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An Arizona law requiring proof of citizenship to register to vote was struck down Tuesday by a federal appeals court because it conflicts with the U.S. National Voter Registration Act.

The U.S. Court of Appeals in San Francisco invalidated parts of Proposition 200, which was approved by voters in 2004.

The case, Gonzalez v. Arizona, 08-17094, was decided, 2-1, by a three judge panel that included former Supreme Court Justice Sandra Day O'Connor. O'Connor was joined by Circuit Judge Sandra Ikuta, while Chief Judge Alex Kozinski dissented on the decision to not require proof of citizenship.

The court upheld the requirement to show identification at the polls.

The National Voter Registration Act — also known as the Motor Voter Act — was passed in 1993 to streamline voter registration procedures nationwide.

The law allows voters to register when applying for a driver's license or social services, and allowed for more accessible voter registration through standardized mail-in forms.

The court held that Arizona's requirement of proof of citizenship violated the federal statute, which lays out what states may and may not require to register to vote. While voters may have to attest to their citizenship, requiring documentary proof falls outside the law, the court said.

The NVRA says states must use the federal voter registration form without additional documentation.

Proposition 200 created an additional hurdle for registration, the court held. The federal law is aimed at reducing "state-imposed obstacles" to voters.

Calling the state's arguments a "creative interpretation," the Ninth Circuit said "states must 'accept and use' the Federal Form as a fully sufficient means of registering to vote in federal elections."

Under the NVRA, prospective voters seeking to register in federal elections need only complete and submit the Federal Form. If this sounds simple, it is by design. Congress enacted the NVRA to increase federal registration by streamlining the registration process and eliminating complicated state-imposed hurdles to registration, which it determined were driving down voter turnout rates. Proposition 200 imposes such a hurdle.

The court declined to rule on constitutional issues of equal protection raised by the plaintiffs, saying that Proposition 200's violation of NVRA made those allegations moot.

The appeals court rejected arguments that Arizona's requirement to show identification at the polls amounts to a poll tax. The plaintiffs, which included the League of United Latin American Citizens, the League of Women Voters, the Hopi Tribe and other voter rights groups, argued that because there's no way to satisfy the voter ID requirement without spending money, that it amounts to a poll tax.

"Although obtaining identification required under Arizona’s statute may have a cost, it is neither a poll tax itself (it is not a fee imposed on voters as a prerequisite for voting), nor is it a burden imposed on voters who refuse to pay a poll tax," the court said.

"The photo identification requirement is not an invidious restriction... and the burden is minimal," the decision said.

O'Connor sat on the panel because retired federal judges, and Supreme Court justices, can be assigned to courts by the Chief Justice if there are vacancies.

O'Connor was a state senator and judge in Arizona before President Reagan nominated her to the Supreme Court. She served from 1981 to 2006 as an Associate Justice.

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