Washington, June 20--(AP) The U.S. Supreme Court has declared that executing mentally retarded murderers is unconstitutionally cruel, offering the possibility of reprieve to scores of inmates in the biggest shift in the court's stance on capital punishment in a quarter-century.

Many inmates in the 20 states that theoretically allow execution of retarded people can be expected to argue that their sentences should be converted to life in prison.

Mentally retarded people should still be tried and punished when they ``meet the law's requirements for criminal responsibility,'' Justice John Paul Stevens wrote for the majority in Thursday's 6-3 ruling.

``Because of their disabilities in areas of reasoning, judgment, and control of their impulses, however, they do not act with the level of moral culpability that characterizes the most serious adult criminal conduct,'' he wrote.

The ruling was part of a piecemeal examination of capital punishment laws the court undertook this year, 26 years after reinstating the death penalty. The court is expected to rule next week on whether judges, not juries, can impose a death sentence. That ruling could affect 800 inmates in nine states. There are more than 3,700 death row inmates in the United States.

The court ruled Thursday in favor of a Virginia inmate, Daryl Renard Atkins, who was convicted of shooting an Air Force enlisted man for beer money in 1996. Atkins' lawyers say he has an IQ of 59 and has never lived on his own or held a job.

``The decision is consistent with increased concern about application of the death penalty,'' said Diann Rust-Tierney, director of the American Civil Liberties Union's capital punishment project.

``It reflects a true consensus that the death penalty should be reserved for the most culpable and a recognition that people with mental retardation do not fit that category.''

Recent concern over the fairness of capital punishment has focused on death row exonerations based on DNA evidence, and questions about the quality of lawyers given to defendants too poor to hire their own. Two states have put executions on hold until more questions are answered.

The high court's most conservative members, all confirmed supporters of capital punishment, filed two overlapping and unusually bitter dissents. They accused the majority of substituting personal views for the law and of relying too heavily on public opinion.

The ruling is confined to mentally retarded defendants convicted of murder and does not address the constitutionality of capital punishment in general.

Justice Antonin Scalia referred to the majority's ``embarrassingly feeble evidence of consensus,'' and accused the majority of ``thrashing about'' to justify the outcome it wanted.

At the White House, spokesman Ari Fleischer pointed to President George W. Bush's previous statement opposing executing the retarded, and reiterated that Bush supports capital punishment ``for those people who commit violent, heinous crimes.''

Opponents of the death penalty say two mentally retarded inmates were executed while Bush was governor of Texas.

In the future, Thursday's ruling will mean that people charged with a killing will not face a death sentence if they can show they are retarded, generally defined as having an IQ of 70 or lower.

The court left it to states to develop their own systems to ensure that mentally retarded people are not executed.

Death penalty supporters and some states that have allowed such executions predicted a tide of lawsuits.

``Do you say that someone is mentally retarded if the IQ is 70 or below? Or do you also use ... behavior of the individual, or both?'' asked Louisiana Attorney General Richard Ieyoub. ``Will a defense attorney question the definition? Certainly.''

The case turned on the Constitution's protection against cruel and unusual punishments, and how to define those terms today.

Now, 18 of the 38 states that allow the death penalty exempt mentally retarded people. Twelve states and the District of Columbia do not impose the death penalty.