LAS VEGAS — When it comes to labor powerhouses in Nevada, few organizations quite match the Culinary Workers Union: 57,000 strong, more than 50 percent Latino, with an 80-year history of labor advocacy on the Strip and a record of turning out its members in political campaigns.

But to the increasing distress of the two Democratic presidential contenders, Senator Bernie Sanders of Vermont and Hillary Clinton, the union has decided to sit out the Democratic presidential caucuses here on Saturday, setting off a free-for-all for its members and adding to the increasingly tense and unsettled political atmosphere here. Both campaigns have pressed union leaders to change their minds and step in at the last minute, though chances of that appear remote.

Union leaders said they were staying on the sidelines because the demands of mobilizing behind either Mr. Sanders or Mrs. Clinton would divert resources, distract members and potentially polarize the union just as they are entering critical contract negotiations. The Culinary Workers will instead focus its resources on the general election, in which Nevada is almost certain to be pivotal.

But the decision to stay out also appears to be, at least to some extent, a legacy of the bitter Democratic caucus battles of 2008, when the union endorsed Barack Obama in the final weeks of the campaign. Mrs. Clinton’s supporters took the Culinary Workers to court to challenge its voting procedures, while an angry Bill Clinton went casino to casino, urging workers to defy their union and support his wife.