Voting rights advocates welcomed the ruling.
"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. "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."
Under Proposition 200 approved in 2004, Arizona officials required an Arizona driver's license issued after 1996, a U.S. birth certificate, a passport or other similar document before the state would approve the federal registration application. It can no longer do that on its own authority.
Less than 5 percent of people registering to vote in Arizona use the federal form, said Matt Roberts, a spokesman for Arizona Secretary of State Ken Bennett. The rest register through the state, meaning they will continue to be asked to provide proof of citizenship when signing up to vote.
But because of the court ruling, people can merely choose the less onerous federal form, which asks people to swear if they are citizens or not, but does not demand proof.
Arizona Attorney General Tom Horne, who argued the case before the Supreme Court, expects the state will ask the Election Assistance Commission to approve the citizenship proof on the federal form and to fight any denial in court the process laid out in Monday's ruling.
"The U.S. Supreme Court has given us a clear path to victory for the people of Arizona, who overwhelmingly approved the state constitutional amendment that was the subject of the legal challenge," Horne said. "Since the U.S. Supreme Court has made it clear that this pathway exists, Arizona should use it. The sanctity of the ballot box is a cherished right for all Americans and it must be protected."
Federal officials deadlocked on Arizona's request in 2005, and the state did not appeal.
In other actions Monday, the court:
Ruled that agreements between the makers of name-brand and generic drugs to delay the generics' availability can be illegal and challenged in court.
Ruled that prosecutors in some instances may use a suspect's silence at an early stage of a criminal investigation against him or her, before the suspect has been arrested or informed of constitutional rights.
Agreed to decide in its next term a new dispute involving race; specifically, whether federal housing law requires proof of intentional discrimination.
The Arizona case is the first of two major voting decisions to be made by the court this month. Justices have yet to say whether a section of the Voting Rights Act of 1965, a law that has helped millions of minorities exercise their right to vote, especially in areas of the Deep South, was still needed, despite several justices voicing deep skepticism during arguments in February.
Arizona has tangled frequently with the federal government over immigration issues involving the Mexican border, health care and more. But the decision on voter registration has broader implications because other states have similar requirements, such as Alabama, Georgia, Kansas and Tennessee, and still others are contemplating such legislation.
Georgia Secretary of State Brian Kemp called the decision disappointing but said he would continue working with state officials to "provide a safe, secure and legal system for voter registration."
Tom Caso, a professor at Chapman University School of Law in California and supporter of the Arizona law, 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. "The notion that the court will not enforce the Constitution unless you first apply to a commission that cannot act because it has no members is mind-boggling."
Currently, the Election Assistance Commission has no active commissioners. The four commissioners are supposed to be nominated by the president and confirmed by the Senate. The last two left in 2011, according to the panel's website.
Kathy McKee, who led the push to get Proposition 200 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."
Justices Clarence Thomas and Samuel Alito were the only two dissenters. Alito said the decision means that Arizona now has two voter registration systems, and that the success of an applicant could come simply by the system he or she chooses. "I find it very hard to believe that this is what Congress had in mind," he said.
Opponents of Arizona's law saw 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 by the state law in the 20 months after it passed. They say about 20 percent of those thwarted were Latino.
Arizona officials say they should be able to pass laws to stop noncitizens from getting on their voting rolls. The Arizona voting law was part of a package that also denied some government benefits to people in the country illegally and required Arizonans to show identification before voting.
Arizona can ask the federal government to include the extra documents as a state-specific requirement, Scalia said, and challenge any adverse decision by the government in court. Louisiana's request already has been granted, Scalia said.
The ruling upholds one by the 9th U.S. Circuit Court of Appeals, which said the 1993 National Voter Registration Act of 1993 trumps Arizona's Proposition 200.
The case is 12-71, Arizona v. Inter Tribal Council of Arizona, Inc.