The chief justice of the Pennsylvania Supreme Court voted in 2014 to reverse a lower court decision that gave relief to a criminal defendant who, the lower court concluded, was the victim of prosecutorial misconduct. The chief justice even wrote separately to make clear just how wrong he thought the lower court decision was.

At first glance this might seem like nothing unusual, but the conduct the chief justice was reviewing was that of lawyers he had supervised as a district attorney.

On Monday, the U.S. Supreme Court heard oral argument in Williams v. Pennsylvania, in which the was asked to decide whether the Pennsylvania chief justice’s decision to participate in that case was lawful.

The facts in Williams are truly stunning. In 1986, Terrance Williams was convicted of murder and sentenced to death in Pennsylvania state court. At the time of the trial and sentencing, Ronald Castille headed the Philadelphia office that prosecuted Williams, and he personally approved the pursuit of a death sentence in that case.

Decades later (and after Williams’ initial appeals were unsuccessful), a post-conviction court found that the prosecutors in the case had “plainly suppressed” evidence that Williams had been the victim of sexual abuse at the hands of the man he killed. Based on this finding, the post-conviction court granted Williams a new penalty hearing.

By the time the state appealed that decision, Castille had become chief justice of the Pennsylvania Supreme Court — the court that was being asked to decide whether prosecutors in his district attorney’s office had engaged in misconduct while prosecuting Williams. Williams asked that Castille recuse himself, but Castille refused. By doing so, Castille created a judicial conflict so extreme that it violated the Constitution’s guarantee of an impartial justice system.

By the time our nation’s founders drafted the Constitution, the importance of impartial adjudicators was well-established. The framers fully embraced this idea as central to a fair justice system, and this commitment was magnified in the years after the Civil War as the framers of the 14th Amendment witnessed, among other things, widespread maladministration of justice in the South.

Rep. Bingham of Ohio, Pennsylvania-born and principal drafter of Section 1 of the 14th Amendment, explained it this way: The 14th Amendment was intended to ensure “due process of law … which is impartial, equal, exact justice.”

The U.S. Supreme Court has repeatedly reaffirmed that promise. Indeed, at oral argument in Williams this week, Justice Sotomayor noted the court’s “jurisprudence that says you can’t be … a prosecutor and a judge.”

And as recently as 2009, in an opinion for the court, Justice Kennedy explained that there is a constitutional violation if “‘under a realistic appraisal of psychological tendencies and human weakness,’ the [judge’s interest in the case] ‘poses such a risk of actual bias or prejudgment that the practice must be forbidden if the guarantee of due process is to be adequately implemented.'”

The interest in this case plainly does. After all, Castille personally approved pursuit of the death penalty in the case; as Justice Kagan noted at argument, he “made the most important decision that could be made in this case.”

And the issue Castille was asked to decide on appeal was whether the trial prosecutor in Williams’ case — that is, an attorney on Castille’s staff and for whose conduct he was ultimately responsible — suppressed evidence in violation of the law, as the lower court found.

An affirmation of the lower court’s order would necessarily impugn the reputation of the office Castille had led and thus his own reputation as well. A judge with that level of personal interest in a case, who participates in deliberations, potentially influences his colleagues’ votes and then votes in the case himself, creates not only the appearance of unfairness but also the probability of it — both of which the Constitution prohibits.

Whether the American people trust in our justice system depends, in significant part, on whether they can trust the judges who make up that justice system to be impartial.

It’s always difficult to predict what the Supreme Court will do based on oral argument, but it’s clear what the justices should do: They should recognize that Chief Justice Castille’s participation in Williams’ case was a clear violation of the Constitution.