The National Labor Relations Board announced on Friday that it planned to sue Arizona, South Carolina, South Dakota and Utah in an effort to invalidate recently approved state constitutional amendments that prohibit private sector workers from choosing a union through a process known as card check.

The labor board asserts that the amendments conflict with federal laws and are pre-empted by those laws.

The state amendments were promoted by various conservative groups concerned that Congressional Democrats and President Obama would enact legislation allowing unions to insist on using card check, in which an employer recognizes a union as soon as a majority of workers sign pro-union cards. That method makes it possible for employees to unionize without elections. But Congressional Republicans blocked such legislation.

Under current law, employers can insist that secret ballots be used when unions are trying to organize private sector employees. But unions had hoped that the card check bill would make it easier to unionize workers because card check lets them gather majority support, often without giving employers the opportunity to campaign against the union.