By Bill Mears, CNN Supreme Court Producer

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."

The appeal was a classic federalism dispute, on the often delicate line between conflict and cooperation between state and federal governments over enforcing voting procedures. During last year's election, there were numerous court challenges to state voter identification laws at the polls. The current fight has produced a range of states, lawmakers and advocacy groups on both sides on the gateway issue of registration. The Obama Justice Department opposed the Arizona law, which went beyond what other states have done to ensure integrity in the registration system.