WASHINGTON – A divided Supreme Court decided Thursday that judges must recuse themselves from cases in which they played a significant previous role in prosecuting the person before them.

The court ruled 5 to 3 that Terrance Williams, an inmate on Pennsylvania’s death row, was right that a former Pennsylvania supreme court justice who had made the decision 30 years earlier to seek the death penalty against him should not have later considered Williams’s appeal.

“Where a judge has had an earlier significant, personal involvement as a prosecutor in a critical decision in the defendant’s case, the risk of actual bias in the judicial proceeding rises to an unconstitutional level,” wrote Justice Anthony Kennedy, who was joined by the court’s liberal justices.

He wrote that former Pennsylvania Chief Justice Ronald Castille – Philadelphia’s former district attorney who was elected to the state’s high court and is now retired – should have stepped aside.

“The due process guarantee that ‘no man can be a judge in his own case’ would have little substance if it did not disqualify a former prosecutor from sitting in judgment of a prosecution in which he or she had made a critical decision,” Kennedy wrote.

Chief Justice John Roberts disagreed. It might have made sense for Castille to recuse, Roberts wrote, but his involvement in the case did not rise to the level of a constitutional violation.

“The majority opinion rests on proverb rather than precedent,” Roberts wrote, ignoring previous rulings that there is “a presumption of honesty and integrity in those serving as adjudicators.”

On the specific procedural claim being considered by the Pennsylvania Supreme Court, Roberts added, “Williams does not allege that Chief Justice Castille had any previous knowledge of the contested facts at issue . . . or that he had previously made any decision on the questions raised by that petition.”

Justice Samuel Alito joined Roberts’s dissent, and Justice Clarence Thomas filed his own objection.

The majority threw out a 2014 ruling by the state high court that upheld Williams’s death penalty, after a lower court had decided he should get a new hearing.

The complicated case began in 1986, when the teenaged Williams, a star football player, killed church deacon Amos Norwood and set his body on fire. Then-prosecutor Castille authorized his lawyers to seek the death penalty, and Williams once came within days of being executed.

Prosecutors said the crime was tied to a robbery. But during the appeals process, lawyers for Williams found evidence that the teenager had been sexually abused by Norwood, something Williams himself had not acknowleged. A lower court found Castille’s prosecutors had withheld that evidence, although there was no claim that Castille knew that.

As part of his campaign for the elected state supreme court, Castille had made an issue of his success in securing the death penalty.

Kennedy said neither the passage of time nor the fact that the rest of the state high court turned down Williams’s appeal alleviated Castille’s duty to recuse.

“The fact that the interested judge’s vote was not dispositive may mean only that the judge was successful in persuading most members of the court to accept his or her position,” Kennedy wrote. “That outcome does not lessen the unfairness to the affected party.”

The decision means that Williams should receive a new hearing. As a practical matter, there is a moratorium on executions in Pennsylvania.

The case is Williams v. Pennsylvania.

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