Mr Obama said Mr Reid had apologised for his "unfortunate" comments
The US Senate majority leader, Harry Reid, has apologised for private comments he made about Barack Obama before the 2008 presidential election.
He is quoted in a new book as saying Mr Obama could win since he was a "light-skinned" African-American "with no Negro dialect", unless he wanted one.
The Democrat said he now regretted "using such a poor choice of words" and apologised to any Americans offended.
President Obama quickly accepted the apology and said "the book is closed".
Correspondents say Mr Reid, 71, is facing a difficult re-election battle in his home state of Nevada in November's mid-term elections.
In a new book published about the presidential election campaign by journalists Mark Halperin and John Heileman, Game Change, the senator is described as somebody whose encouragement of Mr Obama was "unequivocal".
"He was wowed by Obama's oratorical gifts and believed that the country was ready to embrace a black presidential candidate, especially one such as Obama - a 'light-skinned' African American 'with no Negro dialect, unless he wanted to have one,' as he said privately," the authors wrote.
"Reid was convinced, in fact, that Obama's race would help him more than hurt him in a bid for the Democratic nomination."
Mr Reid waited to formally endorse President Obama until after the tough presidential primary battle against Hillary Clinton ended.
After the extract was published by The Atlantic magazine on Friday, the senator confirmed he had made the remarks and apologised.
"I deeply regret using such a poor choice of words. I sincerely apologise for offending any and all Americans, especially African-Americans for my improper comments," he said in a statement.
"I was a proud and enthusiastic supporter of Barack Obama during the campaign and have worked as hard as I can to advance President Obama's legislative agenda. Moreover, throughout my career... I have worked hard to advance issues important to the African-American community."
Mr Obama later issued a statement accepting the apology.
"Harry Reid called me today and apologized for an unfortunate comment reported today. I accepted Harry's apology without question because I've known him for years, I've seen the passionate leadership he's shown on issues of social justice and I know what's in his heart," he said.
"As far as I am concerned, the book is closed."