An adviser to Barack Obama has resigned after a Scottish newspaper quoted her calling rival US Democratic candidate Hillary Clinton "a monster".
Samantha Power said she had tried to retract the comments
Samantha Power has expressed "deep regret" over the comments and said she had tried to retract them.
The Scotsman newspaper quoted Ms Power as saying: "She is a monster, too - that is off the record - she is stooping to anything."
Ms Power is a Harvard professor who has advised Mr Obama on foreign policy.
Announcing her resignation as an adviser, she said: "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."
Ms Power, who won a Pulitzer Prize in 2003, was speaking to the Scotsman about Mrs Clinton's campaign strategy in Ohio, a state the New York senator won in Tuesday's primary elections.
A spokesman for the Obama campaign, Bill Burton, said: "Senator Obama decries such characterisations, which have no place in this campaign."
Shortly before Ms Power stepped down, advisers to Mrs Clinton had held a conference call with reporters in which they called for her resignation.
Ms Power had already issued an apology and Mr Obama's campaign had already condemned her remarks.
'May get nastier'
Comments made by Ms Power about Mr Obama's Iraq strategy in an interview with the BBC earlier this week have also caused a stir.
Democrats fear duelling between the campaigns may hurt the party
Ms Power said the Illinois senator's position that he would withdraw all troops within 16 months was a "best-case scenario" that he would revisit if he became president.
The Clinton camp criticised her comments as inconsistent with those of Mr Obama on the campaign trail.
"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," Mrs Clinton told reporters while campaigning in Mississippi.
Obama campaign manager David Plouffe responded that Mr Obama's plan to withdraw about two brigades a month if elected was a "rock solid commitment" to US voters.
Senior Democrats fear that weeks of attacks and mudslinging between the two camps could damage the party and cost it support in November's presidential election.
Howard Dean, chairman of the national Democratic Party, has warned that the tone of the campaign "may get nastier" and that the party must seek to prevent that happening.
Both Mrs Clinton and Mr Obama have been campaigning in Wyoming which holds its caucus on Saturday.
Mr Clinton has also been in Wyoming, campaigning on behalf of his wife.
Only 12 pledged delegates are at stake in Wyoming, but the closeness of the race means it has gained unexpected importance.
Mr Obama currently has 1,567 delegates against Mrs Clinton's 1,462. It takes 2,025 to secure the party's nomination.
Both candidates have reported massive fund-raising totals for February, with Mr Obama bringing in $55m (£28m) and Mrs Clinton $35m (£17m).
Wyoming's contest will be followed by a primary in Mississippi on Tuesday, in which 33 Democratic delegates will be awarded.
The next major battle will be the Pennsylvania primary on 22 April, with 158 delegates up for grabs.
Michigan and Florida
Republican Ron Paul is "winding down" his presidential campaign
Debate on whether to hold fresh ballots in Florida and Michigan also continues.
Both states were told their delegates would not be seated at the party's August national convention - meaning they cannot vote on who should be the Democratic presidential candidate - after they breached party rules by holding primary elections before 5 February.
But with Mr Obama and Mrs Clinton so close in terms of how many delegates they can claim, the question of whether Florida and Michigan should stage fresh votes has taken on a new urgency.
HAVE YOUR SAY
I'm sure many people will be reluctant to say anything "off the record" in future, which will be detrimental to journalism and news reporting.
Andy Roberts, UK
Aides to Mrs Clinton have indicated they would be open to new elections being held, saying they believe her prospects would be good.
Meanwhile, on the Republican side, a spokesman for presidential hopeful Ron Paul confirmed that he was "winding down" his campaign.
The libertarian congressman from Texas posted a video on his campaign website on Thursday in which he told supporters: "I don't mind playing a key role in this revolution, but it has to be more than a Ron Paul revolution.
"Our job now is to plan for the next phase."
Dr Paul's campaign has been notable for the fervent online following he has attracted and his ability to raise large amounts of cash, much of it through the internet.
John McCain on Tuesday passed the threshold of delegates needed to secure the Republican nomination at the national party convention in September.