The National Labor Relations Board is threatening to sue four states over constitutional amendments that guarantee workers can use secret ballots in union elections.

This action comes as organized labor is attempting to make bigger inroads into healthcare entities. Long-term care operators have been one of their most fertile areas in recent times.

The four states currently with secret ballot amendments on the books are Arizona, South Carolina, South Dakota and Utah.

According to the NLRB, these amendments clash with federal law, which grants employers the option of recognizing a union as long as a majority of workers sign cards in support of it. Under the supremacy clause of the U.S. Constitution, federal law trumps state law when the two conflict, the NLRB asserts.

Anti-union groups and business lobbies have pushed for the secret ballot amendments to the states’ constitutions. These groups, including Americans for Prosperity, insist that the state amendments are there to protect workers against undue influence from unions seeking to coerce individuals into organizing.

On Jan. 27, Sen. Jim DeMint (R-SC) reintroduced the Secret Ballot Protection Act, which would amend the National Labor Relations Act to eliminate the “card-check” and replace it with a federally protected secret ballot process.

After being named a top priority of labor, the Employee Free Choice Act, which would create federal-level protections for workers looking to unionize by card-check, has languished in Congress.

Pro-union groups, however, say secret ballot initiatives restrict workers’ rights. In a letter supporting the NLRB’s threatened legal action, the group American Rights at Work said secret ballot initiatives are “a telling indication of just how far corporate interests will go to maintain a status quo that protects exploitative employers, no matter the cost to ordinary Americans.”