Updated at 1:45 p.m. ET
The Supreme Court ruled Monday that states cannot on their own require would-be voters to prove they are U.S. citizens before using a federal registration system designed to make signing up easier.
The justices voted 7-2 to throw out Arizona's voter-approved 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.
Federal law "precludes Arizona from requiring a federal form applicant to submit information beyond that required by the form itself," Justice Antonin Scalia wrote for the court's majority.
The court was considering the legality of Arizona's requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under the federal "motor voter" registration law. The 9th U.S. Circuit Court of Appeals said that the National Voter Registration Act of 1993, which doesn't require such documentation, trumps Arizona's Proposition 200 passed in 2004.
Arizona appealed that decision to the Supreme Court.
"Today's decision sends a strong message that states cannot block their citizens from registering to vote by superimposing burdensome paperwork requirements on top of federal law," said Nina Perales, vice president of litigation for the Mexican American Legal Defense and Educational Fund and lead counsel for the voters who challenged Proposition 200.
"The Supreme Court has affirmed that all U.S. citizens have the right to register to vote using the national postcard, regardless of the state in which they live," she said.
But Tom Caso, a professor at Chapman University School of Law in California, said the decision "opened the door" to noncitizen voting. "The court's decision ignores the clear dictates of the Constitution in favor of bureaucratic red tape," Caso said.
Kathy McKee, who led the push to get the proposition on the ballot in Arizona, said the ruling makes it harder to combat voter fraud, including fraud carried out by people who don't have permission to be in the country. "To even suggest that the honor system works, really?" McKee said. "You have to prove who you are just to use your charge card now."
The case focuses on Arizona, which has tangled frequently with the federal government over immigration issues involving the Mexican border. But it has broader implications because four other states - Alabama, Georgia, Kansas and Tennessee - have similar requirements, and 12 other states are contemplating such legislation.
Justices Clarence Thomas and Samuel Alito dissented from the court's ruling.
The Constitution "authorizes states to determine the qualifications of voters in federal elections, which necessarily includes the related power to determine whether those qualifications are satisfied," Thomas said in his dissent.
Opponents of Arizona's law see it as an attack on vulnerable voter groups such as minorities, immigrants and the elderly. They say they've counted more than 31,000 potentially legal voters in Arizona who easily could have registered before Proposition 200 but were blocked initially by the law in the 20 months after it passed in 2004. They say about 20 percent of those thwarted were Latino.