Washington (CNN) -- The Supreme Court on Monday tossed out a provision in Arizona's voter registration law that required proof of citizenship.

The 7-2 majority said the state's voter-approved Proposition 200 interfered with federal law designed to make voter registration easier. The state called the provision a "sensible precaution" to prevent voter fraud. Civil rights group countered that it added an unconstitutional and burdensome layer of paperwork for tens of thousands of citizens.

Justice Antonin Scalia said the National Voter Registration Act of 1993 "forbids states to demand an applicant submit additional information beyond that required by the federal form." But in a nod to state authority, Scalia said the federal law "does not prevent states from denying registration based on any information in their possession establishing the applicant's eligibility."