“This was a fraud on the system,” said Art Read, a lawyer with Friends of Farmworkers, one of the groups that filed a complaint last year about the union with the National Labor Relations Board.

The Association of Mobile Entertainment Workers is a new player in a long legal battle between labor advocates and the carnival industry over the pay and working conditions of migrant laborers. In separate lawsuits, migrant laborers from Mexico have charged that carnival companies forced them to work long hours, often at little or no additional pay, and threatened to send them home if they complained.

One laborer, Enrique Vasquez-Alejo, who worked at fairs in Maryland and Virginia, testified in a lawsuit that his bosses told him, “If you don’t like it, get your things together and go back to Mexico.”

The union case is unfolding in the face of a wider national debate about a higher federal minimum wage, a standard from which carnival workers are exempted. Also, in April, the Labor Department proposed new rules to better protect seasonal employees, like those at carnivals, who enter the United States on temporary visas, rules that the industry has filed suits to block.