Supreme Court justices expressed some skepticism on Monday about an Arizona law that requires people registering to vote in federal elections to show proof of citizenship.

The legal question before the nine justices is whether the voter registration provision of the 2004 state law is trumped by a federal law, the 1993 National Voter Registration Act, which outlines various ways in which people can register to vote in federal elections.

The fight over Arizona's strict new voter registration laws arrives at the Supreme Court. NBC News' Brian Mooar reports.

That law requires no proof of citizenship. Would-be voters simply sign a statement saying they are citizens.

Based on Monday's oral argument, it was unclear how the court will rule, but a number of justices, including regular swing vote Justice Anthony Kennedy raised some concerns about the law.

Kennedy noted that the basic registration form required under the federal law was intended to make registering to vote relatively straightforward.

He said that the "whole utility of the form is gone" if it is made too complicated by states requiring additional information.

Justice Sonia Sotomayor appeared to agree on that point.

The federal law is intended to "simplify the process," she said. She questioned how requiring additional information could be "consistent with that objective and purpose."

Arizona, which shares a border with Mexico, has a reputation for passing tough anti-immigration laws that have brought it into conflict with the Obama administration.

Various residents, tribal groups and civil rights organizations objected to the law, including the Inter Tribal Council of Arizona.

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

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