Bill Clinton today accused the union backing Barack Obama of illegally blocking its workers from backing his wife in an orchestrated campaign of "voter suppression" in Nevada's Democratic caucuses.

Mr Clinton, who spent 90 minutes at the Mirage Casino in Las Vegas today shaking voters' hands, was told by several workers that their union, which has backed Mr Obama, has told them they could not register to vote unless they supported the Illinois senator.

Mr Obama was endorsed by the Culinary Workers' Union, the biggest and most powerful trade organisation in Nevada. It carries particular clout in the casinos along the Las Vegas Strip.

Mr Clinton said: "They [the workers in the union] were told to sign up by Wednesday, and told it had to be for Obama, and if they had not signed up, that they had to wait until November to wait for Hillary." He told The Times: "It's really interesting that on the eve of Martin Luther King day we are seeing voter suppression.

He added: "There is a lot of voter suppression going on. Are you getting this?"

The Clintons maintain that caucus sites opened in nine casinos so that employees did not have to leave work to vote gave Mr Obama an unfair advantage because most cleaners, kitchen staff and bell-hops belong to the Culinary Union.

In recent days there have been allegations of the unions intimidating its largely Hispanic membership to back Mr Obama. Mrs Clinton's victory came with large Hispanic support and after many of the union's members broke with their leadership to back the former First Lady.

Outside the Mirage's caucus site, a vast ballroom near the casino floor, Mr Clinton met two dozen culinary union members who said that they were refusing to bow to pressure and were supporting his wife. Several told the former President that they did not think they could vote because they were not voting for Mr Obama.

Francesca Santiago, a casino porter at the Mirage, and a Puerto Rican, told The Times: "The union told everyone that they had to vote for Obama. They didn't tell us we could vote for Clinton. They told us if we registered, we had to go for Obama. The unions said we had to registering advance for Obama."

A spokesman for the Culinary Workers' Union told The Times: "We made it very clear to all the members that they could caucus for any candidate they wanted to, but that the leadership supported Obama."

But inside the caucus site, there were chaotic scenes, with Mr Obama's union supporters facing off against Mrs Clinton's backers. Many of the former First Lady's supporters had defied their union to back her.

As voters entered the caucus site, having registered, they were being physically pushed and pulled in both directions by Clinton and Obama supporters to join their camps. The noise was deafening as both sides aggressively screamed their candidate's name. One union member supporting Mr Obama said of the workers who had defected to Mrs Clinton: "We are going to deal with them when we get back to our respective hotels." Mr Clinton told a group of workers that it was illegal to stop members voting for his wife.

"They've been telling me that if they didn't sign up for Obama they couldn't caucus," he told the workers. "They can't do that", he added, and urged them to vote.

Mr Clinton's comments came before news of his wife's victory, and would clearly have been part of the campaign's post-vote spin if the former First Lady has been defeated.

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