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(The Root) -- In a 7-2 decision announced on Monday in Arizona v. Arizona Inter Tribal Council, the Supreme Court ruled that states cannot require would-be voters to prove that they are U.S. citizens before using a federal voting-registration system. The court agreed with challengers to the law, who argued that the requirement put an extra burden on naturalized citizens, NBC News reports:

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, adopted by Arizona voters in 2004, went further than the federal form by requiring applicants to provide proof of citizenship ...

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 ...

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."

Voting-rights and constitutional advocates consider the decision -- with which Justices Clarence Thomas and Samuel Alito, two members of the court's conservative wing, dissented -- a win. "It took seven years and a series of appeals, but now Arizona has to follow the law. The Supreme Court correctly interpreted Congress' clear intent. Congress saw the federal form as a standalone, uniform document that simplifies voter registration for citizens, not a document that gives the states license to add burdensome requirements," Jon Greenbaum, chief counsel at the Lawyers' Committee for Civil Rights Under Law, said.

Nina Perales, MALDEF vice president of litigation and lead counsel for the voters who challenged Proposition 200, credited the decision with sending "a strong message that states cannot block their citizens from registering to vote by superimposing burdensome paperwork requirements on top of federal law."

Constitutional Accountability Center Chief Counsel Elizabeth Wydra called the ruling "a victory for the federal government's authority to regulate federal elections and protect the right to vote."

The court did not announce its long-awaited decision in Shelby County v. Holder, a major ruling that will determine whether Section 5 of the Voting Rights Act is struck down. The Arizona case involves an entirely different set of laws and issues and doesn't offer much insight into how the court will decide Shelby. Still, says, Lawyers' Committee for Civil Rights President and Executive Director Barbara Arnwine, "It would be a false promise for today's decision to promise equal access to the ballot and for an adverse ruling in Shelby to snatch this away." 

Read more at NBC News.