A key foreign policy advisor to Barack Obama resigned Friday, after calling his White House foe Hillary Clinton a "monster" as the hyper-competitive Democratic race took another negative lurch.
The remarks by Pulitzer prize-winning advisor Samantha Power triggered a fresh firestorm between the two camps, days after Clinton revived her campaign with comeback wins in Texas and Ohio nominating contests.
"We f***d up in Ohio," Power told the Scotsman newspaper, in the latest sign of frustration in the Obama camp over Clinton's resurgence.
"In Ohio, they are obsessed and Hillary is going to town on it, because she knows Ohio's the only place they can win," Power was quoted as saying.
"She is a monster, too -- that is off the record -- she is stooping to anything," Power said.
"You just look at her and think 'Ergh.'"
Power issued a statement through the Obama campaign saying she was sorry, but Clinton's backers pounced in a conference call, and her resignation came within two hours.
"With deep regret, I am resigning from my role as an advisor (to) the Obama campaign effective today," said Power.
"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 in a statement.
"I extend my deepest apologies to Senator Clinton, Senator Obama, and the remarkable team I have worked with over these long 14 months."
The Clinton campaign earlier poured pressure on Obama over the incident.
"Personal attacks are not the way to convince voters that you are capable of being the president of the United States," said congresswoman Nita Lowey, who called the drama a "test" for Obama.
"We call on Senator Obama to make it clear that Samantha Power should not be part of this campaign."
Another Clinton backer, congresswoman Debbie Wasserman Shultz added: "going down in the gutter like that and engaging in grade school name calling is distasteful and inappropriate."
Obama spokesman Bill Burton earlier said his boss "decries such characterizations which have no place in this campaign."
The latest nasty twist to the campaign came a day after the New York senator's camp accused Obama's team of acting like Clinton nemesis, special prosecutor Ken Starr, in its attempts to force her to publish her tax returns.
Power made her comments as she was interviewed in London promoting her book on UN representive Sergio Vieira de Mello, who was killed in a suicide bomb attack in Baghdad in 2003.
Power is a professor of Global Leadership and Public Policy at Harvard University's Kennedy School of Government. Her previous book "A Problem from Hell": America and the Age of Genocide, won the Pulitzer.
As the controversy raged, Clinton and Obama meanwhile held dueling rallies in the western state of Wyoming, a Republican stronghold, which holds the next voting showdown in the Democratic race on Saturday.
The state is holding caucuses, a voting method which normally favors Obama, has only 12 elected nominating delegates available, and will be followed by the more significant Mississippi primary on Tuesday, where Obama is also favored.
The Illinois senator, seeking to erode Clinton's hold on blue collar voters, meanwhile seized on government figures showing the stumbling US economy lost 63,000 jobs in February, to skewer Republican candidate Senator John McCain.
He said Americans who struggle to pay their bills "can't afford John McCain's promise of four more years of the very same failed Bush economic policies that have failed us for the last eight.
In a simultaneous side-swipe at Clinton he added in a statement: "they can't afford another politician who promises solutions but won't change the divisive, lobbyist-driven politics in Washington."
Pressure mounted Friday on Democratic party boss Howard Dean to find a solution to the wrangle over Florida and Michigan, states punished for holding early primaries, by having their convention delegations stripped.
But in an interview in US News and World Report magazine, the former first lady, who won unofficial primaries in both states, said she was against a re-vote in Florida.