The Supreme Court ruled Monday that states cannot require would-be voters to prove they are U.S. citizens before using a federal registration system designed to make signing up easier.

The justices voted 7-2 to throw out Arizona's voter-approved requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under the federal "Motor Voter" voter registration law.

Federal law "precludes Arizona from requiring a federal form applicant to submit information beyond that required by the form itself," Justice Antonia Scalia wrote for the court's majority.

The court was considering the legality of Arizona's requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under the federal "motor voter" registration law. The 9th U.S. Circuit Court of Appeals said that the National Voter Registration Act of 1993, which doesn't require such documentation, trumps Arizona's Proposition 200 passed in 2004.

Arizona appealed that decision to the Supreme Court.

The case focuses on Arizona, which has tangled frequently with the federal government over immigration issues involving the Mexican border. But it has broader implications because four other states — Alabama, Georgia, Kansas and Tennessee — have similar requirements, and 12 other states are contemplating such legislation.