The Supreme Court's refusal to strike down an Indiana law requiring government-issued photo identification at the ballot box could disenfranchise minority and elderly voters at next week's primary and prompt other states to pass similar laws, voting advocates said Monday.

The court, in a splintered 6-3 ruling Monday, said Indiana's law, which took effect in 2006 and requires voters to present a state or federal photo ID card at the ballot box, does not violate the First or 14th amendments. The court said the law served as a justifiable protection to the electoral process.

"It's especially worrisome that the court has sent a signal making it easier to put up barriers to people voting," said Michael Waldman, executive director of the Brennan Center for Justice at New York University's law school. "There's a real risk that people will see this as a green light to pass restrictive voter ID laws in other states."

The ruling means the ID requirement will be in effect for next week's presidential primary in Indiana, where a significant number of new voters are expected to turn out for the Democratic contest between Sens. Hillary Rodham Clinton and Barack Obama.

The results could say something about the effect of the law, either because a large number of voters will lack identification and be forced to cast provisional ballots or because the number turns out to be small.

More than 20 states require some type of identification at the polls. But only Georgia and Indiana require government-issued photo IDs. For the overwhelming majority of voters, an Indiana driver's license serves as the identification.

In recent years, appellate courts have upheld bitterly fought identification laws in Arizona, Georgia and Michigan, but none is as stringent as the Indiana law.

Supporters of the law say it's all about preventing fraud.

Indiana has a "valid interest in protecting 'the integrity and reliability of the electoral process,'" said Justice John Paul Stevens in an opinion that was joined by Chief Justice John Roberts and Justice Anthony Kennedy.

But advocacy groups, including the Brennan Center, say they know of no voter fraud case ever being prosecuted against someone who impersonated another voter at the polls. Indiana's Republican Secretary of State Todd Rokita acknowledged there were no prosecutions in his state for impersonating voters, but said the measure was necessary to protect election integrity.

Indiana Solicitor General Tom Fisher, who argued the state's case before the high court, said Monday's ruling vindicates the law as a "common sense measure to protect the security and integrity of elections.

Of the remaining state primaries, Indiana's vote on May 6 has the most possibility for voter confusion over ID rules, voting advocates say. The remaining states, including Nebraska, Kentucky and Idaho, have much more lax identification requirements.

Those states that worry election advocates because of ongoing efforts to pass strict photo ID laws include Missouri, Kansas and Oklahoma. But it appeared unlikely Monday that legislators in those states would be able to push any such measures through before November's general election.

In Missouri, where the state supreme court overruled a previous photo-ID law, Republican Rep. Stanley Cox earlier this year proposed a constitutional amendment requiring such identification. He'd been waiting on the Supreme Courts decision before aggressively lobbying for it, but with Missouri's legislative session due to end May 16, Cox said Monday that the high court's ruling came too late.

"As a practical matter, the voters probably won't have this choice until 2010," Cox said.

Across the country, as many as 20 million people lack such identification, most of them minorities and the elderly who don't have drivers' licenses or passports and are unable to afford the cost of obtaining documentation to apply for such identification, advocacy groups say.

In Indiana, more than 20 percent of black voters do not have access to a valid photo ID, according to an October 2007 study by the University of Washington.

In Marion County, 34 Indiana voters without the proper identification were forced to file provisional ballots in an offseason local election. According to Indiana's photo law, voters have 10 days to return to the county courthouse with the proper identification. They can also file an affidavit claiming poverty.

"Who's going to do that?" asked Bob Brandon, president of Fair Elections Legal Network, a nonpartisan network of election lawyers. "Who's going to show up and sign an affidavit saying 'I'm poor'?"