WASHINGTON — The Supreme Court will struggle this week with the validity of an Arizona law that tries to keep illegal immigrants from voting by demanding all state residents show documents proving their U.S. citizenship before registering to vote in national elections.

The high court will hear arguments today over the legality of Arizona’s voter-approved requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under the federal “Motor Voter” voter registration law that doesn’t require such documentation.

This case focuses on voter registration in Arizona, which has tangled frequently with the federal government over immigration issues involving the Mexican border. But it has broader implications because four other states — Alabama, Georgia, Kansas and Tennessee — have similar requirements, and 12 other states are contemplating similar legislation, officials say.

The Obama administration is supporting challengers to the law.

If Arizona can add citizenship requirements, then “each state could impose all manner of its own supplemental requirements beyond the federal form,” Solicitor General Donald B. Verrilli Jr. said in court papers. “Those requirements could encompass voluminous documentary or informational demands, and could extend to any eligibility criteria beyond citizenship, such as age, residency, mental competence, or felony history.”

A federal appeals court threw out the part of Arizona’s Proposition 200 that added extra citizenship requirements for voter registration, but only after lower federal judges had approved it.

Arizona wants the justices to reinstate its requirement.

Kathy McKee, who led the push to get the proposition on the ballot, said voter fraud, including by illegal immigrants, continues to be a problem in Arizona. “For people to conclude there is no problem is just shallow logic,” McKee said.

The Associated Press reported in September that officials in pivotal presidential election states had found only a fraction of the illegal voters they initially suspected had existed. In Colorado, election officials found 141 noncitizens on the voter rolls, which was 0.004 percent of the state’s nearly 3.5 million voters. Florida officials found 207, or 0.001 percent of the state’s 11.4 million registered voters. In North Carolina, 79 people admitted to election officials that they weren’t citizens and were removed from the rolls, along with 331 others who didn’t respond to repeated inquires.

Attack on groups?

Opponents of Arizona’s law see it as an attack on vulnerable voter groups such as minorities, immigrants and the elderly. They say Arizona’s law makes registering more difficult, which is an opposite result from the intention of the 1993 National Voter Registration Act.

Proposition 200 “was never intended to combat voter fraud,” said Democratic state Sen. Steve Gallardo of Phoenix. “It was intended to keep minorities from voting.”

With the additional state documentation requirements, Arizona will cripple the effectiveness of neighborhood and community voter registration drives, advocates say. More than 28 million Americans used the federal “Motor Voter” form to register to vote in the 2008 presidential elections, according to the U.S. Election Assistance Commission.

More restrictions

An Arizona victory at the high court would lead to more state voting restrictions, said Elisabeth MacNamara, the national president of the League of Women Voters.

Opponents of the Arizona provision say they’ve counted more than 31,000 potentially legal voters in Arizona who easily could have registered before Proposition 200 but who were blocked initially by the law in the 20 months after it passed in 2004. They saidabout 20 percent of those thwarted were Latino.

Arizona officials say they should be able to pass laws to stop illegal immigrants and other noncitizens from getting on their voting rolls.

The Arizona voting law was part of a package that also denied some government benefits to illegal immigrants and required Arizonans to show identification before voting. The 9th U.S. Circuit Court of Appeals upheld the voter identification provision. The denial of benefits was not challenged.

Opponents “argue that Arizona should not be permitted to request evidence of citizenship when someone registers to vote, but should instead rely on the person’s sworn statement that he or she is a citizen,” Arizona Attorney General Thomas C. Horne said in court papers.

“The fallacy in that is that someone who is willing to vote illegally will be willing to sign a false statement. What (opponents) are urging is that there should be nothing more than an honor system to assure that registered voters are citizens. That was not acceptable to the people of Arizona.”

The Arizona proposition was enacted into law with 55 percent of the vote.

This is the second voting issue the high court is tackling this session. Last month, several justices voiced deep skepticism about whether a section of the Voting Rights Act of 1965, a law that has helped millions of minorities exercise their right to vote, especially in areas of the Deep South, was still needed.

This case involves laws of more recent vintage.

The federal “Motor Voter” law, enacted in 1993 to expand voter registration, allows would-be voters to fill out a mail-in voter registration card and swear they are citizens under penalty of perjury, but it doesn’t require them to show proof.

Under Proposition 200 approved in 2004, Arizona officials require an Arizona driver’s license issued after 1996, a U.S. birth certificate, a passport or other similar document, or the state will reject the federal registration application form.

This requirement applies only to people who seek to register using the federal mail-in form. Arizona has its own form and an online system to register when renewing a driver’s license. The court ruling did not affect proof of citizenship requirements using the state forms.

State officials say more than 90 percent of those Arizonans applying to vote using the federal form will be able to simply write down their driver’s license number, and all naturalized citizens simply will be able to write down their naturalization number without needed additional documents.

Former Arizona Senate President Russell Pearce, a leading Republican proponent of Proposition 200, strongly disputed claims that Arizona doesn’t have voter fraud problems. “They turn a blind eye,” Pearce said of the state’s election officials.

But Karen Osborne, elections director for Maricopa County, where nearly 60 percent of Arizona’s voters live, said voter fraud is rare, and even rarer among illegal immigrants.

“That just does not seem to be an issue,” Osborne said of the claim that illegal immigrants are voting. “They did not want to come out of the shadows. They don’t want to be involved with the government.”

The main legal question facing the justices is whether the federal law trumps Arizona’s law. A 10-member panel of the 9th Circuit in San Francisco said it did.

The appeals court issued multiple rulings in this case, with a three-judge panel initially siding with Arizona. A second panel that included retired Supreme Court Justice Sandra Day O’Connor, who from time to time sits on appeals courts, reversed course and blocked the registration requirement. The full court then did the same, and that decision will be reviewed by the justices in Washington.