Samantha Power, a Dublin-born, Pulitzer Prize-winning author, yesterday stepped down from her role as Mr Obama's foreign policy adviser after her off-the-record comment made to a Scottish newspaper was made public.

Speaking to a reporter in London earlier this week, Ms Power told the newspaper, "We f****d up in Ohio", referring to Obama's loss to the former first lady in the midwestern state.

"In Ohio, they are obsessed and Hillary is going to town on it, because she knows Ohio's the only place they can win.

"She is a monster, too -- that is off-the-record -- she is stooping to anything . . . You just look at her and think 'Ergh'," Ms Power was quoted as saying.

She added: "Here, it looks like desperation, I hope it looks like desperation there, too."

The Clinton camp have jumped on Ms Power's remark, saying they show Mr Obama's side is resorting to the "usual attack style politics".

In a statement published yesterday, she announced her resignation and apologised to both Mr Obama and Mrs Clinton, saying her comments had no place in American politics.

Inexcusable

"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.

"These comments do not reflect my feelings about Senator Clinton, whose leadership and public service I have long admired. I should not have made these comments and I deeply regret them. It is wrong for anyone to pursue this campaign in such negative and personal terms," she added.

Ms Power was in Ireland and Britain to promote her new book on Sergio Vieira de Mello, the UN Human Rights Commissioner who was killed in a suicide bomb blast in Baghdad.

She had taken a year's leave of absence from her job as a professor at Harvard University to join the Obama campaign.

The Scotsman defended its release of the comments, saying the interview was clearly conducted on-the-record.

"Our job was to put that interview before the public as a matter of public interest. It was for others to judge whether the remarks were ill-judged or spoke of the inexperience in the Obama camp," said the paper's editor, Mike Gilson.

After weeks in which Mr Obama had shrugged off Mrs Clinton's flailing attacks, he is now the candidate who appears most rattled and prone to making mistakes.

Earlier this week, Mr Obama fled a press conference where he was asked awkward questions about his relationship with former fundraiser, Tony Rezko. David Axelrod, Mr Obama's strategist, responded by threatening to dredge up material about Mrs Clinton, but her campaign was swift to compare him to Ken Starr, the notorious investigator who hounded the White House in the 1990s.

Instead of coasting down the road to the convention in Denver as the Democratic frontrunner -- a realistic possibility until this week -- he now faces a rocky road to the nomination. He will be harried at every stage by Mrs Clinton, who predicts some "interesting twists and turns".

First stop is the suitably mountainous state of Wyoming which holds it caucuses today, followed by a vote in Mississippi on Tuesday. Mr Obama expects to win both states, cancelling out most of the gains Mrs Clinton made on Tuesday night.

He has a seemingly insurmountable lead of 140 over Mrs Clinton in elected delegates, a margin which would narrow to an estimated 84 in the unlikely event of Mrs Clinton winning each remaining primary by 10 points.

The blizzard of memos coming from his aides, explaining why he cannot be caught, reflects how Mr Obama has been changed from a candidate of hope - to one remorselessly grinding out the logic of what Mrs Clinton calls a "mathematical calculation".

Her campaign, though still riven by internal dissent, appears to have recovered some of its former self-belief and discovered a source of passion.

Advisers say she can win the nomination by raising doubts in the minds of voters and Democratic party super-delegates about whether Mr Obama is ready to be commander-in-chief -- or strong enough to withstand attacks from Republicans in November's general election.

Mrs Clinton's aides are pointing to the next big contest in Pennsylvania, where she is backed by popular politicians and which has a similar blue-collar demographic to Ohio.

They tell super-delegates to think twice before picking a candidate who has failed to win in any major states, except for his home base of Illinois. And there are already suggestions that Mrs Clinton may overtake Mr Obama in the popular vote before the convention, giving the 795 super-delegates a reason to defy the will of voters from caucuses and primaries.

But the concern in the party is that Mrs Clinton will succeed in wounding Mr Obama without quite killing him off. That task, warned a senior Democrat, would be "left to Senator McCain in the general election".