The network is mostly black-based and was founded by the Rev Al Sharpton, who was also on the stage. The joke brought the house down. Senator Obama is laying siege to the black support in New York state that - until recently - Mrs Clinton could take for granted.

It is a development that may cost the former first lady dearly. Roughly one in five Democrats who vote on primary day in New York next February will be black and any suggestion of an erosion of their support will force her to spend time and money where she thought she would not need to.

Signs that black elected officials in the state are drifting into the Obama camp come as his campaign continues to build momentum. In the first quarter of this year he matched Mrs Clinton in raising funds, while a new poll released on Monday showed him dead-even with her nationally for the first time.

Loyalty among black leaders in the state can be traced back to the high popularity of President Bill Clinton in the African American community, but the sudden rise of Mr Obama appears to be stirring a degree of ethnic pride and excitement that is straining those old assumptions.

"I would have supported Hillary if it were not for Barack Obama," Adam Clayton Powell IV of the state assembly told The New York Times. "He can identify with my African American community in a way that no other candidate can." Jeffrion Aubry, also an assemblyman, concurred: "His presence as a legitimate politician at a national level brings a certain pride. It makes you have to make a choice."

Dismissing reports of a mass desertion, however, is Congressman Charles Rangel, the most senior black elected official in New York state, who is working hard to bolster Senator Clinton. "I don't know Obama supporters in New York," he said.

The ability of Senator Obama to draw black voters away from his rivals, including the former senator John Edwards, could prove crucial to his campaign in other states too, for instance in South Carolina where African Americans are likely to determine the outcome on its primary day.

The state university in Orangeburg, South Carolina, is staging a first debate between all the Democrat ic runners tomorrow evening.

Senator Obama, meanwhile, told the National Action Network last week that he did not expect people to choose candidates only on ethnic lines. "I don't care whether they are white or black or they are male or female - if there is somebody who has been more on the forefront on behalf of the issues you care about and has more concrete accomplishments on behalf of the things you're concerned about, I'm happy to see you endorse them. But I am absolutely confident you will not find that," he said.

In the wake of the dismissal 10 days ago of the radio shock-jock Don Imus for insulting a predominantly black women's college basketball team with a racial slur, Senator Obama warned that blacks themselves showed too little respect for their own community.

"I've heard those words around the kitchen tables," he said. "All of us have been complicit in diminishing ourselves, and engaging in the kind of self- hatred that keeps our young men and young women down. That's something we have to talk about in this election."