And yet we know that Mr. Trump’s speech and tweets led to a violent insurrection. Now that Mr. Trump knows he could get away with that, how much worse would things get in a second term? The most urgent danger is his possible abuse of the legal system, because as the dissent suggests, if every conversation between the president and the Justice Department is considered a protected official act, there is no limit to the kinds of illegal conduct that could be plotted, even fabricating evidence.

What doesn’t count as an official act? The justices in the majority would not say, but it is hard to identify any clear guiding principle — perhaps because they couldn’t find any.

Prior to this decision, there was no grant of criminal immunity to presidents; though the authors of the Constitution gave a form of that privilege to members of Congress, they declined to do so for the chief executive. For a conservative majority that pretends to rely on historical precedent, the newly created standard is remarkable for its lack of basis in the Constitution, law or any precedent of the court. It was made up out of thin air.

The product of the majority’s invention runs counter to the entire notion of a government based on the rule of law. It also runs counter to the long-settled understanding of a president’s exposure to criminal prosecution, regardless of whether his acts were considered “official.” As Justice Sotomayor pointed out, why would Richard Nixon have accepted a pardon for his role in the Watergate scandal if not because everyone agreed that he could otherwise be prosecuted for his actions?