By Tim Gaynor

PHOENIX (Reuters) - Arizona will argue before the Supreme Court on Monday that it is within its rights to demand voters show proof of U.S. citizenship to register to vote in federal elections, while opponents will argue it is an overreach of its powers.

The legal dispute over the registration requirement dates back to 2004 when Arizona voters passed a ballot initiative, Proposition 200, designed to stop illegal immigrants from voting.

The measure amended state election laws to require voters to show proof of citizenship to register to vote, as well as identification to cast a ballot at the polls in the desert state which borders Mexico.

"It is needed to preclude non-citizens from voting," said Tom Horne, Arizona's Republican attorney general who will argue the case for the state before the Supreme Court justices on Monday.

The voter registration measure is one of many nationwide championed by Republicans and put in place at the state level that Democrats say are intended to make it more difficult for certain voters who tend to vote Democratic to cast ballots.

While a 1993 federal law requires would-be voters to fill in a postcard-sized form with their details and swear under penalty of perjury to its truth, the state's registration law requires them to present "satisfactory evidence" of U.S. citizenship, including a driver's license number, naturalization papers, birth certificate or passport.

Arizona residents, civil rights groups and Indian tribes sued to challenge the measure, which they say discriminates against otherwise eligible voters - among them members of more than a score of Native American tribes across the rugged desert state, some of whom struggle to meet additional requirements.

"Indian reservations for the most part are remote. ... Lots of Indians have no street address ... many are born at home and they have no birth certificates, even fewer have passports," said Joe Sparks, attorney for the Inter Tribal Council of Arizona, one of the plaintiffs in the lawsuit.

"The result is disenfranchisement of those who cannot muster up those kinds of IDs," he added.

The top court battle is the latest for Arizona's Republican state leadership, which won a partial victory over Democrat Barack Obama's administration last year in a mixed Supreme Court ruling on SB 1070, its 2010 state crackdown on illegal immigration.

In a landmark ruling, the justices allowed a state provision on immigration status checks by police. But it also struck down rules in the state's measure that would, among other things, ban illegal immigrants from soliciting work in public places.

CONFLICT WITH FEDERAL LAW?

The heart of the argument for the Supreme Court appeal is whether federal law trumps state law on the issue of voter registration.

Horne will argue that the federal law, enshrined in the 1993 National Voter Registration Act, and the state's own measure, are not in conflict, but are in fact complementary.

"The federal law doesn't preclude us from asking for additional evidence," Horne told Reuters. "The federal law requires us to accept and use the federal form, but it doesn't preclude us from also asking for information to demonstrate that the person is a citizen," he added.

But opponents will argue that the federal law, designed to make it easier for people to register to vote in federal elections by using a federal registration form, holds sway over state law on voter registration.

"We believe that it is in conflict with federal law and that federal is supreme in this instance," said Nina Perales, vice president of litigation for the Mexican American Legal Defense and Educational Fund, which is lead counsel in the case.

While the 9th U.S. Circuit Court of Appeals upheld Arizona's right to require voter identification at polling places, the court in April found that the citizenship requirement conflicted with the federal law.

A Supreme Court decision in the case is expected by the end of June. Horne says that, while it is hard to predict the outcome of a Supreme Court case, Arizona "deserves to win."

But Perales said she believed the momentum was with the plaintiffs.

"We feel strong coming into this argument that, given the history of success of these claims up until now, the Supreme Court will generally agree with the logic of the 9th circuit in this case."

The case is Arizona v. The Inter Tribal Council of Arizona Inc et al, U.S. Supreme Court, No. 12-71.

(Editing by Howard Goller and Xavier Briand)