The U.S. Supreme Court Monday struck down Arizona's voter-registration identification law, saying states cannot require would-be voters to prove they are U.S. citizens before using a federal registration system.

The justices voted 7-2 to throw out Arizona’s 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.

The 9th U.S. Circuit Court of Appeals said that the 1993 National Voter Registration Act, which doesn’t require such documentation, trumps Arizona’s Proposition 200, which was passed by the voters in 2004.

The proposition was championed by Republican Gov. Jan Brewer, who then was secretary of state. She and other proponents said it was designed to prevent voter fraud.

That is despite a lack of evidence of widespread or even moderate levels of fraud. Current Arizona Secretary of State Ken Bennett reported 15 instances of potential voter fraud in the 2008 and 2010 elections combined. Those involved people who voted in Arizona and in another state, rather than people who voted despite not being citizens.

Those 15 cases were out of an estimated 2 million Arizonans casting ballots in 2008 and an estimated 1.5 million in 2010.

Minorities said Proposition 200 was designed to suppress their voting rights and those of the elderly, who could find it difficult to prove citizenship.

Read the Supreme Court's ruling on Arizona's voter law: View at Google Docs | Download File