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 signed a letter Thursday pledging to defend changes to their state constitutions approved by voters on Nov. 2.

"These state laws protect long-existing federal rights and we will vigorously defend any legal attack upon them," the attorneys general said in a letter to the National Labor Relations Board.

Earlier this month, the labor board threatened to sue the states, saying the constitutional amendments conflict with federal law.

NLRB spokeswoman Nancy Cleeland said the board is reviewing the letter.

Business groups sought the state measures because they fear Congress could pass a new "card check" law requiring every employer to recognize a union if a majority of workers simply sign cards instead of holding secret ballot elections. Democrats failed to muster enough votes in the Senate last year to pass such a measure.

Unions have pushed the card check process in hopes of making it easier to form unions when businesses actively resist organizing efforts.

The labor board's acting general counsel, Lafe Solomon, claims the state amendments already clash with federal law that gives employers the option to voluntarily recognize a union through the card check process.

The attorneys general call the government's legal threat misguided, claiming the amendments do not interfere with voluntary card check. They also ask the labor board to respect the will of voters in their states. The amendments passed as ballot initiatives with 61 percent approval in Arizona, 86 percent in South Carolina, 79 percent in South Dakota and 60 percent in Utah.

"South Carolina voters spoke overwhelmingly to ensure that their ballot votes are kept between them and their maker _ not to be influenced by union bosses," said South Carolina Attorney General Alan Wilson.

Catherine Fisk, a labor law professor at the University of California at Irvine, gives the states little chance of success.

"In every area where federal labor law extensively regulates the field of labor management relations, the Supreme Court has concluded that federal law pre-empts state regulation," Fisk said.

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Associated Press writer Jim Davenport in Columbia, S.C., contributed to this report.