The Arizona requirement applies only to people who seek to register using the federal mail-in form. The state has its own form and an online system to register to vote when renewing a driver's license. The appeals court ruling did not affect proof of citizenship requirements using the state forms.
Justice Samuel Alito asked if Arizona kept two different voter rolls, one for people who used the state system and one for those who use the federal. The answer was no.
That means that some people face one set of requirements to vote, and others a different set, he said. "This seems to me like a crazy system," Alito said.
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.
But Arizona officials say they should be able to pass laws to stop illegal immigrants and other non-citizens 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.
Checks since last fall by The Associated Press showed that in Colorado, election officials found 441 non-citizens on the voter rolls out of nearly 3.5 million voters. Florida officials found 207, or 0.001 percent of the state's 11.4 million registered voters. In North Carolina, 79 people admitted to election officials that they weren't citizens and were removed from the rolls, along with 331 others who didn't respond to repeated inquires.
Horne compared the Arizona system to an airline sending out e-tickets instead of paper tickets but asking for identification before allowing passengers to board the airplane. "That would not contradict the statement that they are accepting and using e-tickets," he said.
But Justice Elena Kagan didn't accept that analogy, saying Arizona went further. "Wouldn't it contradict it if instead of saying 'Well, we'd like you to offer identification,' saying, 'Well, we'd like you also to have a paper ticket'?" she said.
Arizona asked the federal government to add the state's citizenship eligibility requirements to the federal form but was turned away. Scalia said the state should have sued to overturn that decision. "Why didn't you do that?" said Scalia, who indicated that he would look favorably on such a challenge.
The decision not to challenge was his predecessor's, Horne said.
The case is 12-71, Arizona v. Inter Tribal Council of Arizona, Inc.
Associated Press writer Jacques Billeaud contributed to this story from Phoenix.