Stephanie Power, who advised the Illinois senator on foreign policy matters, said her remarks were "inexcusable" and said she was leaving the campaign "with deep regret," The Washington Post reported.
Powers, in an interview with The Scotsman, said the New York senator "is a monster, too -- that is off the record -- she is stooping to anything."
Powers said she apologized to both Clinton and Obama. Obama was preparing to campaign in Wyoming, which has its Democratic primary Saturday, when informed of Power's decision.
"She made a decision to resign and we accepted it," said Obama spokesman Robert Gibbs.
Earlier Friday, several members of Clinton's campaign called upon Powers to resign.