Mr. Taylor, of Unite Here, said he had not ruled out backing either Ms. Warren or Mr. Sanders.

The union has a history of effective turnout. In 2008, Unite Here endorsed Barack Obama soon after he lost in New Hampshire, providing the campaign with a key boost and helping him amass more delegates in Nevada, though he lost the popular vote to Ms. Clinton.

During the 2018 midterms, about 250 culinary workers took leave from their jobs to focus on elections, registering 10,000 new voters and knocking on 200,000 doors. Democrats won several key races, flipping a Senate seat and capturing the governor’s office for the first time since 1999.

In dozens of interviews with rank-and-file members, they voiced a broad range of opinions about the candidates: Many said they were undecided; some favored Mr. Sanders for his populist appeals and immigration proposals, others Mr. Biden because they liked his health care plans and because they believe he has the best chance to win in the general election. Ms. Warren won over some women here Monday, though several said they were disappointed she did not answer questions more directly, sticking instead to more familiar talking points that are part of her stump speech.

“Mr. Biden inspires hope for me, for the Latino community,” said Roselia Mendoza, a 49-year-old housekeeper at the Wynn hotel and casino, one of the toniest on the strip. “He both will have a kind heart and give us more rights. What we really need is hope.”

But Mr. Biden faced some skepticism for his record on immigration, with one member asking him how he would approach deportations differently than he did under the Obama administration, which deported roughly three million people.

The former vice president began by citing the “incredible pain” caused by those deportations and assured the crowd that those without criminal records would not be deported in his administration, as part of an immigration plan he announced Wednesday.

Union members repeatedly raised immigration and health care as the most important issues. Even those who were pleased with their own health plan spoke about friends and family members who have gone without needed medication or gone bankrupt to pay for hospital bills.