Among those bringing suit was Jesus Gonzalez, a public school employee in Yuma, Arizona, who tried to register to vote the day he became a citizen. His application was twice rejected when his separate naturalization and driver's license numbers were improperly "red-flagged" by state databases that initially indicated he was a noncitizen.
The Mexican American Legal Defense and Educational Fund, which helped bring the original lawsuit against the state regulations, said 31,000 potential voters had their applications rejected in the two years after the Arizona law took effect. MALDEF said 90% of those were born in the U.S.
The group's Nina Perales said voter registration drives at county fairs, church services and similar venues have dropped, since many potential voters don't bring the necessary citizenship documents -- like a birth certificate -- to these community events. One estimate found a 45% reduction in Maricopa County, the state's largest county and the seat of Phoenix, the capital.
The ACLU said about 13 million people nationwide lack documents proving their citizenship, and it praised Monday's ruling.
"This decision reaffirms the principle that states may not undermine this critical law's effectiveness by adding burdens not required under federal law," said Laughlin McDonald, of the group's Voting Rights Project. "In doing so, the court has taken a vital step in ensuring the ballot remains free, fair, and accessible for all citizens."
The Obama administration said that if the provision in Arizona's law were allowed to continue, it would create a mishmash of regulations across the county. "Each state could impose all manner of its own supplemental requirements beyond the federal form," Solicitor General Donald Verrilli Jr. said.
What supporters have to say
But Proposition 200 supporters say the state needs the power to keep illegal immigrants and those ineligible to vote in the U.S. from getting a ballot.
"The integrity of our nation's elections suffered a blow today from the Supreme Court," Tom Fitton, president of the conservative Judicial Watch, said Monday. "This issue takes on increasing urgency with the prospect of 11 million illegal immigrants being given amnesty. It is essential that our elections be secured by ensuring that only citizens register to vote."
Some Arizona activists agreed. "I believe we must go out of our way to protect the integrity of America's elections, to avoid the fraud we see regularly in other nations, and which if not checked will rise up here in the United States," said Russell Pearce, a former state Senate president, who helped spearhead Proposition 200's passage. "It ought to be common sense that proof of citizenship be required for voter registration, especially given the concrete evidence we've seen that illegal aliens are indeed both registered and voting. But common sense and America's judicial system don't always see eye to eye, and this is one area we'll just have to keep working."
O'Connor has a professional stake in the current high court fight. As a retired justice, she can sit on lower appeals court cases, and she was part of a 9-2 majority to rule in 2010 that Arizona's citizenship requirement conflicted with federal law.
The Supreme Court case is Arizona v. Inter Tribal Council of Arizona (12-71).