WASHINGTON — Arizona may not require documentary proof of citizenship from prospective voters, the Supreme Court ruled in a 7-to-2 decision on Monday.

Justice Antonin Scalia, writing for the majority in Arizona v. Inter Tribal Council of Arizona, No. 12-71, said a federal law requiring states to “accept and use” a federal form displaced an Arizona law.

The federal law, the National Voter Registration Act of 1993, allows voters to register using a federal form that asks, “Are you a citizen of the United States?” Prospective voters must check a box for yes or no, and they must sign the form, swearing under the penalty of perjury that they are citizens.

The state law, by contrast, required prospective voters to prove that they were citizens by providing copies of or information concerning various documents, including birth certificates, passports, naturalization papers or Arizona driver’s licenses, which are available only to people who are in the state lawfully.

A divided 10-judge panel of the United States Court of Appeals for the Ninth Circuit had ruled that the two sets of requirements “do not operate harmoniously” and “are seriously out of tune with each other in several ways.”

The decision from that panel effectively affirmed a 2010 ruling from a three-judge panel that included Justice Sandra Day O’Connor, who retired from the Supreme Court in 2006 but occasionally acts as a visiting appeals court judge. She joined the majority in ruling that the state law was inconsistent with the federal one and so could not survive.

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