PHOENIX - Claiming pre-emption by federal law, the National Labor Relations Board filed suit Friday to void a voter-approved state constitutional amendment saying unions can be formed in Arizona only by secret ballot.

The lawsuit, filed in federal court, says the measure mandating elections by affected workers would eliminate the option that now exists for employers to recognize a union without having an election. That, according to attorneys for the NLRB, violates federal law.

The lawyers say in cases of such a conflict, federal statute prevails.

Arizona Attorney General Tom Horne said nothing in the state constitutional amendment conflicts with the federal law. Instead, he argued, it merely says that if a company chooses not to voluntarily allow a union to be formed, then only a secret vote of the employees can force the issue.

The measure, Proposition 113 on the November ballot, was pushed by business interests as a pre-emptive strike against anticipated federal legislation that would have allowed union organizers to demand employer recognition once they had cards with the signatures of at least half the workers. That procedure, known as "card check," would have been an alternative to a process where workers petition for an election by secret ballot.

Clint Bolick of the Goldwater Institute, who helped push the issue, said employers fear a card check system could result in coercion by union organizers, as they would know who does and doesn't support organizing.

As it turned out, the federal legislation faltered. And the chance of approval now, with the House in Republican hands, is virtually nil.

But Arizona voters, by a 61-39 margin, approved the state constitutional change.

Nancy Cleeland, director of public affairs for the NLRB, said even with the failure of the federal legislation, the amendment violates existing labor laws that already allow a union to be formed without an election simply by having cards with the consent of half the workers.

Horne said that's not the case, saying the federal law and Arizona constitutional provision can work together.

But Horne also said he would defend the Arizona amendment even if it would preclude the alternate method of forming a union.

"I would ask you to ask your readers to consider how they would feel if their secret ballot was taken away from them when they elect their congressmen?" he said. "And then consider how workers would feel in what is really the same situation - even more so, because they have more at stake."