Federal labor regulators have sued the state of Arizona over a state constitutional amendment that limits how workers can choose whether they want union representation.
The amendment passed by voters last year requires workers to hold secret ballot elections before a company can be unionized.
The National Labor Relations Board says the amendment interferes with federal law. Current law gives employers the choice of recognizing a union if a majority of workers sign cards, a process known as card check.
Arizona's attorney general has vowed to fight the lawsuit. Business groups that pushed the ballot measure say they fear Congress will require all businesses to recognize card check.
The NLRB says it plans to sue South Dakota in the coming weeks over a similar constitutional amendment.