Washington, Nov 7 (IANS): Defeated Republican challenger Mitt Romney congratulated President Barack Obama on his re-election and vowed to reach across the aisle to meet the critical challenges faced by the nation.

"This is a time of great challenges for America and I pray that the president will be successful in guiding our nation," the former Massachusetts governor told cheering supporters in Boston in a short concession speech.

"At a time like this we can't risk partisan bickering and political posturing," he said shortly before 1 a.m.

"Our leaders have to reach across the aisle to do critical work," Romney said urging an end to partisan gridlock as he thanked his family, staffers and supporters.

Romney also thanked his wife saying "she would have been a wonderful first lady", his sons, his campaign staff, his volunteers and his running mate, Paul Ryan.

"Paul and I have left everything on the field. We have given our all to this campaign. I so wish I had been able to fulfil your hopes to lead the country in a different direction, but the nation chose another leader," Romney said. "Ann and I join with you to earnestly pray for him and this great nation."

He ended with a "You guys are the best" and a "Thanks, guys" to the crowd.

Romney's speech came after the US media projected Obama as getting about 300 electoral votes, a comfortable victory from the magic figure of 270 needed to win.

His wife, children and grandchildren came up on stage after the speech.