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WASHINGTON - A former adviser to Barack Obama, who resigned Friday after calling rival Hillary Rodham Clinton "a monster," said Obama may not be able to withdraw all U.S. combat troops from Iraq within a year as he has promised on the campaign trail.

Samantha Power, a Pulitzer Prize-winner author, made the comments in two separate interviews with foreign media while promoting her latest book. The comment that led to her resignation came in an interview with The Scotsman, and she immediately 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. A few hours after the comments were published, Power, an unpaid adviser and Harvard professor, announced her resignation in a statement distributed by the Obama campaign.

"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," the statement said.

Power's comments about Iraq came in an interview with the BBC. She said Obama's position is that withdrawing all U.S. troops within 16 months is a "best-case scenario" that he will revisit if he becomes president.

"He will, of course, not rely on some plan that he's crafted as a presidential candidate or a U.S. senator," she said. "He will rely upon a plan -- an operational plan -- that he pulls together in consultation with people who are on the ground to whom he doesn't have daily access now, as a result of not being the president."

Obama has actually shortened his original 16-month commitment to say he'll end the war in 2009. Obama advisers say President Bush's plan to draw troops down to 15 brigades this year means Obama could complete the removal in a year.

In Mississippi, Clinton questioned the Iraq comments based on Obama's public statements.

"He has attacked me continuously for having no hard exit date, and now we learn he doesn't have one, in fact he doesn't have a plan at all," Clinton told reporters while campaigning in Mississippi. Her campaign sent out a fundraising appeal based on the Power's resignation.

Obama campaign manager David Plouffe said Obama's plan to draw down approximately two brigades a month upon becoming president is "a rock solid commitment."

"He has been and will continue to be crystal clear with the American people," Plouffe told reporters in a conference call.

Plouffe noted that one of Clinton's military advisers, retired Gen. Jack Keane, said last week that she would not remove troops immediately upon taking office.

Keane told The New York Sun, "I have 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 said Power's comment on Iraq is reminiscent of Obama's senior economic adviser, Austan Goolsbee, telling Canadians privately that the debate over free trade in Ohio had was "political positioning" and not an indicator of policy plans. The description appeared in a Canadian government memo, but Goolsbee and the Canadian Embassy later said Goolsbee never suggested Obama's public and private positions are different.

Recent reports in Canada said an official in the Clinton campaign also gave Canada back-channel assurances that her harsh words about the North American Free Trade Agreement were for political show. The campaign denied the report.

Clinton said the two instances suggest "he keeps telling people one thing, while his campaign tells people abroad something else. I'm not sure what the American people should believe."

Power's resignation came a day after Clinton spokesman Howard Wolfson compared Obama to former special prosecutor Kenneth Starr by criticizing Clinton. Starr's investigation led to Bill Clinton's impeachment, and he is an unpopular figure among Democratic voters.

"I for one do not believe that imitating Ken Starr is the way to win a Democratic primary election for president," Wolfson said.

Clinton told reporters the two cases were different because "one is an ad hominem attack and one is a historical reference."

She also said Wolfson's criticism "is a true statement."

It was unclear whether her comment about a "true statement" also applied to the material Wolfson cited such as an Obama campaign memo referring to the Clinton's tax returns and Hillary Clinton's cattle-futures trading profits.