Arizona Attorney General Thomas C. Horne told the Supreme Court on Monday that states carry the "burden" of determining voter eligibility and they can demand residents prove their citizenship before registering to vote.
Conservative-leaning Justice Antonin Scalia seemed to agree with Arizona, saying a sworn oath of citizenship "is not proof at all."
Arizona and its supporters -- including other states with plans for similar laws -- say a signature isn't enough to combat voter fraud. And they argue the federal voting-rights law doesn't say that states can't impose additional citizenship requirements.
"The [federal law] should not be construed to pre-empt Arizona's Proposition 200," Mr. Horne told the court.
But lawyer Patricia Millett, representing those challenging the law, which requires residents to show "satisfactory evidence" of citizenship -- such as a driver's license -- said a signature is more than enough to combat voter fraud.
Criminal courts routinely accept sworn oaths as statements of proof, she said. "It's a very serious oath."
The dispute centers on Arizona's Proposition 200 referendum approved by voters in 2004 that requires residents to show "satisfactory evidence" of citizenship -- such as naturalization papers, a birth certificate, passport or Indian tribal identification -- before registering to vote. A standard Arizona driver's license also is accepted because the state requires proof of citizenship to obtain one.
An appeals courts ruled the citizenship requirement conflicted with the National Voter Registration Act of 1993, also known as the "Motor Voter Law," that was drafted in part to make it easier for people to register to vote. The law requires a signed sworn oath -- not but a document -- to verify citizenship.
The law's critics, including Hispanic-rights organizations and civil-rights groups, say it imposes an unfair burden on residents and threatens to disenfranchise minorities, the poor, the elderly and students, all of whom generally vote Democratic.
Several justices suggested they were sympathetic to the law's detractors. Justice Ruth Bader Ginsburg seemed to reject the state's argument that federal law allows states legal wiggle room to determine who is a citizen.
"Congress did specify how citizenship was to be handled. And it was to be an attestation, a signed attestation, subject to the penalty of perjury," she said. "It said this is the way we deal with citizenship. Then Arizona adds something else."
The Constitution allows states to set their own elections laws but gives Congress the power to intervene.
Myrna Perez, senior counsel at the Brennan Center for Justice at the New York University School of Law, said -- judging from the justice's questions and comments -- that the appeal's court decision will be upheld and the Arizona law struck down.
"It seemed like there was more common ground among the judges from differing perspectives on the court," said Ms. Perez, who attended the oral arguments.
The Brennan Center has filed a brief urging the Supreme Court to reject the Arizona law.
The courts in recent years have upheld several challenges to state laws that have strengthened rules requiring voters to show identification before casting ballots, including a landmark 6-3 Supreme Court decision in May 2008 that upheld a photo ID requirement in Indiana.
But because the Arizona's voting requirement centers around proof of citizenship -- not just a photo ID -- many legal analysts say it's difficult to guess what the court will decide.
The Supreme Court last month heard oral arguments in a separate case challenging a key provision in the 1965 Voting Rights Act that requires all or parts of 16 states with a history of discrimination in voting to get federal approval before making any changes in the way they hold elections.
The court is expected to announce its decision in both cases in June.
This article is based in part on wire service reports.
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