Four states are vowing to fight the federal government in a bid to preserve state measures that guarantee workers the right to secret ballots in union elections.
Attorneys general from Arizona, South Carolina, South Dakota and Utah are pledging in a joint letter to "vigorously defend" changes to their state constitutions approved by voters on Nov. 2.
The National Labor Relations Board has threatened to sue the states, saying the constitutional amendments conflict with current federal law.
Business groups sought the constitutional amendments because they fear Congress could pass a new law requiring every employer to recognize a union if a majority of workers simply sign cards.
The attorneys general call the legal threat misguided. They say the federal agency should respect the will of voters in their states.