Washington ? Juries, not judges, must make the crucial decisions that determine whether a convicted killer lives or dies, the Supreme Court ruled Monday, a finding that could overturn scores of death sentences nationwide.

The ruling capped a court term that death penalty opponents called the most promising in a quarter-century.

The high court declared the sentencing laws of five states unconstitutional. The ruling called into question whether 168 death row inmates in those states will be put to death, and it also could affect some inmates sentenced in four other states, lawyers said.

The 7-2 ruling affects systems in which judges have the final say after a jury convicts someone of murder.

The Constitution guarantees a trial by a jury, and that right extends to weighing whether a particular killing merits death or life in prison, Justice Ruth Bader Ginsburg wrote for an unlikely alliance of conservative and liberal-leaning justices.

The court rejected arguments from Arizona, where Monday’s case originated, that judges could be more evenhanded and less emotional than juries.

“The Sixth Amendment jury trial right … does not turn on the relative rationality, fairness or efficiency of potential fact-finders,” Ginsburg wrote for herself and Justices John Paul Stevens, Antonin Scalia, Anthony M. Kennedy, David H. Souter and Clarence Thomas.

Justice Stephen Breyer wrote a separate opinion to agree with the outcome.

Neither this case nor last week’s ruling exempting mentally retarded people from the death penalty directly attacks the constitutionality of capital punishment for the general population.

Still, these and other rulings this term questioned the way states carried out the death penalty, and might open the door for wider examination of the practice, lawyers and death penalty opponents said.

“When you take last week’s ruling and this one combined, this is the most favorable Supreme Court term in a quarter century,” for those who want to see the practice ended, said David Elliot, spokesman for the National Coalition to Abolish the Death Penalty.

The Supreme Court allowed states to resume executions in 1976 after a brief national hiatus.

Nationwide, about 3,700 inmates await execution in the 38 states that allow capital punishment.