LOS ANGELES (Reuters) - Nevada Democratic Party officials said on Friday they were canceling a presidential debate co-sponsored by Fox News, following a joke chairman Roger Ailes made about Democratic candidate Barack Obama.

Roger Ailes, chairman and CEO of Fox News and Fox Television Stations, attends a panel discussion at the Television Critics Association summer press tour in Pasadena, California July 24, 2006. Nevada Democratic Party officials said on Friday they were canceling a presidential debate co-sponsored by Fox News, following a joke chairman Roger Ailes made about Democratic candidate Barack Obama. REUTERS/Fred Prouser

In a letter sent to Fox, Nevada State Democratic Party Chairman Tom Collins and Senate Majority Leader Harry Reid said Ailes “went too far” with comments made the night before.

The letter makes no reference to a crusade by the liberal activist group MoveOn.org to boycott Fox, which it calls a “right-wing mouthpiece.” Democratic presidential candidate John Edwards dropped out of the debate on Thursday, citing in part Fox’s participation.

The letter also does not specify which comments by Ailes lead to the decision, but a Democratic source told Reuters it was a joke Ailes made about Obama and President Bush during a speech on Thursday night.

“We cannot, as good Democrats, put our party in a position to defend such comments,” Collins and Reid said in the letter. “We take no pleasure in this, but it the only course of action.”

Fox News Vice President David Rhodes responded with a written statement criticizing the Democrats for caving in to MoveOn.org.

“News organizations will want to think twice before getting involved in the Nevada Democratic Caucus, which appears to be controlled by radical fringe out-of-state interest groups, not the Democratic Party,” David Rhodes said in the statement.

“In the past, MoveOn.org has said they ‘own’ the Democratic Party. While most Democrats don’t agree with that, its clearly the case in Nevada,” he said.

The joke by Ailes came during a speech to the Radio and Television News Directors Foundation First Amendment Dinner on Thursday night and -- while playing on similarity between Obama’s name and Osama Bin Laden -- appears to be directed more at Bush than the senator.

“It’s true that Barack Obama is on the move,” Ailes said during the speech. “I don’t know if it’s true that President Bush called Musharraf and said ‘Why can’t we catch this guy?’”

During his remarks, Ailes also took indirect swipes at both MoveOn.org and Edwards, saying pressure groups were now urging candidates to “only appear on those networks and venues that give them favorable coverage.”

Though he didn’t refer to Edwards by name, Ailes said “any candidate of either party who cannot answer direct, simple, even tough questions from any journalist runs a real risk of losing the voters.”