A top foreign policy adviser to Barack Obama resigned Friday after she called Hillary Rodham Clinton a "monster" and said that Obama's Iraq war withdrawal plan was a best-case scenario rather than a firm commitment.

Samantha Power, a 37-year-old Harvard professor and Pulitzer Prize-winning author, expressed her "deepest apologies" to Clinton, Obama and the Obama campaign team as she left the unpaid post that she had held for 14 months.

"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," said Power, who is known for her outspokenness.

The Clinton campaign quickly seized upon Power's comments, asking supporters for contributions to "show the Obama campaign that there is a price to this kind of attack politics."

Clinton said the flap demonstrated her rival's inexperience and untrustworthiness.

Obama "has attacked me continuously for having no hard exit date [from Iraq], and now we learn he doesn't have one," Clinton told reporters in Mississippi.

Obama responded in Casper, Wyo., that Clinton has no standing to question his resolve on Iraq because she voted in 2002 to authorize the war. "If it had been up to me, we would have never been in this war," Obama said, his voice rising. "It was because of George Bush, with an assist from Hillary Clinton and John McCain, that we got into this war.

"I will end it in 2009," he said.

In an interview with The Scotsman newspaper, Power said the Clinton campaign had become "obsessed" with winning last week's Ohio primary.

"She is a monster, too -- that is off the record -- she is stooping to anything," Power told the newspaper. "You just look at her and think, 'Ergh.' ... The amount of deceit she has put forward is really unattractive."

The newspaper followed a standard journalistic practice in requiring interview subjects -- particularly political operatives -- to reach agreement in advance if they want to keep matters out of the public domain.

Power made the remarks in Scotland on a publicity tour to promote her latest book on a U.N. envoy killed in Baghdad in 2003.

Obama campaign manager David Plouffe said Power had to step aside because Obama "cannot condone" the name-calling.