"We remain more than a collection of red states and blue states, we are, and forever will be, the United States of America," Mr. Obama said during his victory speech in Chicago.
The president faces a still-sharply divided Congress, with a Democratic Senate and a Republican House, so pushing his second-term agenda will be difficult. Those agendas include addressing the Bush era tax cuts, which are set to expire at the end of the year. Mr. Obama wants to end them for those making over $250,000, but Republicans strongly oppose that.
If they can't come to a bipartisan agreement, the nation could fall off what's being called a "fiscal cliff," where automatic spending cuts and tax increases go into effect.
Stocks plunged on Wall Street the day after the elections in one of the sharpest sell-offs of the year. Investors fretted that a package of tax increases and government spending cuts could stall the economic recovery unless Congress acts to stop it by Jan. 1.
It's still unclear who will be on the president's team in his second term. Hillary Clinton on Wednesday stood by her decision to leave her post as secretary of state. Defense Secretary Leon Panetta also said he's ready to retire and Treasury Secretary Tim Geithner said he's on his way out.
The president will likely be pressed harder on questions he has avoided over the embassy attack in Benghazi. He could have some Supreme Court vacancies to fill, and there will likely be more discussion of immigration reform.
National exit polls show Mitt Romney winning among white voters 58 to 40, but that's no longer enough to win the election. In 1996, the percentage of non-whites voting was 10 percent. On Tuesday night, it was a record 21 percent, and the president dominated with those groups.
"It doesn't matter whether you're black or white or Hispanic or Asian or Native American or young or old or rich or poor, enabled, disabled, gay or straight, you can make it here in America if you're willing to try," the president told a cheering crowd of supporters.
The outcome became clear before 9 p.m. PT, as the president's Midwestern firewall all went blue: Wisconsin, then Iowa, and finally Ohio, leading the networks to make the call.
"I don't think anybody expected that the projected winner would be made this early," said Los Angeles Mayor Antonio Villaraigosa.
On Wednesday evening, Florida was still too close to call. The unofficial count has Mr. Obama with a narrow lead, but even if Romney won the state, the president retains the Electoral College vote majority.
"I return to the White House more determined and more inspired than ever about the work there is to do and the future that lies ahead," Mr. Obama said during his victory speech.
ABC News and the Associated Press contributed to this report.