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

The Obama campaign immediately forced the resignation of Harvard professor Samantha Power, a human rights advocate and unpaid campaign adviser, and affirmed Obama's troop-withdrawal plan. But Hillary Clinton said that 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 while campaigning in Mississippi, according to The Associated Press. She linked the incident to Obama's recent stumble over whether one of his aides had told Canadian officials that his stand on NAFTA was political posturing rather than a solid commitment.

"I don't want anybody here to be confused," Obama, said in Wyoming of his Iraq war stance. "I have been against it in 2002, 2003, 2004, 5, 6, 7, 8, and I will bring this war to an end in 2009." Noting that Clinton voted in 2002 to authorize the war, he said that, "She doesn't have standing to question my position on this issue."

Power, a Pulitzer prize-winning author, said of Clinton in an interview with a Scottish newspaper, The Scotsman: "She is a monster, too — that is off the record — she is stooping to anything."

Obama campaign manager David Plouffe said that Power had to step aside because Obama "cannot condone" her name-calling against Clinton. Power issued a public apology.

Separately, Power told the BBC that Obama "will try to get U.S. forces out as quickly and as responsibly as possible," but that the two-brigade per month plan he has outlined is "the best case estimate of what it would take."

She said he wouldn't pull troops out based on a campaign plan alone, but would draw an "operational plan" based on military consultations he doesn't yet have access to.

Clinton linked Power's comments to a recent gaffe involving Obama economic policy adviser Austan Goolsbee, who held a closed-door chat about the North American Free Trade Agreement with officials from the Canadian consulate in Chicago.

Initially, Obama denied that Goolsbee had met with Canadian officials, only to acknowledge after a memo surfaced that he had.

The leaked memo indicated that at least one Canadian official thought Goolsbee was signaling that Obama's criticism of NAFTA was at least partly for show, but Obama and the Canadian Embassy deny that.

A separate report in the Canadian press, however, said that a Clinton campaign adviser also told the Canadians not to overreact to the anti-NAFTA rhetoric in the Democratic presidential campaign. Clinton spokesman Phil Singer on Friday again denied that assertion.

Nevertheless, the Clinton campaign went into overdrive, organizing two conference calls with reporters Friday to hammer Obama.

Obama aides said that Clinton's professed outrage about Power's withdrawal remarks seemed disingenuous, given what retired Army Gen. Jack Keane, an architect of the surge in Iraq, recently said about Clinton and withdrawal from Iraq.

Keane told the New York Sun that he has "no doubts whatsoever that if she were president in January '09, she would not act irresponsibly and issue orders to conduct an immediate withdrawal from Iraq, regardless of the consequences, and squander the gains that have been made."

Clinton aides argued Friday that Keane wasn't speaking as a formal campaign adviser and insisted that Clinton's rhetoric has been consistent.