“I have so far chosen not to!” President Trump tweeted Friday morning, describing his asserted right to interfere with criminal cases, and even to order up criminal prosecutions. So far. Chilling words. So much for his attorney general’s admonition to cut out the tweeting about the Justice Department.

Perhaps no president — yes, including Richard M. Nixon — has been so convinced that he is entitled to weaponize criminal law to punish his political enemies, or to use his power to shield allies from criminal consequences. For years, Trump has lamented that he is limited in doing so by prevailing norms of presidential behavior.

“You know, the saddest thing is that because I’m the president of the United States, I am not supposed to be involved with the Justice Department,” Trump said in 2017. “Look at what’s happening with the Justice Department. Well, why aren’t they going after Hillary Clinton with her emails? . . . I’m very unhappy with it.”

Now, the president’s unhappiness persists, and the constraints on his behavior have lessened if not vanished, with Trump surrounded by enablers and emboldened by acquittal. According to a report in The Post, the president “has publicly and privately raged in recent months about wanting investigations of those he sees as enemies, including former vice president Joe Biden and his son Hunter Biden, former FBI director James B. Comey and former FBI deputy director Andrew McCabe.”

We are left with what passes for a firewall: Attorney General William P. Barr. “I’m happy to say that in fact, the president has never asked me to do anything in a criminal case,” Barr assured ABC News’s Pierre Thomas on Thursday. “If he were to say, you know, go investigate somebody . . . and you sense it’s because they’re a political opponent, then an attorney general shouldn’t carry that out, wouldn’t carry that out.” Perhaps to drive that point home, Barr’s Justice Department announced Friday that it would not pursue charges against McCabe for allegedly lying to investigators.

But do not delude yourself. All is not well.

First, we have a president who is an autocrat, one who does not share the common understanding, expressed by Barr, that the mechanisms of justice should not be used against political opponents. As Trump tweeted, he may as president have “the legal right” to interfere in a criminal case in the sense that nothing in the Constitution prohibits him from doing so.

But previous presidents have understood that it is wrong to pervert the criminal-justice system — our own or that of a foreign country — to their own political ends. As Susan Hennessey and Benjamin Wittes explain in their new book, “Unmaking the Presidency: Donald Trump’s War on the World’s Most Powerful Office,” Trump “has never made the slightest pretense of respecting his highest prosecutors’ autonomy. The most remarkable feature of his behavior toward law enforcement is how overt it is. Where mid-century presidents struck a pose of virtue in public and quietly tolerated or encouraged abuses, Trump openly calls for the abuses.”

Second is the scary concept of Barr as firewall. Let’s assume that Barr, as he asserted, would not accede to an outright order by the president to pursue a political opponent. Of course, Trump could fire Barr — as he did his previous attorney general, as he did the director of the FBI. If past performance is any guide, congressional Republicans are too cowed to respond.

In any event, there are more subtle ways in which the attorney general can bend federal prosecution to presidential desires. One was on display in recent days, as Barr took the extraordinary step of withdrawing the sentencing recommendation of career prosecutors in the case of Trump crony Roger Stone.

Barr insisted that he acted on his own and that he reached the decision before the president tweeted about the proposed seven- to nine-year sentence as a “horrible and very unfair situation.” But Barr was fully aware of the president’s fury over Stone’s conviction for lying to Congress and witness tampering. The attorney general could channel the president’s desires without them having been explicitly communicated to him.

Perhaps, as Barr argued, the prosecutors recommended too harsh a sentence. But it is not the attorney general’s job to flyspeck every sentencing memorandum. There seems little doubt that Barr was reviewing the Stone recommendation precisely because of Stone’s association with the president — which is precisely why he should not have inserted himself into the matter. Nor was it necessary to ensure that justice was done: The sentencing decision is up to the judge.

Barr’s involvement was no aberration. Since taking office a year ago, the attorney general has energetically inhabited the role of presidential wingman, including pre-spinning the special counsel’s report to Trump’s benefit. Now comes news that Barr has asked outside prosecutors to review the prosecution of former Trump national security adviser Michael Flynn, about whom the president once asked FBI director James B. Comey: “Can you give him a break?”

How convenient for Trump. He doesn’t have to order his attorney general to do anything if Barr is going to do Trump’s bidding without having to be asked.

Read more: