The Supreme Court will struggle this week with the validity of an Arizona law that tries to keep illegal immigrants from voting by demanding all state residents show documents proving their U.S. citizenship before registering to vote in national elections.

The high court will hear arguments Monday over the legality of Arizona's voter-approved requirement that prospective voters document their citizenship in order to use a registration form produced under the federal "Motor Voter" voter registration law that doesn't require such documentation.

This case focuses on voter registration in Arizona, which has tangled frequently with the federal government over immigration issues involving the Mexican border. But it has broader implications because four other states - Alabama, Georgia, Kansas and Tennessee - have similar requirements, and 12 other states are contemplating similar legislation, officials say.

The Obama administration is supporting challengers to the law.

If Arizona can add citizenship requirements, then "each state could impose all manner of its own supplemental requirements beyond the federal form," Solicitor General Donald B. Verrilli Jr. said in court papers.

A federal appeals court threw out the part of Arizona's Proposition 200 that added extra citizenship requirements for voter registration, but only after lower federal judges had approved it.

Arizona wants the justices to reinstate its requirement.

Kathy McKee, who led the push to get the proposition on the ballot, said voter fraud, including by illegal immigrants, continues to be a problem in Arizona.

Opponents of Arizona's law see it as an attack on vulnerable voter groups such as minorities, immigrants and the elderly.

The case is 12-71, Arizona v. Inter Tribal Council of Arizona, Inc.

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