Special Counsel Jack Smith does not want the justices of the Supreme Court to worry that prosecuting presidents will one day be as American a pursuit as baking apple pie. That is because he hopes that they will let him prosecute President Trump, who is claiming “absolute” presidential immunity. Not to fret, Mr. Smith reassures — there is no danger of these kinds of prosecutions becoming de rigueur.

Color us unconvinced. As our A.R. Hoffman reports, Mr. Smith’s promise in his brief that his prosecution will not be precedent could be more honored in the breach than the observance. It could also fail to quell a parade of horribles from dancing before the justices’ eyes. The special counsel cites “strong institutional checks to ensure evenhanded and impartial enforcement of the law.” Mr. Smith’s argument for a narrow immunity depends on these “checks.” 

That is because courts have long held that the prerogatives of the presidency are deserving of protection. The Constitution provides that he “shall take Care that the Laws be faithfully executed,” and the executive is the only branch of government in which the Framers vested all the power in one person. The Department of Justice does not prosecute sitting presidents. Whether former ones are so protected is now the issue before the Nine.

Mr. Trump contends that the chilling effect on what Justice Antonin Scalia called the “boldness of the president” will be no less frigid if prosecutions can be brought the moment a term ends. That’s where Mr. Smith’s “checks” come in. He cites grand and petit juries, the presumption of innocence, and other “protections to guard against politically motivated prosecutions.” Is this laundry list, though, a constitutional firewall or a prosecutorial turnstile?

Take grand juries. Chief Justice Warren once described grand juries as a protection of the accused. Yet a study in 2014 disclosed that these juries hand up indictments in 99.99 percent of federal cases. Who thinks that the trial-jury pools at, say, New York City, Atlanta, and the District of Columbia will be free from political bias? Fani Willis could yet have to step down from her prosecution of Mr. Trump for trying to poison the jury pool.

These protections assume greater import because precedent appears to cut against Mr. Trump. In United States v. Nixon, the high court held that “neither the doctrine of separation of powers nor the generalized need for confidentiality of high-level communications, without more, can sustain an absolute, unqualified Presidential privilege of immunity.” Mr. Trump has his arguments for why that does not pertain, but his sledding will be uphill.   

Mr. Smith’s mention of “politically motivated prosecutions” telegraphs that he is aware the critique could have teeth when oral arguments are made before the justices on April 25. Ms. Willis’ erstwhile special prosecutor and boyfriend, Nathan Wade, twice met with lawyers at the White House before handing up an indictment. Mr. Smith was at the White House, too, before his charges were filed. That seems neither checked nor balanced.

Our point here is that if, when it comes to the presidency, one has a cramped vision of immunity, one needs to have greater safeguards against prosecutorial abuse. In Nixon, the Supreme Court opened the door to abuse, and within a generation, and several independent counsels or special prosecutors later, Congress simply decided to let the independent counsel statute expire, unmourned. They had stopped trusting the concept.

It’s not that the independent counsels — or, in Mr. Smith’s case, special counsels — are bad persons. It’s that in cases where the prosecutorial power is checked neither by politics or immunity, the temptation to abuse would be hard to avoid, even by an alabaster saint. What American watching the progress of the campaign to convict Mr. Trump of something — anything — will be comforted by Jack Smith’s argument that adds up to “trust us”?