Washington ? In a landmark decision reflecting a new national debate on the death penalty, the Supreme Court on Thursday categorically ruled out the execution of the mentally retarded.

The sweeping and unqualified 6-3 decision was the first in a generation to narrow the death penalty substantially, and legal experts said it signaled that the justices may curtail its use in some other situations as well.

The ruling’s forceful tone indicates that “the court is saying it is going to require more justification from states generally, not just with the retarded, for their broad death penalty statutes,” said Michael Millemann, a law professor at the University of Maryland. But there is no indication the court is ready to outlaw the death penalty or to prevent its application to children over 15, he said.

The opinion by Justice John Paul Stevens said that executing the retarded was “cruel and unusual punishment” proscribed by the Eighth Amendment to the Constitution. Just 13 years ago the court reached the opposite conclusion, but since then the number of states prohibiting the execution of the retarded has risen from two to 18, reflecting a changed national consensus, Stevens said. Some 14 other states reject capital punishment completely.

The ruling gives a new chance at life to Daryl Atkins, who was sentenced to death in Virginia for kidnapping an Air Force enlisted man at a convenience store in 1996, robbing him with an accomplice, and then driving him to a deserted area and shooting him eight times, killing him, while he begged for his life.

A defense psychiatrist testified that Atkins, with an IQ of 59, was mildly retarded, which some medical texts define as having an IQ between 50 and 69, corresponding to a mental age of 9 to 12 years. But a prosecution expert said Atkins was of “average intelligence.”

The court did not define mental retardation Thursday, and sent his case back to the lower courts. Atkins’ ultimate fate will likely take years of litigation to decide.

Regardless of what happens to Atkins, the decision in his case Thursday could give new momentum to the anti-death penalty movement, which had been revived recently by cases where inmates convicted of killings were found by DNA or other evidence to be innocent.

“We cannot ignore the fact that in recent years a disturbing number of inmates on death row have been exonerated,” Stevens said Thursday in an unusual footnote. Justice Sandra Day O’Connor, who in the past has voted to uphold most death penalty cases, raised the possibility that innocent people have been put to death in an anguished speech last year. O’Connor and fellow swing conservative Anthony Kennedy both voted Thursday with Stevens, joining without reservation in an opinion that some experts said sounded like those of the liberal Warren Court era.

Stevens cited public opinion polls, the views of professional and religious organizations and even the laws of European countries in broadly asserting that the justices have a personal role in deciding what is cruel and unusual where some evidence of a consensus exists.

Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas dissented.

Scalia’s dissent dripped with sarcasm and anger. He asserted that the decisions of a small number of states hardly establish a consensus, and that the ruling’s majority were simply asserting their personal preferences. The Eighth Amendment, he said, was meant to outlaw punishments like the rack and the thumbscrew.

“The arrogance of this assumption of power takes one’s breath away,” Scalia wrote.

In 1972 the court threw out all existing death penalty laws, saying they were arbitrary and irrational. But in 1976 it approved new state laws designed to meet the court’s objections, and since then had gradually broadened the permissible use of the ultimate penalty.But the decision was welcomed by some key Democrats and Republicans.

“This is another sign of the fundamental re-examination that is under way at all levels of government about how the death penalty is being carried out today,” said Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee.