The Supreme Court overturned the death sentences of more than 150 murderers on Monday, ruling that only juries and not judges may make the life-or-death decision to condemn a killer.

The 7-2 ruling affects the way death sentences are imposed in at least five states and means that scores of death sentences must be reconsidered or commuted to life in prison.

Monday's ruling concerned instances in which juries determined defendants' guilt or innocence and judges alone decided their punishment. The court held that such a sentence imposed by a judge violates a defendant's constitutional right to a trial by jury.

It was the second major Supreme Court ruling in less than a week affecting the ways that states sentence people to death. Last week, the justices divided bitterly in exempting mentally retarded people from execution.

None of the cases attacks the basic constitutionality of capital punishment for the general population.

The court has also agreed to hear an appeal in the fall from Tennessee death row inmate Abu-Ali Abdur'Rahman. That case could have far-reaching effects if the justices decide to loosen the rules for when condemned inmates can get new evidence before a judge.

Nationwide, about 3,700 people await execution for crimes committed in the 38 states that allow the death penalty.

In some states juries determine guilt or innocence, but a judge then can base a death sentence on aggravating factors such as the heinous nature of a murder or whether it was committed for monetary gain.

Monday's ruling turned on the Constitution's guarantee of a jury of one's peers and a Supreme Court ruling two years ago that struck down another kind of sentence determined by a judge instead of a jury.

Justice Ruth Bader Ginsburg, writing for a majority that included an unusual alliance of conservative and liberal-leaning justices, said the court's 2000 ruling in a case called Apprendi v. New Jersey cannot be reconciled with the death penalty sentencing laws in Arizona and four other states in which one or more judges impose the sentence.

The Apprendi case concerned a judge's ability to lengthen a sentence by two years if a crime was determined to be a hate crime. The high court struck down that sentencing law.

"The right to trial by jury guaranteed by the Sixth Amendment would be senselessly diminished if it encompassed the factfinding necessary to increase a defendant's sentence by two years, but not the factfinding necessary to put him to death," Ginsburg wrote. "We hold that the Sixth Amendment applies to both."

Ginsburg was joined by Justices John Paul Stevens, Antonin Scalia, Anthony M. Kennedy, David H. Souter and Clarence Thomas. Justice Stephen Breyer wrote separately to agree with the outcome.

The case concerned an Arizona inmate, and the ruling will immediately apply in that state and in Idaho and Montana, where a single judge decides the sentence. It will also apply immediately in Colorado and Nebraska, where a panel of judges makes the sentencing decision.

It was not immediately clear what will happen to inmates in those states. Some lawyers have said death row inmates' sentences could be commuted to life in prison, as was done when the Supreme Court put a temporary halt to the death penalty in the 1970s. Or the inmates could be resentenced, with some receiving death sentences all over again.

Also unclear was whether the ruling will have a spillover effect in four other states in which juries only recommend whether a convicted murderer should receive the death penalty or life in prison: Florida, Alabama, Indiana, and Delaware.

A judge makes the final call in those states. Indiana, however, recently passed a law that will require judges to follow a jury's sentencing recommendations.

In dissent, Justice Sandra Day O'Connor predicted that many inmates in the additional four states will challenge their sentences now.

The earlier Apprendi ruling "had a severely destabilizing effect on our criminal justice system," O'Connor wrote in a dissent joined by Chief Justice William H. Rehnquist "The decision today is only going to add to these already serious effects."

Arizona has 129 people on death row, Idaho 21 and Montana six. Colorado has five, and Nebraska seven. Florida has 383, Alabama 187, Indiana 39 and Delaware 20.

Timothy Stuart Ring was convicted of killing an armored car driver during a 1994 robbery in Phoenix.

Ring challenged his sentence and Arizona's law on grounds that his constitutional right to a jury was violated when a judge held a separate hearing after the jury that convicted Ring was dismissed.

The judge heard testimony at a sentencing hearing from an accomplice who said Ring planned the robbery and murdered the guard. The judge then determined that the aggravating factors warranted death.

"I was essentially given two trials," Ring said in an interview. "One before a jury and then one before a judge."

The Arizona Supreme Court rejected Ring's constitutional challenge last year.

Ring's case put the court in an awkward position. The high court had already upheld the constitutionality of Arizona's law in 1990, but that was before its ruling in Apprendi v. New Jersey.

Finding the two rulings irreconcilable, the high court took the rare step of overturning one of its own fairly recent decisions. The first decision was written by O'Connor, who defended it in her dissent Monday.

The case is Ring v. Arizona, 01-488.

The Associated Press contributed to this report.