On Wednesday, Mr. Trump retweeted calls for Mr. Barr to “clean house” at the Justice Department, apparently referring to anyone with the temerity to maintain allegiance to the Constitution rather than to the president.

Characteristically, while interfering in the justice system, Mr. Trump is both insisting he’s doing nothing of the sort and asserting his right to do so. “Just so you understand, I chose not to be involved,” he said Tuesday in response to a question about his meddling in Mr. Stone’s case. “I’m allowed to be totally involved.”

This is in the style of autocrats across the globe, who weaponize the law to help themselves and their friends and hurt their enemies. The nation’s legal system is now run by a man who has spent his life mocking it.

He is trying to turn the people charged with federal law enforcement into presidential fixers, starting with Mr. Barr, who finds himself in a tight spot. Whether or not you believe him when he says the president’s tweets make his job “impossible,” the fact is that he said it, and that Mr. Trump ignored him. And if you do believe him, then you have to ask why, if his job has become impossible, he hasn’t resigned already. (You might also ask just what Mr. Barr considers his job to be.) Mr. Barr becomes only the latest in a long line of powerful officials Mr. Trump has seemed to take delight in publicly humiliating, including, to name just a few: Jeff Sessions, Rex Tillerson, H.R. McMaster, Jim Mattis, Kirstjen Nielsen and John Kelly.

This is the second time in half a century that a lawless chief executive has tested the nation’s fundamental constitutional design. The first time, we passed. But now we know that the mechanisms put in place after Watergate were not sufficient. One potential new safeguard would be for Congress to make the Justice Department more independent, like the Federal Reserve, thus insulating it from the self-interested meddling of unethical presidents. The Supreme Court is currently considering the scope of Congress’s power to establish independent agencies. However it rules, the current checks are demonstrably not strong enough.

If there is anything useful to draw from Mr. Trump’s degradation of the rule of law and the powers of his office, it’s that he is exposing a critical vulnerability in the Constitution’s design, which anticipated presidents behaving badly, but not this badly. To be fair, the founders did include two backstops to guard against a lawless chief executive: One is impeachment. The other is a quadrennial election.