The U.S. Supreme Court on Monday upheld a tough state law requiring voters to show photo identification, a decision critics say could keep some blacks, poor people and other traditional Democratic supporters from voting in the November election.

Stepping into a partisan political battle, the nation's high court voted 6-3 to reject a challenge to Indiana's toughest-in-the-nation voter identification law.

Democrats and other opponents had argued the law was unconstitutional because it made it too difficult for some people to vote, especially minorities, the poor, the disabled and the elderly. Those groups are most likely not to have government identification and also tend to vote for Democrats.

Supporters, mainly Republicans, defended the law as necessary to prevent voter fraud and to heighten public confidence in the integrity of elections. The Bush administration supported the law.

The court handed down the ruling just eight days before the crucial Indiana presidential primary election featuring Democratic candidates Barack Obama and Hillary Clinton. If elected in November, Obama would be the first black U.S. president and Clinton would be the first woman U.S. president.

"The effect of the loss ... will begin to be felt next week when Indiana holds its presidential primary using the voter ID law the court has just upheld," said Nathaniel Persily, an election law expert at Colombia University in New York.

The decision could have broad national significance because more than 20 states have adopted voter identification laws and other states are considering similar legislation.

The law requires a photo ID such as a driver's license to vote in federal, state and local elections. Those without identification may vote using a provisional ballot, which is counted only if the voter travels to a government office to prove his or her identity within 10 days of the election.

U.S. Sen. Charles Schumer of New York, chairman of the Senate Democratic campaign committee, criticized the ruling.

'OBSTACLES TO VOTE'

"This decision is a body blow to what America stands for -- equal access to the polls," he said. "From time immemorial, states have placed obstacles in the way of people voting. The evidence shows that many voter ID laws are no different. The Indiana law purports to solve a problem that does not exist and it could very well disenfranchise many, many citizens."

The lead opinion, written by Justice John Paul Stevens and joined by Chief Justice John Roberts and Justice Anthony Kennedy, held the evidence in the record did not support an attack now on the law's validity.

Three other court members -- conservative Justices Antonin Scalia, Clarence Thomas and Samuel Alito, concurred in the judgment and issued a separate opinion that the law should be upheld because its overall burden is minimal and justified.

Stevens wrote that Indiana has a valid interest in deterring voter fraud, in addressing the problem of a large number of people on voter registration rolls who are dead or no longer live in the state and in safeguarding voter confidence.

He said the law may place a small burden on a limited number of people -- the elderly born out-of-state who may have difficulty in getting the required documents, the homeless or people with a religious objection to being photographed.

Stevens wrote that politics may have been a factor in adopting the law.

"But if a nondiscriminatory law is supported by valid neutral justifications, those justifications should not be disregarded simply because partisan interests may have provided one motivation for the votes of individual legislators," he said in the 20-page opinion.

Justices David Souter, Ruth Bader Ginsburg and Stephen Breyer dissented. Souter said the law threatened to impose major burdens on the voting rights of tens of thousands of Indiana residents, especially the poor and the elderly.