WASHINGTON -- The Supreme Court ruled Monday that states can require voters to produce photo identification without violating their constitutional rights, validating Republican-inspired voter ID laws.

In a splintered 6-3 ruling, the court upheld Indiana's strict photo ID requirement, which Democrats and civil rights groups said would deter poor, older and minority voters from casting ballots. Its backers said it was needed to deter fraud.

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Analysis: Capitol WatchBlog

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It was the most important voting rights case since the Bush v. Gore dispute that sealed the 2000 election for George W. Bush.

The law "is amply justified by the valid interest in protecting 'the integrity and reliability of the electoral process,"' Justice John Paul Stevens said in an opinion that was joined by Chief Justice John Roberts and Anthony Kennedy.

Justices Samuel Alito, Antonin Scalia and Clarence Thomas also agreed with the outcome, but wrote separately.

Justices Stephen Breyer, Ruth Bader Ginsburg and David Souter dissented.

More than 20 states require some form of identification at the polls. Courts have upheld voter ID laws in Arizona, Georgia and Michigan, but struck down Missouri's. Monday's decision comes a week before Indiana's presidential primary.

The case concerned a state law, passed in 2005, that was backed by Republicans as a way to deter voter fraud. Democrats and civil rights groups opposed the law as unconstitutional and called it a thinly veiled effort to discourage elderly, poor and minority voters -- those most likely to lack proper ID and who tend to vote for Democrats.

"It's not unreasonable to ask for people to show a photo identification when they show up to exercise our most fundamental right," said Indiana House Minority Leader Brian Bosma. "We're asked at Blockbuster. We're asked at the airport. We're asked to enter some buildings. Certainly, it's appropriate to guarantee that each vote counts."

There is little history in Indiana of either in-person voter fraud -- of the sort the law was designed to thwart -- or voters being inconvenienced by the law's requirements.

"We cannot conclude that the statute imposes 'excessively burdensome requirements' on any class of voters," Stevens said.

Stevens' opinion suggests that the outcome could be different in a state where voters could provide evidence that their rights had been impaired.

But in dissent, Souter said Indiana's voter ID law "threatens to impose nontrivial burdens on the voting rights of tens of thousands of the state's citizens."

Scalia, favoring a broader ruling in defense of voter ID laws, said, "The universally applicable requirements of Indiana's voter-identification law are eminently reasonable. The burden of acquiring, possessing and showing a free photo identification is simply not severe, because it does not 'even represent a significant increase over the usual burdens of voting."'

The Indiana law was upheld by a federal judge and by the 7th U.S. Circuit Court of Appeals in Chicago. Before the law passed, Indiana voters only had to sign poll books at the polling place, where photocopies of their signatures were kept on file for comparison.