WASHINGTON — The Supreme Court on Monday upheld Indiana’s voter-identification law, declaring that a requirement to produce photo identification is not unconstitutional and that the state has a “valid interest” in improving election procedures as well as deterring fraud.

In a 6-to-3 ruling in one of the most eagerly awaited election-law cases in years, the court rejected arguments that Indiana’s law, probably the strictest in the country, imposes unjustified burdens on people who are old, poor or members of minority groups and less likely to have driver’s licenses or other acceptable forms of identification.

The ruling, coming just eight days before the Indiana primary and at the height of a presidential election campaign, upheld rulings by a federal district court and the United States Court of Appeals for the Seventh Circuit, which had thrown out challenges to the 2005 law.

Justice John Paul Stevens, who announced the judgment of the court and wrote an opinion in which Chief John G. Roberts Jr. and Anthony M. Kennedy joined, alluded to — and brushed aside — complaints that the law benefits Republicans and works against Democrats, whose ranks are more likely to include poor people or those in minority groups.

The justifications for the law “should not be disregarded simply because partisan interests may have provided one motivation for the votes of individual legislators,” Justice Stevens wrote.

Justice Stevens and the two court members who joined him found that the Democrats and civil rights groups who attacked the law, seeking a declaration that it was unconstitutional on its face, had failed to meet the heavy burden required for such a “facial challenge” to prevail. Perhaps, they suggested, the outcome could be different in another voter-rights case, one in which a plaintiff could show that his or her rights had been violated.

Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. concurred in the judgment of the court, but went further in rejecting the plaintiffs’ challenge. In an opinion by Justice Scalia, the three justices said, “The law should be upheld because its overall burden is minimal and justified.”

Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer dissented. Justice Souter, in an opinion joined by Justice Ginsburg, said the Indiana law “threatens to impose nontrivial burdens on the voting rights of tens of thousands of the state’s citizens.”

Some Democrats have complained that those who succeeded in passing the law and fought on its behalf were citing problems that did not exist, because prosecutions for impersonating a registered voter are exceedingly rare, or non-existent. The real motivation of those behind the law was to hamper Democrats, those foes of the law have argued.

“This decision is a body blow to what America stands for — equal access to the polls,” said Senator Charles E. Schumer, Democrat of New York.

When the case was argued before the Supreme Court in January, there was considerable back-and-forth over how much of a burden the Indiana law could be in an age when an overwhelming majority of people old enough to vote also possess a driver’s license or other form of photo identification.

There was also discussion over how much voter fraud really exists, with some suggestions that the reason it has apparently never been prosecuted in Indiana is because those who commit fraud are good at it.

But, as Justice Stevens noted, there have been flagrant examples of voter fraud in American history. He cited the 1868 New York City elections, in which a local tough who worked for Tammany’s William (Boss) Tweed explained why he liked voters to have whiskers: “When you’ve voted ’em with their whiskers on, you take ’em to a barber and scrape off the chin fringe. Then you vote ’em again with the side lilacs and a mustache. Then to a barber again, off comes the sides and you vote ’em a third time with the mustache. If that ain’t enough and the box can stand a few more ballots, clean off the mustache and vote ’em plain face.”

In 2004, Justice Stevens noted in a footnote, the hotly contested gubernatorial election in Washington State produced an investigation that turned up 19 “ghost voters” and at least one confirmed instance of voter fraud.

Indiana usually goes Republican in presidential elections. Republicans control the State Senate, while Democrats hold a narrow advantage in the House. The governor, Mitch Daniels, is a Republican. When the 2005 law was passed, Republicans controlled both houses and were unanimously behind the law — while Democrats were unanimously opposed.

Lawyers who challenged the case cited the experience of one would-be Indiana voter, Valerie Williams, who was turned away from the polling place in November 2006 by officials who told her that a telephone bill, a Social Security letter with her address and an expired driver’s license were no longer sufficient.

“Of course, I threw a fit,” she said in a January interview with The New York Times, recalling how she cast a provisional ballot which was never counted. Ms. Williams, in her early 60’s, is black — and is a Republican.