In a 5-4 vote, the court said the Louisiana law allowing the death penalty to be imposed in such cases violates the Constitution's ban on cruel and unusual punishment.

"The death penalty is not a proportional punishment for the rape of a child," Justice Anthony Kennedy wrote in his majority opinion. His four liberal colleagues joined him, while the four more conservative justices dissented.

There has not been an execution in the United States for a crime that did not also involve the death of the victim in 44 years.

Patrick Kennedy, 43, was sentenced to death for the rape of his 8-year-old stepdaughter in Louisiana. He is one of two people in the United States, both in Louisiana, who have been condemned to death for a rape that was not also accompanied by a killing.

The Supreme Court banned executions for rape in 1977 in a case in which the victim was an adult woman.

Forty-five states ban the death penalty for any kind of rape, and the other five states allow it for child rapists. Montana, Oklahoma, South Carolina and Texas allow executions in such cases if the defendant had previously been convicted of raping a child.

The court struggled over how to apply standards laid out in decisions barring executions for the mentally retarded and people younger than 18 when they committed murder. In those cases, the court cited trends in the states away from capital punishment.