The ruling comes after an election year in which judges blocked a number of state laws that would have voters show proof of identity before they cast ballots. The judges said these laws, if rigidly enforced on election day, could turn away eligible and registered voters.

The Arizona law, part of an initiative from 2004, seemed to stand on stronger ground because it set standards for registering to vote. And only U.S. citizens are eligible.

But a lawsuit brought by the Mexican American Legal Defense and Educational Fund revealed that registration applications from 31,000 prospective voters were turned down by Arizona because they did not have driver's licenses or U.S. passports. Ninety percent of them were citizens born in the United States.

In Monday's decision, Justice Antonin Scalia said the Constitution gives Congress broad power to regulate federal elections. And the Motor Voter Act says states must "accept and use" the simplified form that allows applicants to submit basic information on who they are and where they live. They also sign a sworn oath that they are U.S. citizens who are eligible to vote.

Scalia insists on closely following the words of the law, and in this instance, the words of the federal measure were clear in their meaning, he said. As written, the federal law "forbids states to demand that an applicant submit additional information beyond that required by the federal form," he said.

Voting rights advocates hailed the ruling as an important victory.

"For two decades, the motor voter law has made it dramatically easier for Americans to register to vote by a standard, uniform voter registration form nationwide," said Laughlin McDonald, a veteran voting rights lawyer for the American Civil Liberties Union. The law got its name because it was designed to allow new residents in a state to register to vote at the same time they obtained driver's licenses.

But Scalia emphasized that states could still prevent noncitizens from getting onto the voter rolls. State officials may check other information and databases to see whether a new registrant is indeed a U.S. citizen.

"Not every submitted federal form will result in registration," Scalia wrote. Nothing in the federal law "precludes states from denying registration based on information in their possession establishing the applicant's ineligibility."

Arizona officials indicated they would take advantage of a provision in the ruling allowing the state to seek to continue requiring the added documentation while it appeals to a federal agency.

"The next step is going to the EAC," Arizona Atty. Gen. Tom Horne said, referring to the federal Elections Assistance Commission, the agency that developed the federal form. After that appeal, the state can go back to the courts, where it believes it can win and keep the requirement of added documentation on citizenship, Horne said.

The Arizona case was one of two voting rights disputes before the high court. The other, and far more significant case, involves the fate of Section 5 of the Voting Rights Act of 1965, which requires much of the South and a few other states such as Arizona to seek advance clearance from Washington before they change their laws on voting and elections. A ruling on that case is expected by the end of the month.

Scalia's opinion could give hope to advocates on both sides of that case. On the one hand, he emphasized that Congress had the constitutional power to regulate elections. On the other hand, he said states played the lead role in deciding the qualifications for voting.

The Arizona provision was adopted by voters in 2004 as part of an initiative intended to "combat voter fraud." The Supreme Court cleared it to go into effect in 2006, but it has been tied up in litigation since. Last year, the court struck down much of an Arizona law that made it a state crime for immigrants to be in the state illegally.

Chief Justice John G. Roberts Jr. and Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan agreed with Scalia in the case of Arizona vs. Inter-Tribal Council of Arizona. Justice Anthony M. Kennedy concurred in a separate opinion.

Dissenting were Justices Clarence Thomas and Samuel A. Alito Jr. They said federal law and the Constitution leave states free to check the citizenship of would-be voters.

In reaction to the ruling, Sen. Ted Cruz (R-Texas) said he would seek to amend the pending immigration bill to "close this hole" in the law and permit states to require identification from those who wish to register to vote.