accomplishments.

In Colorado, Representative Cory Gardner knocked off Mr. Udall in a state that President Obama had won twice and where Democrats had pointed to a strong ground game that they said would hold off the Republican challenge.

In Arkansas, Representative Tom Cotton, the Republican candidate, defeated Mr. Pryor after hammering the Democrat’s ties to Mr. Obama in a state where the president is deeply unpopular. Mr. Cotton, an Iraq combat veteran and a first-term congressman, won despite feverish campaigning by former President Bill Clinton on behalf of Mr. Pryor, according to projections by The Associated Press.

In West Virginia, Shelley Moore Capito, a Republican, easily defeated her Democratic opponent to replace Senator Jay Rockefeller, who is retiring after 30 years in the Senate.

Mike Rounds, a former Republican governor of South Dakota, won the third pickup to add to the Republican gains, according to projections by The A.P. And in Montana, Representative Steve Daines, a Republican, won a seat that had been in Democratic hands for decades.

In an outcome that diminished the chances of the Democrats’ clinging to the majority, the Republican businessman David Perdue defeated Michelle Nunn, a Democrat, and avoided a runoff in Georgia’s Senate race, The A.P. reported. Democrats had hoped that Ms. Nunn, the daughter of former Senator Sam Nunn, could grab a Republican seat.

And the Democrats lost their opportunity to pick up a Republican-held seat in Kansas when Senator Pat Roberts, the longtime incumbent, was projected by the news networks to be the victor over Greg Orman, a businessman who ran as an independent.

In New Hampshire, Senator Jeanne Shaheen, a Democrat and former governor, beat back a challenge from Scott Brown, a former Republican senator from Massachusetts, The A.P. projected.

Mr. McConnell of Kentucky handily won re-election, giving the Republicans the first victory. The five-term senator fought an intense and spirited campaign against Alison Lundergan Grimes, Kentucky’s secretary of state, who Democrats hoped would tap into voter frustration with Washington to end the career of the longtime master of the Capitol’s legislative process.

Well into a suspense-filled evening, some of the most crucial Senate and governor’s races remained excruciatingly close, a reflection of the electorate’s refusal to reward either party with their loyalty after two years of gridlock in Washington and crisis around the world. As expected, the Senate race in Louisiana was headed to a runoff in December after the incumbent, Mary L. Landrieu, and her Republican opponent, Representative Bill Cassidy, appeared unlikely to capture at least 50 percent of the vote.

Republicans emerged victorious in one hotly contested governor’s races, with Rick Scott, the incumbent, defeating his Democratic rival, Charlie Crist, after a bitter contest that featured personal attacks and a bizarre fight over a small electric fan. The Republicans also avoided defeat in Wisconsin, where Gov. Scott Walker won re-election in a hard-fought race in which labor unions worked furiously to oust him.

A consensus of election-eve polls had bolstered Republican spirits, giving the party a clear advantage in most of the competitive Senate races. Democrats, meanwhile, clung to hope that an urgent and robust turnout effort, modeled on Mr. Obama’s presidential campaigns, would rally their supporters to save the party’s majority in the Senate.

But even Mr. Obama, the party’s top cheerleader, appeared dispirited late Tuesday. Calling into a Hartford radio show, the president seemed ready to concede defeat in an election that will shape the balance of his time in the White House and could constrain his legacy.

“This is probably the worst possible group of states for Democrats since Dwight Eisenhower,” Mr. Obama said. “There are a lot of states that are being contested where they just tend to tilt Republican, and Democrats are competitive, but they tend to tilt that way.”

In early results, Senator Lindsey Graham, Republican of South Carolina, easily won re-election after earlier fighting back a primary challenge, The A.P. reported. Senator Tim Scott, a South Carolina Republican who was appointed in 2013 to replace a retiring senator, won another two years in office. And Nikki Haley, the state’s Republican governor, won another term.

In other Senate races, Senator Susan Collins, a three-term Republican from Maine, won re-election. Senator Cory Booker, a Democrat from New Jersey, was elected to his first full term after winning a special election last year. Senator Thad Cochran, a Republican from Mississippi, barely survived a contentious primary and won re-election, while Senator Edward J. Markey, Democrat of Massachusetts, who was appointed last year, won election to his first full six-year term.

In preliminary results from exit polls, voters who cast ballots on Tuesday expressed deep frustration about Mr. Obama, Congress and the overall direction of the country.

Nearly half of all voters said the economy — a subject for which candidates offered few serious solutions on the campaign trail — was the issue most on their minds, almost double the number who picked health care. Roughly two-thirds of the voters in North Carolina said the country was “seriously off track,” and six in 10 expressed negative opinions of the Obama administration.

Early exit poll results showed that a majority of voters disapprove of the job Mr. Obama is doing as president, and most said they are dissatisfied or angry with his administration. Disapproval of the president looks similar to what it was in the 2010 midterm elections, when 55 percent disapproved.