LAS VEGAS  Nevada’s state teachers union and six Las Vegas area residents filed a lawsuit late Friday that could make it harder for many members of the state’s huge hotel workers union to vote in the hotly contested Jan. 19 Democratic caucus in Nevada.

The 13-page lawsuit in federal district court here comes two days after the 60,000-member Culinary Workers Union Local 226 in Nevada endorsed Senator Barack Obama, a blow to Mrs. Clinton. Mr. Obama addressed the Culinary Union at their hall earlier Friday.

The lawsuit argues that the Nevada Democratic Party’s decision, decided late last year, to create at-large precincts inside nine Las Vegas resorts on caucus day violates the state’s election laws and creates a system in which voters at the at-large precincts can elect more delegates than voters at other precincts. The lawsuit employs a complex mathematical formula to show that voters at the other 1,754 precincts would have less influence with their votes.

The at-large precincts are being established because thousands of hotel workers cannot leave work to participate in the midday caucuses in their home precincts. The Nevada State Education Association has said it would not endorse any Democrat, but some of its top officials have endorsed Mrs. Clinton. The association’s deputy executive director, Debbie Cahill, for instance, was a founding member of Senator Clinton’s Nevada Women’s Leadership Council.