Will the United States Supreme Court act to reinstate what advocates call “the first significant sexual assault conviction of the #MeToo era” and send Bill Cosby back to prison? That’s the question after a Pennsylvania prosecutor petitioned the Nine to reverse the decision of Pennsylvania’s Supreme Court to vacate the conviction of Mr. Cosby for sexual assault. Or will Mr. Cosby stay free and his prosecutor remain under a cloud?

Mr. Cosby’s wrongful conviction on sexual assault charges was brought in in 2018. He was serving in prison a sentence of three-to-ten years when, in June of this year, Pennsylvania’s Supreme Court reversed all of the convictions he was dealt. The high court concluded that Mr. Cosby’s rights were violated when the local district attorney’s office broke a 2005 pledge it had made not to prosecute him.

That pledge was made by the prosecutor, Bruce Castor, and for a reason. He had concluded that he couldn’t win the case. To salvage something of it, we wrote in an editorial called “Redeeming Pennsylvania’s Promise,” Mr. Castor promised Mr. Cosby that if he cooperated in the civil case brought by Mr. Cosby’s accuser, Pennsylvania would never levy criminal charges in the matter.

“Fair enough,” we wrote. Mr. Cosby forsook his Fifth Amendment right against self-incrimination and testified in the civil case, and, we wrote, “The result was not nothing.” His accuser won a civil settlement of $3.38 million. Years later, though, as sexual assault emerged as a national issue, Pennsylvania broke its agreement, or so the Quaker State’s top court concluded.

“There was,” we noted at the time, “a good bit of dispute about how serious an agreement it was — and whether it was properly laid before a judge, and the like.” The question, though, couldn’t be more serious, involving, as it does, our most fundamental constitutional principles of due process. When the Cosby case finally got to the state’s supreme court, the justices ruled six to one that the agreement was binding.

“We hold,” they wrote in no uncertain terms, “that, when a prosecutor makes an unconditional promise of nonprosecution, and when the defendant relies upon that guarantee to the detriment of his constitutional right not to testify, the principle of fundamental fairness that undergirds due process of law in our criminal justice system demands that the promise be enforced.”

The plea for a reversal of Mr. Cosby’s conviction was filed by Kevin Steele, the district attorney of Montgomery County, which is adjacent to Philadelphia County, where the Constitution was framed in a proceeding so sacred that the people of Philadelphia spread dust on the streets outside Constitution Hall so passing carriages would make less noise. They didn’t want the Framers to be disturbed in their work.

Two centuries later, Mr. Steele is now arguing that the appeal is necessary to avoid setting a precedent “that prosecutors’ statements in press releases now seemingly create immunity.” Yet where is the logic of that? Why should the solution be to deny due process to Mr. Cosby? Why shouldn’t the solution be for the prosecutors to stand by the words they said to the press? Should prosecutors be protected from public prevarication?

Americans are appalled at the things Mr. Cosby was accused of. That’s no excuse for forsaking the Bill of Rights, which ordains that no person shall be “deprived of life, liberty, or property, without due process of law.” When county prosecutors get to the Supreme Court, let the justices deliver a stern defense on due process and the importance of keeping their word. It’s the best thing for all, including victims of sexual violence.