The National Labor Relations Board is taking aim at newly adopted constitutional amendments that bar a federally recognized path for workers to unionize: signing cards authorizing a union.

The NLRB has threatened to file lawsuits against Arizona, South Carolina, South Dakota and Utah to block the amendments. The board contends the measures conflict with the National Labor Relations Act and are pre-empted by the U.S. Constitution.

The amendments all were approved on Nov. 2. South Dakota’s took effect on that date; Utah’s took effect on Jan. 1. The North Dakota and South Carolina measures are to take effect soon.

In Jan. 13 letters to the states’ attorneys general, NLRB acting General Counsel Lafe E. Solomon said the amendments allow “only one path to union representation”—a secret-ballot election. He says the NLRB also permits a union to be certified by “voluntary recognition based on other convincing evidence of majority support.”

But the NLRB may face a tough road. Mark Plowden, spokesman for South Carolina Attorney General Alan Wilson, says the state’s voters “spoke overwhelmingly [an 86.2% majority] to ensure that their ballot votes are not to be influenced by union bosses. If that right is challenged, our office is prepared to defend it in court.”