LAS VEGAS — A federal judge refused yesterday to shut down nine casino-based sites for tomorrow's caucuses, delivering a victory to Sen. Barack Obama in what has become an increasingly bitter Democratic contest here.

U.S. District Judge James Mahan rejected the argument that conducting some of the caucuses in casinos would give Obama an unfair advantage because he has been endorsed by the state culinary workers' union, which represents thousands of casino workers. Siding with lawyers for the Democratic National Committee, the judge said that federal law "recognizes the parties have the right to determine how to apportion delegates."

The DNC, working with Nevada Democratic officials, approved the at-large precincts last summer to accommodate people who will be working when the hour-long caucuses are held at noon tomorrow. Any shift worker employed within a 2 1/2-mile radius of the Strip can attend, but those sites are expected to be dominated by culinary workers, many of whom are Latino. State party officials estimate that casino caucusgoers could account for up to 10 percent of the total turnout.

The lawsuit, brought by a state teachers' union that has endorsed Sen. Hillary Rodham Clinton, has led to a nasty dispute between her campaign and Obama's, escalating tensions just days after the two tried to defuse a racially tinged dispute.

The Clinton campaign has denied playing any formal role in the lawsuit but has been critical of the casino caucuses.

Unite Here, which represents 440,000 textile and hotel workers and is the parent union of the culinary workers' organization, began running a Spanish-language radio spot in which, according to a translation provided by the Clinton campaign, a narrator says: "Hillary Clinton does not respect our people. Hillary Clinton supporters want to prevent people from voting in their workplace on Saturday. This is unforgivable. Hillary Clinton is shameless."

The ad also touts Obama's defense of workers' rights.

Chris Bohner, speaking for the culinary workers' union, said its leaders were offended by the lawsuit. "We can't think of a more negative and disgraceful political tactic than publicly supporting a lawsuit that would disenfranchise thousands of workers — bellhops, dishwashers, housekeepers, recent immigrants who've just become American citizens," Bohner said. "The ad intends to point out the fact that the Clinton campaign is supporting this lawsuit, which is entirely appropriate, and we completely stand by the ad. We've waited for the Clinton campaign to denounce the lawsuit and they didn't."

Clinton aides said Obama, who had criticized ads from outside groups affiliated with former Sen. John Edwards when they ran in the Iowa caucuses, had been "strangely silent now that a labor union is attacking the Clinton campaign.

"In Iowa, Senator Obama and his campaign went out of his way to attack labor unions for independently promoting other candidates," said Phil Singer, a Clinton campaign spokes-man. "But in Nevada, he's looking the other way as they falsely attack his opponents."

Bill Burton, Obama's campaign spokesman, said in an e-mail that the Clinton objections "take some chutzpah."

"The fact is their camp clearly would like to have workers' voices silenced and they need to live with that unfortunate position," Burton wrote.

A newspaper's poll set to be released today is expected to show Clinton with a nine-point lead in the state.

Republicans will also caucus tomorrow, although only former Massachusetts Gov. Mitt Romney is making an aggressive play in the state.