Arizona’s attorney general on Monday told the U.S. Supreme Court that states could demand more proof of citizenship than federal law does to determine who is eligible to vote.

The justices heard arguments in the closely watched case of Arizona v. The Inter Tribal Council of Arizona in which Arizona Attorney General Thomas Horne defended the state’s so-called Proposition 200 from claims that it conflicts with the National Voter Registration Act.

Arizona’s challenge is the second voting rights case being heard by the justices this term. In February, the court heard arguments in Shelby County, Ala. v. Holder, challenging the constitutionality of Section 5 of the Voting Rights Act. Both cases have as their backdrop serious voting problems and accusations of vote suppression during the 2012 elections.

The National Voter Registration Act, passed by Congress in 1993, directs the U.S. Election Assistance Commission to create a national uniform voter registration form that applicants can use to register by mail or in person. The federal form asks applicants if they are U.S. citizens and if they are 18 years old or will be by Election Day. It requires applicants to attest to their eligibility with their signatures. The law requires states to "accept and use" the federal form, but also allows states to create their own registration forms but those cannot conflict with the federal one.

"The federal government can prescribe the form, but it can’t make it exclusive," argued Horne, explaining that the federal law contemplates the traditional role of the states in determining voter eligibility. An applicant’s signature attesting to eligibility, he added, is "not a sufficient way of testing eligibility."

In 2004, Arizona voters approved Proposition 200, which added a requirement for proof of citizenship to voter registration. The state law requires applicants to provide a driver’s license number, a nonoperating identification number, a birth certificate or passport, a certificate of naturalization number or a number associated with membership in a Native American tribe.

Although the Election Assistance Commission rejected Arizona’s request that its citizenship requirement be included as state-specific instructions on the federal form, the state implemented the requirement. Following a legal challenge, a panel of the U.S. Court of Appeals for the Ninth Circuit (with retired Justice Sandra Day O’Connor sitting) ruled that the state requirement conflicted with federal law and was pre-empted under the Constitution’s elections clause.

During Monday’s arguments, Horne fielded skeptical questions from a number of justices who focused on the meaning of the federal law’s requirement that states "accept and use" the federal form.

"Congress did specify how citizenship was to be managed," said Justice Ruth Bader Ginsburg, noting the signed attestation. And the lower court held that the form must be used, not added to with other requirements, she said.

Horne said someone can accept and use a form without the form being sufficient for a particular purpose. He gave the example of an airline e-ticket being accepted and used for flying while the government still requires official identification. However, a paper ticket is not required in addition to the e-ticket, countered Justice Elena Kagan, and she suggested that Arizona’s requirement creates a new federal form.

Horne said Arizona was not creating an additional form but requiring information consistent with the purpose of the form. The federal form’s attestation signature "is essentially an honor system that does not do the job," he added.

Representing the challengers, Patricia Millett of Akin Gump Strauss Hauer & Feld said, "Arizona simply disagrees with the balance that Congress drew." Congress, she added, "confronted a situation in which 40 percent of eligible voters were not registered, because state procedures and burdens were standing as an obstacle, a barrier in the direct line of accountability between individual citizens and their federal government."

Justice Antonin Scalia questioned whether giving a driver’s license number or a naturalization certificate number was "an immense barrier." Millett responded that the district court found that 31,500 people were denied voter registration for failing to provide the additional documentation; 11,000 of those subsequently registered but "had to do the double gauntlet that Congress was trying to eliminate," and 20,000 swore under oath they were citizens and Arizona conceded it had no evidence to the contrary.

Deputy Solicitor General Sri Srinivasan, supporting the challengers, argued that the Election Assistance Commission determines the contents of the federal form. "States have a consultative role at the front end."

If the federal form served only to set a floor on registration requirements, he said, "then each of the 50 states could superimpose whatever additional requirements they wanted to. I think that would largely defeat the entire purpose of the Federal form."

The Mexican American Legal Defense and Educational Fund filed the original lawsuit. "The citizen’s right to participate and vote is too important to permit ill-founded state measures to undermine our presidential and congressional elections," said Thomas Saenz, MALDEF’s president and general counsel in a statement.

The state’s law has made it more difficult to conduct voter registration drives, according to the League of Women Voters, which filed an amicus brief in the case. "States should not be allowed to play politics with the voter registration process, the key entry point for political participation in our democracy," said League President Elisabeth MacNamara after the arguments. "This case could send us down a slippery slope toward more and more state restrictions on the voter registration process and the right to vote."