A short drive from the Las Vegas Strip but a world away from its luxury hotels, roulette wheels and scantily clad cocktail waitresses, Barack Obama stood in a stuffy union hall before a crowd of dishwashers, casino cleaners and bartenders.

“Sí, se puede!” — “Yes, we can!” — the Mexicans, Guatemalans and Puerto Ricans yelled at the candidate before them, a man whom their Culinary Workers Union, the biggest and most powerful in Nevada, had just endorsed before the Silver State's Democratic caucuses on Saturday.

Hours earlier Hillary Clinton was in a heavily Hispanic neighbourhood in eastern Las Vegas, home to many of the union members that Mr Obama is wooing. She talked to the families of the kitchen porters, bellhops and maids who service The Strip's 26 hotel-casinos, empires that generate $6.5 billion (£3.3 billion) a year for Sin City and the giant corporations that run it. She ate salsa, and equated America's problems to guacamole and chips.

After their contests in freezing — and overwhelmingly white — Iowa and New Hampshire, Mrs Clinton and Mr Obama are suddenly locked in a very different battle: for the votes of the Hispanic workers of Las Vegas, a fight that will be decided largely by the machine politics of The Strip, perhaps the world's most heavily unionised four-mile boulevard.

This latest chapter in their extraordinarily close contest, in the heart of the American West, is unprecedented. Until now, the Nevada caucuses came so late in the nominating season that they were largely irrelevant. This year, however, the Democratic party catapulted the state up the calendar, to give its booming Hispanic population their first real say in choosing a White House candidate. Of Nevada's 2.5 million people — 35 per cent of whom are Hispanic — two million live in the greater Las Vegas area. The key to victory will be getting voters to caucus sites here, a test of organisational muscle that is pitting union against union in a fight for the city's Latino workers.

After Mr Obama's win in Iowa and Mrs Clinton's stunning comeback in New Hampshire, victory on Saturday will give one of them a real boost before their next showdown, in South Carolina on January 26. It will also provide bragging rights before the contests in California, New York and New Mexico, three of the biggest prizes on February 5 — “Super Tuesday” — when the Hispanic vote will again be crucial. Both candidates are broadcasting Spanish television and radio advertisements in Nevada. Posters in Obama offices declare “Tell Mama Vote Obama”. The candidates have also developed an appetite for tacos, after weeks of extolling the virtues of Iowa corn.

The culinary union's decision to back Mr Obama is a huge boost for a candidate who in a televised debate on Tuesday night was confronted with internet rumours that he is secretly a Muslim who follows the Koran. “I am a Christian. I have been sworn in by the Bible,” he said. The trade organisation's 60,000-strong, membership dominates the workforce on The Strip. It is so powerful that it has managed to get caucus sites set up in nine casinos, including the Mirage, the Luxor and Caesar’s Palace, so that its members do not have to leave work to vote.

Mrs Clinton held a double-digit lead in the autumn. A new poll puts Mr Obama two points ahead and he received a further fillip yesterday with the endorsement of the state's biggest newspaper. John Edwards is also in contention. A Nevada win for him would revitalise his campaign.

The union's backing is important for Mr Obama as he tries to woo Hispanics, a voting bloc traditionally hostile to African-Americans. José Maldonado, an electrician holding a “Latinos for Obama” sign outside the debate, said: “I don't look at race any more. We need change. He has given me hope.”

Yet Mrs Clinton, who, like her husband, Bill, has campaigned heavily in Las Vegas this week, is not ready to cede the culinary union vote to Mr Obama. She has been endorsed by eight smaller unions, much of the state's Democratic Establishment and by Richard Chávez, brother of the late César Chávez, the iconic Mexican-American labour leader.

In east Las Vegas, crowds seemed thrilled to see her. She was chaperoned by Ruben Kihuen, a local politician, who whispered in her ear: “The Hispanic workers are loyal to Culinary, but they will vote for you.” At one point, an Hispanic man holding a “Culinary Workers for Hillary” placard said that she would win.

Whether the union leadership can hold its members in line against the political muscle of the Clintons is something on which Mr Obama is gambling heavily.

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