Democrats will hold on to the Senate seat in Colorado, avoiding another embarrassing defeat in a state that voted for President Obama in 2008.
Incumbent Sen. Michael Bennet will retain his seat, defeating conservative Ken Buck who had led in virtually all the polls leading up to the election, the Associated Press projected.
Bennet, who replaced Ken Salazar last year after Salazar was tapped for interior secretary, struggled to energize independents and Democrats that turned out in high numbers for then-candidate Obama in 2008.
At a time when discontent against Washington is high, Buck, a Tea Party favorite, attacked Bennet as a Washington insider closely aligned with President Obama's agenda.
Obama carried Colorado with 53 percent of the vote in 2008; fewer voters now -- 47 percent -- said they approved of the job he is doing as president.
Democrats will retain control of the Senate, and in one of the few major victories for the party, Sen. Harry Reid will continue in his position as majority leader. But Democrats face an uphill battle as they try to advance their agenda.
One key race has yet to be called. In Washington, three-term Democratic incumbent Patty Murray faces a close race against Republican Dino Rossi.
Republicans didn't win in enough blue states to gain control of the Senate, but with the House now under GOP control and Republicans gaining a much stronger foothold in the Senate, Democrats will have to retool their agenda, especially on issues such as energy and jobs, where the two parties have failed to come to a consensus.
Republicans were quick to paint Tuesday night's results as a referendum on President Obama's policies.
"We're determined to stop the agenda Americans have rejected," Senate Minority Leader Mitch McConnell said in a press conference with members of the Republican leadership. "We will work with the administration when they agree with the people and confront them when they don't."
"I think what our friends on the other side learned is that choosing the president over your constituents is not a good strategy," he said. "Ignoring the voters and their wishes, as you could see during the entire two-year period, produces predictable results."
The president today urged both parties to work together on issues of concern to the American people.
"I'm not going to pretend this is going to be easy," he said. "What I think the American people are expecting and what we owe them is to focus on those jobs that affect their jobs."
"I do believe there is hope for progress," he added.
In a signal of how important it will be for Democrats to reach across the aisle, the White House is working on a plan for a retreat with GOP and Democratic leaders of the House and Senate.
There had been little bipartisan cooperation in the Senate in the past year.
Not one Republican voted for the health care bill, the flagship legislation of the president and congressional Democrats. Today, McConnell called it "a metaphor of government excess."
The two parties couldn't find middle ground on other important legislation, either, particularly on energy and immigration.
This is the first time since 1930 that one chamber of Congress has changed hands without the other.