WASHINGTON – An Arizona law requiring would-be voters to show proof of U.S. citizenship seemed to divide Supreme Court justices Monday in a case important to many states that want to stiffen their own voting standards.

Conservative justices sounded sympathetic to Arizona's proof-of-citizenship requirement, while more liberal justices suggested the measure might conflict with a 1993 law passed by Congress called the National Voter Registration Act. The eventual ruling will define when federal law pre-empts state efforts, a legal determination that accompanies political controversies ranging from illegal immigration to allegations of voter suppression.

"Many people don't have the documents that Arizona requires," Justice Sonia Sotomayor noted pointedly at the start of the hourlong oral argument Monday.

Sotomayor and Justice Elena Kagan, a fellow appointee of President Barack Obama, pushed back vigorously against the Arizona law.

From the other side, though, Republican appointees, including Chief Justice John Roberts and Justice Samuel Alito, pressed questions seemingly supportive of Arizona's actions.

"The state has a very strong and vital interest in the integrity of its election … perhaps especially when those are elections of federal officials," said Justice Anthony Kennedy, a frequent swing vote on close cases, adding that a lower appellate court "did not give sufficient weight to that interest" when it struck down Arizona's law.

The case, called Arizona v. Inter Tribal Council of Arizona, is the latest Supreme Court dispute arising from the state's politically charged concerns over illegal immigration. Last year, the court in a 5-3 decision struck down portions of an Arizona immigrant-control law on the grounds it was pre-empted by the federal government's responsibilities.

The current Arizona case also arises amid increasingly aggressive nationwide efforts to impose photo identification or other requirements on voters. These broader stakes were underscored by numerous friend-of-the-court briefs.

Kansas Secretary of State Kris Kobach, who has made his mark nationally as a border-security hard-liner, filed a brief supporting the Arizona law. Kansas has imposed its own proof-of-citizenship requirements, effective this year.

Separately, Kansas joined Texas, Georgia and three other states in filing a similar brief supporting Arizona, arguing that "states' control over elections will be diminished in law and in practice" if the court strikes down Arizona's law.

Groups such as the League of Women Voters and the NAACP Legal Defense and Education Fund have joined in backing the Obama administration and other opponents of the Arizona law.

"Thirty-one thousand five hundred and fifty people were rejected from voting" by the Arizona law, attorney Patricia Millett, arguing for the law's opponents, said Monday, adding that many of those then "had to do the double gauntlet that Congress was trying to eliminate" in order to finally register.

The legal dispute centers on the relationship between the National Voter Registration Act and Arizona's Proposition 200, passed with 55 percent of the state vote in 2004.

The federal law, sometimes called the "Motor Voter Act," was passed by a Democratic-controlled Congress whose members said they wanted to boost voter participation by easing registration procedures.

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