Democrats picked up two U.S. Senate seats on an Election Day that the most optimistic conservatives hoped would favor the GOP.
Republicans, meanwhile, maintained their hold on the House of Representatives, virtually ensuring that the partisan gridlock consuming Washington will continue into President Barack Obama's second term.
With several races still too close to call, Republicans had won 234 House seats while Democrats captured 189, according to the Associated Press.
The changes in the Senate expand Democrats' control, bringing it to 53-45, with the certainty that Vermont independent Bernie Sanders will align with the Democrats and the expectation that Maine independent Angus King, a social liberal who was elected to fill the seat of retiring Republican Olympia Snowe, will do the same to give Democrats an effective 55-45 majority.
While the numbers didn't change much, the political dynamics are expected to.
Voters returned the most conservative Republicans to office. Meanwhile, both chambers' Democratic caucuses became more liberal with the addition of socially liberal Mr. King, Elizabeth Warren of Massachusetts and Tammy Baldwin of Wisconsin, the first openly gay senator.
"America has said extremes are OK and we're going to leave them there," said Arnold Shober, associate professor of government at the Lawrence University in Appleton, Wis. "There's not much incentive for any member of Congress to move to the middle."
Some say the changes will embolden Republicans, while others say it will force both parties to work together, particularly as the fiscal cliff looms.
Mr. Obama began to make inroads during an acceptance speech that emphasized unity as it acknowledged ideological differences.
"Democracy in a nation of 300 million can be noisy and messy and complicated," he said in his speech in Chicago early Wednesday morning.
"Each of us has deeply held beliefs. And when we go through tough times, when we make big decisions as a country, it necessarily stirs passions, stirs up controversy," Mr. Obama said. "That won't change after tonight, and it shouldn't."
He said Republicans and Democrats all want freedom, national security, dignity, generosity, compassion and tolerance.
"We will disagree, sometimes fiercely, about how to get there," he said. "By itself, the recognition that we have common hopes and dreams won't end all the gridlock, or solve all our problems ... but that common bond is where we must begin."
Republican House leaders put the onus on Mr. Obama to end the logjam on Capitol Hill.
"It's time for the president to propose solutions that actually have a chance of passing the Republican-controlled House of Representatives and a closely divided Senate, step up to the plate on the challenges of the moment and deliver in a way that he did not in his first four years in office," Senate Minority Leader Mitch McConnell, R-Ky., said. "To the extent he wants to move to the political center which is where the work gets done in a divided government, we'll be there to meet him half way."
Mr. Obama, in his acceptance speech, said he would work with both parties to reduce the deficit, reform the tax code, fix the nation's immigration system and end reliance on foreign oil.
Both branches of government will face their first test when a lame duck Congress gets to work next week on a plan to avoid effects of the expiration of the 2011 Budget Control Act, which would end business tax breaks and institute deep automatic spending cuts to Medicare, defense and hundreds of other government programs.
Democrats want a solution that includes tax increases on the wealthy, which Republicans oppose.
Lawmakers had put off action until after the election.
Senate Majority Leader Harry Reid, D-Nev., said constituents are tired of obstruction and delay.
"Now that the election is over, it's time to put politics aside and work together to find solutions," Mr. Reid said Wednesday. "This is no time for excuses. This is no time for putting things off until later. We can achieve big things when we work together. And the middle class is counting on us to achieve big things in the months ahead."
Correction/Clarification: (Published November 9, 2012) Mitch McConnell, a Republican from Kentucky, is minority leader of the U.S. Senate. His leadership position was incorrect in an election followup story Thursday. Also, Arnold Shoper, who was quoted in the story is an associate professor of government at Lawrence University in Appleton, Wis. His school was reported incorrectly. And Olympia Snowe is a Republican senator from Maine. Her last name was misspelled.
Washington Bureau Chief Tracie Mauriello: email@example.com or 703-996-9292.