WASHINGTON -- The Supreme Court appeared divided along familiar lines on Monday as it heard arguments over whether Arizona can require proof of citizenship from people seeking to register to vote in federal elections.
Several of the court's more liberal justices sounded doubtful about a state law that imposes requirements beyond those called for by a federal law.
"Many people don't have the documents that Arizona requires," Justice Sonia Sotomayor said.
The Arizona law, enacted in 2004 by a ballot initiative, requires prospective voters to prove that they are citizens by submitting copies of or information concerning various documents, including birth certificates, passports, naturalization papers or Arizona driver's licenses, which are available only to people who are in the state lawfully.
The question for the justices was whether that state law conflicted with the National Voter Registration Act of 1993, which allows voters to register using a federal form that asks, "Are you a citizen of the United States?" Prospective voters must check a box yes or no, and they must sign the form, swearing that they are citizens under penalty of perjury.
Several members of the court's conservative wing indicated that the state was free to impose additional requirements to make sure only citizens vote.
Justice Antonin Scalia said the federal form was inadequate. "So it's under oath," he said. "Big deal. If you're willing to violate the voting laws, I suppose you're willing to violate the perjury laws."
"Under oath," he added, "is not proof at all. It's just a statement."
Patricia A. Millett, a lawyer for several groups challenging the Arizona law, responded that "statements under oath in criminal cases are proof beyond a reasonable doubt" sufficient to lead to the death penalty.
She added that tens of thousands of people had been rejected from the registration rolls thanks to the Arizona law, though there was no evidence that they were not citizens.
The United States Court of Appeals for the Ninth Circuit, in San Francisco, blocked the state law last year, saying it conflicted with the federal one. That decision, from a 10-judge panel, effectively affirmed a 2010 ruling from a three-judge panel that included Justice Sandra Day O'Connor, who retired from the Supreme Court in 2006 but occasionally acts as a visiting appeals court judge.
Justice O'Connor, who was in the courtroom on Monday, joined the majority in 2010 in ruling that the state law was inconsistent with the federal one and so could not survive.
Justice Anthony M. Kennedy, who may hold the decisive vote, asked questions on Monday sympathetic to both sides.
Under Arizona's interpretation, he said, "it seems to me the federal form, as some of my colleagues have indicated, is not worth very much."
Later, though, he said the appeals court had not given enough weight to Arizona's concerns. "The state has a very strong and vital interest in the integrity of its elections," he said, "even when those, and perhaps especially when those, are elections of federal officials."
Thomas C. Horne, Arizona's attorney general, said the federal approach was insufficient to protect the integrity of federal elections in his state.
"It's essentially an honor system," he said of the statement required by the federal form. "It does not do the job."
Mr. Horne said Arizona should be free to supplement the federal form even though the federal law required state officials to "accept and use" the federal form.
An airline may "accept and use" an electronic ticket, he said, and yet still require identification.
Justice Elena Kagan said that was the wrong analogy. Arizona's system is akin, she said, to requiring a paper ticket in addition to the electronic one. "That would be inconsistent with the 'accept and use' language," she said.
Though Justice Scalia appeared attentive to the state's concerns, he suggested that it may have fumbled the litigation by not going to court after losing before the federal Election Assistance Commission. "You should have challenged the commission's refusal to place that evidence in the federal form," Justice Scalia told Mr. Horne.
Justice Samuel A. Alito Jr. inquired whether state election officials would be free to ask a youth who was plainly too young to vote for proof of his age when he submitted an otherwise unobjectionable federal form in person. Ms. Millett said no, though she added that officials remained free to gather their own evidence "including from their own eyeballs." The question in the case, she said, was whether the state could place the burden of proof on the applicants.
Justice Alito also noted that people seeking to register to vote in Arizona need not use the federal form and might choose to use a more demanding state one instead.
"This seems to me like a crazy system," he said. "This is like the I.R.S. creating two different tax returns with different tax rates and different tax tables, and how much you pay would depend on which particular form you happened to pick up and submit."
Toward the end of the argument in the case, Arizona v. The Inter Tribal Council of Arizona, No. 12-71, Justice Scalia said states should be free to step in when a federal agency did not ensure the integrity of state's voting system.
"When the commission fails to do what enables the state to assess qualifications, the state will do it," he said. "No problemo."
This article originally appeared in The New York Times.