Samantha Power, an unpaid foreign policy adviser and Harvard professor, announced her resignation in a statement provided by the Obama campaign in which she expressed "deep regret."
"Last Monday, I made inexcusable remarks that are at marked variance from my oft-stated admiration for Senator Clinton and from the spirit, tenor, and purpose of the Obama campaign," she said. "And I extend my deepest apologies to Senator Clinton, Senator Obama and the remarkable team I have worked with over these long 14 months."
Power's interview Monday was published Friday in a Scottish newspaper, even though she tried to keep it from appearing in print.
"She is a monster, too — that is off the record — she is stooping to anything," The Scotsman quoted her as saying.
As U.S. news media picked up on the remark, Power issued a statement of apology and the campaign said Obama decried the characterization.
The Clinton campaign held a conference call with several of the former first lady's congressional supporters calling for Power to be fired.
"Senator Obama has called for change, and a new kind of politics," said New York Rep. Gregory Meeks. "This is the worst kind of politics."
Clinton spokesman Howard Wolfson noted that those involved in the Clinton campaign had been removed when they spoke of Obama's teenage drug use or helped spread the false rumor that the Illinois senator is a Muslim.
He defended his own comparison of Obama to independent prosecutor Kenneth Starr, saying he'd been responding to "attacks" from the Obama campaign regarding Clinton's tax returns and real estate transactions. That, he said, was a clear reference to Whitewater and so it was appropriate to bring up Starr in that context.