Buried in a footnote in the Supreme Court’s landmark immunity decision in Trump v. United States is a loophole that could result in the prosecution of a president — possibly Donald Trump. 

The high court, by a 6-to-3 margin, held that a president’s official acts are presumptively immune, and unofficial ones do not possess immunity. Actions that are at the heartland of presidential prerogative, like dispensing pardons, are “absolutely” immune.

The court went further and ruled that official acts are not only non-prosecutable, but also can’t be used as evidence. Justice Amy Coney Barrett departed from her conservative colleagues on this head, writing that the “Constitution does not require blinding juries to the circumstances surrounding conduct for which Presidents can be held liable.” She did not carry the day.  

That is not the end of the story, though. In the opinion’s third footnote, Chief Justice Roberts carves out an exception. He writes that “the prosecutor may admit evidence of what the President allegedly demanded, received, accepted, or agreed to receive or accept in return for being influenced in the performance of the act.” While a pardon, say, is likely still protected, evidence can be adduced for the surrounding machinations. Mr. Smith could work to extend that logic to the acts at the heart of his prosecution. 

The high court remanded the case to Judge Tanya Chutkan, the jurist overseeing Mr. Smith’s January 6 prosecution, to determine which of the acts in the indictment are official, a process that will likely unfold over months of litigation, across so-called mini-trials where Trump’s team and the government will wrangle over what can be charged and what is immune. The justices mandated that all of this be ironed out before a trial begins.

Expect oral arguments around Trump’s interactions with Vice President Pence and the plan to use “alternate electors” to be especially pitched. Mr. Smith will bear the burden — a high one — of showing that any act he cites is “unofficial.” The Supreme Court instructs lower courts to carve out a wide berth of protection for presidential activity, lest what Justice Antonin Scalia called the “boldness of the president” be impaired.  

The chief justice explains that a “prosecutor may point to the public record to show the fact that the President performed the official act,” but he may not “admit testimony or private records of the President or his advisers probing the official act itself.” If that evidence is fair game, it could “threaten the independence or effectiveness of the Executive.”

Chief Justice Roberts’s loophole closely tracks the federal bribery statute. That law is not unknown to either Mr. Smith or the chief justice. It surfaced in McDonnell v. United States, from 2016. A unanimous court, in a decision written by the chief, vacated the convictions of Governor McDonnell and his wife, Maureen, finding that the relevant statute’s reference to an “official act” had been read too broadly by prosecutors and lower courts.

In McDonnell, Chief Justice Roberts wrote that “there is no doubt that this case is distasteful; it may be worse than that. But our concern is not with tawdry tales of Ferraris, Rolexes, and ball gowns. It is instead with the broader legal implications of the Government’s boundless interpretation of the federal bribery statute.” Here, he protects the possibility of a bribery prosecution.

A decade ago, before Mr. Smith decamped to the Hague to prosecute war crimes, he was in charge of the Department of Justice’s Public Integrity Section, which brought the case against the McDonnells, who are now divorced. Now, Chief Justice Roberts appears to be signaling that evidence of a bribe could be admissible, notwithstanding the new broad rule of immunity.

The origin of the chief’s footnote could lie in a hypothetical he posed in oral arguments, in April. He asked Trump’s attorney, John Sauer, whether immunity would attach if “the president appoints a particular individual to a country, but it’s in exchange for a bribe.” The Constitution ordains that the president “shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors.” 

In a fiery dissent, Justice Sonia Sotomayor wonders about the license given by immunity to abuse of the president’s core responsibilities. Under the majority’s holding, she asks what would happen if a president “Orders the Navy’s Seal Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune.”

Justice Sotomayor’s mention of a corrupt pardon underscores the exalted place the Constitution — and now, the high court — affords to the pardon power. Article II Section 2 vests the “Power to Grant Reprieves and Pardons for Offences against the United States” in the president. The Trump court holds that the power to dispense pardons is one that is in the “exclusive and preclusive” purview of the president and entitled to “absolute” immunity.

The possibility of litigation over a pardon has come into high relief with the inevitability that Mr. Smith’s four January 6 charges against Trump will not be resolved until after — possibly well after — November’s elections, and a potential victory for the 45th president. While a self-pardon would not appear to be comprised in Chief Justice Roberts’s exposition of bribery, his footnote suggests that it possibly be used as evidence as part of a renovated case.  

At the moment, Trump would have no need of a pardon for Mr. Smith’s Mar-a-Lago case because Judge Aileen Cannon has dismissed those charges. On Thursday, though, the United States Court of Appeals for the 11th Circuit ordered the government’s brief to be submitted by August 27, to be followed a month later by Trump’s. Throw in reply briefs, and the riders have laid out a schedule through the middle of October. The special counsel, though, could yet move for an expedited appeal.