A booth in 2010 Reuters

The Supreme Court on Monday held that it’s illegal for states to require would-be voters to prove their U.S. citizenship before using a registration form under the federal “motor voter” law, by voting 7-2 to strike down an Arizona provision seeking to do so.

The justices decided the federal law takes precedence over Arizona's law. The decision came a year after the court rejected much of the state’s immigration law. Arizona officials had said the law is necessary to prevent people who aren’t Americans from voting.

Arizona officials had appealed a 9th U.S. Circuit Court of Appeals decision that said the state’s Proposition 200, which was passed in 2004, didn’t undo the 1993 National Voter Registration Act.

The federal “motor voter” law requires states to provide voter registration when an individual applied for a driver’s license of certain type of benefits. The law also requires states to allow would-be voters to complete mail-in registration cards and swear they are citizens. However, the provision doesn’t require showing proof. This is the part that is at issue here as Arizona is demanding that proof by way of a driver’s license, birth certificate or other form of identification.