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WASHINGTON - Lawyers for Arizona, a state that has clashed repeatedly with the federal government on the issue of immigration, will be back at the Supreme Court on Monday defending a state law that requires proof of citizenship in order to register to vote in elections.

Critics of the law say that it conflicts with federal law - the National Voter Registration Act (NVRA), which is sometimes referred to as the Motor Voter law. It was enacted in 1993 to establish uniform procedures to vote in federal elections. .

The NVRA provides a federal form for registration in which the registrant is required to check a box indicating U.S. citizenship and to sign the form under penalty of perjury.

But the state law, Proposition 200, which passed in 2004, requires any registrant who does not have a driver's license issued after 1996 or a non-operating license to provide documents such as a copy of a birth certificate or a passport.

"The case is intrinsically important," says election law expert Edward B. Foley of the Moritz School of Law at Ohio State University, "because it asks whether a state can add a requirement to prove U.S. citizenship at the time of voter registration, beyond what the federal government requires under the NVRA".

"This is an important case that is under the radar screen, because it involves not only the issue of immigration but also the regulation of voting rights," Foley says. "More broadly, how do we make the rules of elections and who gets to write the rules - the federal government or the states?"

Groups such as the Mexican American Legal Defense and Educational Fund (MALDEF) say the law puts an additional burden on voters. They argue that a voter registrant who submits the federal form but does not provide additional documentation required by the state law is rejected for voter registration.

In court briefs, lawyers for MALDEF say: "Although states are authorized to design and use their own mail voter registration form, nothing in the statute permits states to use their forms to the exclusion of the federal form."

Nina Perales, MALDEF's Vice President of Litigation says, that Arizona is trying to portray the dispute as a "David and Goliath battle between a state and an overbearing federal government," but she says the "bottom line, is that Arizona's law excludes U.S. citizens from registering to vote and it conflicts with federal law. Federal law says an individual can register to vote using a federal voter registration form. Arizona rejects the federal voter registration form unless they meet additional documentation requirements."

She says that following the enactment of Proposition 200, more than 31,000 individuals were rejected for voter registration and less than one-third of the rejected registrants were subsequently successfully registered to vote.

A federal appeals court ruled against Proposition 200 and blocked the provision at issue.

Arizona Attorney General Thomas C. Horne argues in court briefs that Proposition 200 is necessary to preserve the integrity of elections. He says the state law does not conflict with the NVRA.

"The requirement that applicants provide additional evidence to support their application does not constitute a 'rejection' of the Federal Form any more than an identification check at an airport gate entrance constitutes a 'rejection' of a passenger's ticket," Horne says in the court briefs.

"Arizona, like other States, has experienced fraud in voting with regard to both registration and casting ballots," Horne argues.

Alabama , Georgia, Kansas, Michigan, Oklahoma and Texas have filed a brief in support of Arizona saying that the state law "works hand in glove" with the federal law at issue and that Proposition 200 "is itself designed to protect the electoral process for the benefit of eligible citizens."

But the federal government has filed a brief in support of the challengers, arguing that the NVRA "prohibits a state from imposing additional requirements on applicants who seek to register for federal elections through the federal form."