NBC's Pete Williams reports on the 7-2 decision that knocked down Arizona's requirement for would-be voters to show proof of U.S. citizenship.

The Supreme Court on Monday struck down an Arizona law that requires people to submit proof of citizenship when they register to vote.

The vote was 7-2. Justice Antonin Scalia, writing for the majority, said that a 1993 federal law known as the Motor Voter Act takes precedence over the Arizona law because of its requirement that states “accept and use” the federal voter registration form.

Justices Clarence Thomas and Samuel Alito, two members of the court’s conservative wing, dissented.

Only a handful of states have similar laws, which the states say are meant to reduce voter fraud. Civil rights groups said the Arizona law was an effort to discourage voting by legal immigrants, and they worried that more states would have followed if the Supreme Court had upheld it.

Groups opposed to the Arizona law said that the court had blocked an attempt at voter suppression.

“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 Education Fund.

Citizenship is a requirement to vote in any federal election, and the federal registration form requires people to state, under penalty of perjury, that they are American citizens. States can use their own forms, but they must be equivalent to the federal form.

The Arizona law, known as Proposition 200 and adopted by Arizona voters in 2004, went further than the federal form by requiring applicants to provide proof of citizenship. Arizona has used the law to reject 30,000 voter applications, according to the Brennan Center for Justice.

Challengers to the law argued that it put an extra burden on naturalized citizens. Using a naturalization document as proof would require an applicant to register in person, as opposed to through the mail, because federal law prohibits copying the document.

A federal appeals court said that Arizona had gone too far and essentially rejected the federal form. Arizona said it was not a rejection of the federal form any more than asking for ID at an airport is a rejection of a plane ticket.

The Supreme Court ruling pointed out that Arizona still has an option: It can ask the federal government to include state-specific instructions on the federal form, and go to court if the government says no.

Three other states — Alabama, Georgia and Kansas — have laws almost identical to Arizona’s and joined it in urging the court to uphold the additional requirement for proof of citizenship.

At an oral argument in March, Thomas Horne, a lawyer for Arizona, told the justices that the state was within its rights to ask for additional information beyond the simple federal form.

“It’s extremely inadequate,” Horne said. “It’s essentially an honor system. It does not do the job.”

“Well,” answered Justice Sonia Sotomayor, “that’s what the federal system decided was enough.”

The court’s conservatives had appeared sympathetic to the Arizona side. Scalia said during the argument that federal law clearly empowers the states to take additional action to assess a potential voter’s eligibility.

“Under oath is not proof at all,” he said. “It’s just a statement.”

Patricia Millett, a lawyer for groups opposed to the law, countered: “Statements under oath in a criminal case are proof beyond a reasonable doubt” in criminal cases that result in execution.

“It’s a very serious oath,” she said.

Arizona is known for its tough stance on immigration. Last year, the Supreme Court struck down some key provisions of a state law meant to crack down on illegal immigration.

But it let stand the most controversial part — a requirement that police making traffic stops check the immigration status of anyone they suspect of being in the country illegally.

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