WASHINGTON –  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 Antonia 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.

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.

Barbara Arnwine, president and executive director of the Lawyers' Committee for Civil Rights Under Law, called the decision a victory. "The court has reaffirmed the essential American right to register to vote for federal election without the burdens of state voter suppression measures," she said.

But Arizona officials say they should be able to pass laws to stop illegal immigrants and other noncitizens from getting on their voting rolls. The Arizona voting law was part of a package that also denied some government benefits to illegal immigrants and required Arizonans to show identification before voting.

The federal "motor voter" law, enacted in 1993 to expand voter registration, requires states to offer voter registration when a resident applies for a driver's license or certain benefits. Another provision of that law -- the one at issue before the court -- requires states to allow would-be voters to fill out mail-in registration cards and swear they are citizens under penalty of perjury, but it doesn't require them to show proof. Under Proposition 200, Arizona officials require an Arizona driver's license issued after 1996, a U.S. birth certificate, a passport or other similar document, or the state will reject the federal registration application form.

While the court was clear in stating that states cannot add additional identification requirements to the federal forms on their own, it was also clear that the same actions can be taken by state governments if they get the approval of the federal government and the federal courts.

Arizona can ask the federal government to include the extra documents as a state-specific requirement, Scalia said, and take any decision made by the government on that request back to court.  Other states have already done so, Scalia said.

The Election Assistance Commission "recently approved a state-specific instruction for Louisiana requiring applicants who lack a Louisiana driver's license, ID card or Social Security number to attach additional documentation to the completed federal form," Scalia said.

The case is 12-71, Arizona v. Inter Tribal Council of Arizona, Inc.