“Lock him up.” That was the hope expressed by President Biden in respect of his predecessor during a campaign stop to rally those working to make Vice President Harris his successor. It could be the 46th president’s final time on the trail, which now appears to have curved around full circle. Mr. Biden ran as a guardian of democratic norms and a restorer of faith in government. Now, he’s swiped the same language Trump chanted at Secretary Clinton.

The reversal is a moment to mark how the criminal prosecutions of the 45th president have mixed law and politics to a degree hitherto unseen in American politics. That Mr. Biden added “Politically lock him up. Lock him out. That’s what we have to do” suggests that Mr. Biden understood he had erred. His edit, though, does little to allay the due process concerns that arose when Special Counsel Jack Smith was hired to pursue Trump.

The entanglement between Mr. Smith’s January 6 prosecution and the presidential campaign reached new levels when Judge Tanya Chutkan released the special counsel’s 165-page immunity opus. Trump, calling that document “politically motivated,” and to persuade her that its publication would violate the Department of Justice’s guidelines against election interference. She insisted that those guidelines were not hers to enforce.

The appointment of Mr. Smith was meant to distance the White House from the prosecution of its former occupant, though too remote a relationship between president and prosecutor would, as Justice Antonin Scalia argued in Morrison v. Olson, present its own problem to separated powers and leave exposed what Scalia called the “boldness of the president.” Mr. Biden appointed Mr. Smith two days after Trump declared his intention to regain the White House.

At times the patina of arm’s length distance that Mr. Smith was intended to provide has flaked off. After Judge Aileen Cannon ruled that Mr. Garland’s appointment of Mr. Smith was unlawful and dismissed the Mar-a-Lago case, the attorney general took to television to, in a fit of pique, accuse her of making “a basic mistake about the law.” The New York Times reported that Mr. Garland’s slowpoke cases against Trump have incurred his boss’s ire.

Then there was Mr. Smith’s rush to try Trump before the election, which involved petitioning the Supreme Court for an expedited hearing that he admitted was “extraordinary.” The special counsel argued that the public possessed a “compelling” interest in the case’s outcome before the election. That deadline is a political, not a legal one — and anyway, the Constitution grants the right to a “speedy” trial to the “accused.”

We noted a crossing of the Rubicon when Ms. Harris featured Mr. Smith’s immunity opus — full of allegations Trump has not yet had the opportunity to rebut — in a campaign advertisement called “Bombshell.” The fate of Mr. Smith’s cases cannot help but be tied to the outcome of the election. A victory for Trump would likely end them, while a triumph for Ms. Harris could give the special counsel another four years with which to work.

It is no coincidence that reams of the evidence adduced by Mr. Smith matriculated to his brief from the work of the House January 6 committee. That body, by convening a trial the Constitution forbids to Congress, flirted with the Constitution’s prohibition on attainder. Its work could be, as lawyers say, fruit of the poisonous tree. No wonder Mr. Biden and Ms. Harris are turning to Mr. Smith for their closing argument.

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Correction: Fruit of the poisonous tree is the legal doctrine that could implicate evidence derived from the January 6 Committee. The term was rendered incorrectly in the bulldog.