LAS VEGAS — Maria Mendoza, a Mexican-born hotel housekeeper, was out of work and worried about supporting her two daughters when she said she attended a job fair about six years ago and glimpsed her future in a gold-windowed, 64-story hotel just off the Las Vegas Strip. Its name alone sold her.

“I thought, ‘Wow, Trump,’” Ms. Mendoza, 52, said. “I felt proud. I was working for Trump. I thought he was a good businessman. I thought that by working for him, my life was going to change.”

Today, she and a vocal group of fellow workers at the Trump International Hotel have gone to battle with Donald J. Trump, the Manhattan businessman and Republican presidential nominee, waging a raucous and rowdy effort to form a union that has run parallel to their boss’s campaign for the White House.

About 500 employees who clean the hotel and prepare and deliver food and drinks to guests narrowly approved unionizing in a vote in December. But the hotel has been challenging the vote, contending that it “was anything but free and fair,” and appealing a regional National Labor Relations Board decision in March that certified the union.