An ex-employee of a company with links to Democratic presidential candidate Barack Obama has said he created a video attacking Hillary Clinton.
Hillary Clinton is shown as a dictator
But Philip de Vellis said he produced the ad outside work hours and that neither his company nor the Obama campaign was aware of his role in it.
"I did it. And I'm proud of it," Mr de Vellis wrote in a blog for news and opinion site HuffingtonPost.com.
He said he had resigned so as not to harm the firm, "even by implication".
Thomas Gensemer, the managing director of Blue State Digital, where Mr de Vellis worked as a strategist, said he had been dismissed.
The work had been done without the knowledge of the management, he said.
The Obama campaign also denied any knowledge of the creation of the video.
The video shows Mrs Clinton as a "Big Brother" figure and urges support for Mr Obama.
It was posted on the YouTube video website in early March and has attracted 1.5 million viewings so far.
The move prompted the posting of an anti-Obama video on the same site.
The anti-Clinton video, Vote Different, shows virtual clones with shaved heads marching with military precision while she praises the start of her presidential bid, which she has dubbed a "Conversation with America".
As Mrs Clinton speaks, armed guards chase a woman through the crowd as she throws a sledgehammer shattering the screen.
"On 14 January, the Democratic Primary will begin. And you'll see why 2008 won't be like '1984'," says the video, which is based on an advertisement by Apple Computer attacking its rival IBM.
The anti-Obama video, Barack 1984, posted on YouTube a few weeks later, is based on the same advertisement.
The identity of the creator of the anti-Clinton video remained a mystery for some weeks.
Mr de Vellis said the ad had been a private initiative "because I wanted to express my feelings about the Democratic primary, and because I wanted to show that an individual citizen can affect the process".
"The underlying point was that the old political machine no longer holds all the power," he said.
Ten months before the presidential primary campaign opens, some observers see the rival videos as the first salvo in an internet campaign in support of rival Democratic presidential candidates.
"This is the opening round," Carol Darr, director of the Institute for Politics, Democracy and the Internet at George Washington University, said.
"The internet is going to be the main event ... anything that gets the attention of millions of eyeballs - and particularly millions of eyeballs of people who at this early stage are watching - matters," she said.
The stir caused by the ad is evidence of the rapidly expanding political influence of sites like YouTube.
Unlike newspaper or broadcast adverts in the US, the sources of internet "attack ads" can remain anonymous.