"This is a time of great challenges for America, and I pray that the president will be successful in guiding our nation," Romney said.
Romney spoke to cheering supporters in a hotel ballroom in Boston. The former Massachusetts governor said he was proud of the campaign he had run but that voters had made another choice.
Romney thanked his running mate Paul Ryan, saying that besides his wife, Ann, the Wisconsin congressman was "the best choice I've ever made." Romney predicted Ryan would have a bright future serving the nation.
Romney struck a note of bipartisanship after the bitterly fought contest, saying elected officials must find common ground.
"We can't risk partisan bickering and political posturing. Our leaders have to work across the aisle to do the people's work," Romney said.
It was a bitter loss for Romney, who lost decisively to Obama after portraying the Democrat as out of his depth and unsuited to steer the nation's limping economy. Romney hoped to convince voters that his background as a successful private equity investor gave him the experience Obama lacked to boost the slow economic recovery.
Instead, Romney saw nearly all the major battleground states tip Obama's way including Iowa, Wisconsin, Virginia and Ohio, the hardest fought prize.
Romney supporters cheered a win in North Carolina, which Obama captured four years ago. But it was a rare prize in an evening that broadly favored the president.
The Republican nominee spent Election day campaigning in Ohio and Pennsylvania, a Democratic-leaning state that Republicans hoped to put in play. Obama won Pennsylvania Tuesday.
Romney told reporters he had written a victory speech that he thought would conclude his yearslong quest for the presidency.
"I feel like we put it all on the field. We left nothing in the locker room. We fought to the very end," Romney told reporters aboard his plane.
