WASHINGTON (AP) The Supreme Court rejoined the heated national debate over the death penalty yesterday, announcing it will decide whether the Constitution"s ban on "cruel and unusual punishment" bars execution of mentally retarded people.

The justices said they will hear an appeal by North Carolina death-row inmate Ernest McCarver, whose lawyers say he is retarded. The justices halted his execution early this month just hours before he was to have been put to death.

The justices decided in 1989 the Constitution allows execution of mentally retarded killers. McCarver"s lawyers say Americans" standards of decency have changed since then.

"There has been a substantial change in American society," his lawyers wrote in his appeal. "The penalty of death is plainly cruel when imposed on those whose culpability is lessened by their inability to reason."

The Constitution"s Eighth Amendment bans "cruel and unusual punishments." The question, McCarver"s lawyers contend, is whether a punishment offends contemporary standards.

Prosecutors said considerable evidence showed that McCarver was not mentally retarded, but they added that even if he was, his execution would not violate the Constitution.

North Carolina Gov. Mike Easley denied clemency, saying McCarver planned the 1987 stabbing and choking death of a co-worker, motivated by revenge.

This month, the justices blocked the execution of another man said by his lawyers to be borderline retarded. Antonio Richardson had been scheduled to be put to death March 7 in Missouri.

The court also plans to hear arguments today in another case involving a death-row inmate whose lawyers say he is mentally retarded.

The case involving Johnny Paul Penry of Texas does not ask whether the Constitution prohibits executing the mentally retarded. Instead, Penry"s lawyers say jurors who sentenced him to death for murder did not have the chance to properly consider his mental capacity.

It was not immediately clear how the justices" decision to hear McCarver"s case this fall will affect the Texas appeal.

But Penry"s lawyer, Robert S. Smith, said that if the Supreme Court decides the mentally retarded cannot be executed, "I would hope and believe that that decision will be applied to Penry."

The Supreme Court used Penry"s case in 1989 to rule that the Constitution allows the execution of mentally retarded killers.

There have been 702 inmates executed nationwide since a Supreme Court-ordered moratorium ended in 1977. Of those, about 35 had showed evidence of mental retardation in tests, said Richard Dieter of the Death Penalty Information Center, a group critical of how capital punishment is administered.

The 1972 ruling that halted executions nationwide said existing laws made capital punishment too arbitrary. Executions resumed in 1977, and during recent years the nation"s highest court and Congress have moved to limit and speed up death row inmates" appeals.

Recent debate over the death penalty has centered on arguments that some death-row inmates are innocent. Last year, Republican Gov. George Ryan of Illinois imposed a moratorium on capital punishment in his state after 13 inmates were exonerated.

McCarver, 40, was convicted of the January 1987 stabbing and choking death of Woodrow Hartley, a 71-year-old worker at the Concord, N.C., cafeteria where McCarver had worked.

The inmate"s lawyers say he has the mind of a 10-year-old and reads at third-grade level. The Supreme Court halted his execution on March 1 after he had been served his last meal.

In denying clemency, Easley said McCarver was competent enough to gain employment and earn driving privileges, and no court had found him incompetent.

Thirteen capital-punishment states prohibit execution of the mentally retarded, McCarver"s lawyers said. Another 12 states do not have capital punishment. Those states, combined with the federal government and District of Columbia, mean that most U.S. jurisdictions bar such executions, his lawyers said.

McCarver"s most recent IQ test, arranged by the defense team, put his score at 67, but his IQ was measured at between 70 and 80 before his 1988 trial.

Kent Scheidegger of the Criminal Justice Legal Foundation said he believed most people would oppose executing the mentally retarded, but he believed it was preferable to act through legislation rather than a court ruling.

Diann Rust-Tierney, director of the American Civil Liberties Union capital punishment project, said,"The time is ripe for the court to take a look at this. We"re hopeful that the court will see that the public"s views on this have changed, and that under evolving standards of decency it is untenable to execute the mentally retarded."