Senate Democratic leader Harry Reid has apologised to Barack Obama for pre-election comments describing the president as a "light skinned" African-American "with no Negro dialect, unless he wanted to have one".
Mr Reid made the comments in private during the long 2008 campaign when discussing why Mr Obama should seek, and could win, the presidency.
Last night Mr Obama quickly accepted the apology, saying: "As far as I am concerned, the book is closed."
Mr Reid's comments appeared in a new book about that election, which elevated Mr Obama from first-term Illinois senator to America's first black president.
After excerpts from the book appeared on the website of The Atlantic, Mr Reid released a statement expressing regret for "using such a poor choice of words".
He said: "I sincerely apologise for offending any and all Americans, especially African-Americans for my improper comments."
Mr Obama issued a statement saying he had spoken with Mr Reid, who faces a difficult re-election amid frustration from both liberals and conservatives with his leadership in the Senate and his agenda.
Mr Reid's office also said he had phoned several civil rights leaders to apologise.
The revelations about his language, included in the book Game Change by Time Magazine's Mark Halperin and New York magazine's John Heilemann, are based on interviews with more than 200 people involved in the campaign, including Mr Obama.
The writers' sources were granted anonymity and the writers reconstructed much of the narrative from interviews with those involved with direct knowledge of events, notes and transcripts.
The book will be released on Monday.
"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," Mr Reid said in his apology.
Mr Reid was neutral during the bitter Democratic primary that became a marathon contest between Mr Obama and then-Senator Hillary Clinton. Mr Obama picked her as his secretary of state after the election.
The book, obtained by The Associated Press yesterday, also says Mr Reid urged Mr Obama to run for president, perceiving the freshman senator's impatience in Congress.
"You're not going to go any place here (the Senate)," he told Mr Obama. "I know that you don't like it, doing what you're doing."
The book also dealt with the Republican campaign, claiming aides to Republican nominee John McCain described the difficulties they faced with their vice presidential pick, then-Alaska governor Sarah Palin.
Steve Schmidt, a senior adviser to Mr McCain, is quoted telling Mrs Palin's foreign policy tutors: "You guys have a lot of work to do. She doesn't know anything."
While flying home to Alaska to see her son off to war, she is said to have told her team: "I wish I'd paid more attention to this stuff."
But Mrs Palin's spokeswoman, Meg Stapleton, disputed the version presented in the reporters' book.
"The governor's descriptions of these events are found in her book, Going Rogue. Her descriptions are accurate," Ms Stapleton said in a statement to the 60 Minutes TV programme.
"She was there. These reporters were not."Reuse content