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During oral arguments, conservative and liberal-leaning justices engaged in often terse back-and-forth, suggesting a 5-4 decision.

Justice Anthony M. Kennedy, considered the swing vote on the case, suggested that while preclearance has had a positive effective in the fight against voter discrimination, it may also have outlived its usefulness, saying “times change.”

Regarding which way a 5-4 decision would swing, Mr. Raskin joked it “depends on which side of the bed Justice Kennedy walks up on that day.”

The Arizona case, meanwhile, involves a law designed to stop illegal immigrants from voting. The law, approved by state voters in 2004, 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.

A group of Arizona residents, Indian tribes and civil rights organizations challenged the law, and an appeals court ruled in their favor, saying the citizenship requirement conflicted with the 1993 federal law known as the “Motor Voter Law,” designed to make it easier for people to register to vote. Arizona appealed and the Supreme Court agreed to take the case.

Arizona and other supporters of the law — mostly Republicans — say the citizenship requirement is a valuable tool to combat voter fraud.

But the law’s critics, mostly Democrats, Hispanic rights organizations and civil rights groups, say the law imposes an unfair burden on residents and threatens to disenfranchise minorities, the poor, the elderly and students — typically Democratic-leaning voters.

“No one challenges in theory the idea that people’s identify and relevant citizenship needs to be established before voting,” said Mr. Raskin, a Democratic Maryland state senator. “But the question is whether the government sets up so many thresholds and barriers that it acts as a deterrent to participation.”

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 Indiana case [and the Arizona case] are apples and oranges,” Ms. Perez said. “They’re different claims under different statues with different jurisprudential and precedential histories.”