Roy Cohn would be proud.

The lizard-eyed lawyer who perched in his silk bathrobe at the intersection of New York's political, criminal, financial and sexual underworlds during Donald Trump's formative years taught his most celebrated client one lesson above all others.

Typical of Cohn's unvarnished approach, it was the first advice out of his mouth. Trump was a young man embroiled in a conflict with the Justice Department. (A recurring motif, it turns out.) In 1973, Trump and his father, an apartment house mogul, were charged with illegal housing discrimination against African Americans. Encountering the notorious Cohn at a Manhattan club, Trump introduced himself and asked for advice.

"Tell them to go to hell," Cohn replied.

In those days, the best Trump could manage along those lines was a feeble countersuit, quickly tossed out of court, followed by a raft of baseless allegations against the feds and, ultimately, a consent decree to go and sin no more.

But today, Trump is president of the United States. And when a president tells the Justice Department to go to hell, the whole country gets dragged down. By the time you read this, Trump may well have ordered the release of a classified memo prepared by partisan House staffers, cherry-picking undisclosed sources, alleging that some of the nation's leading law enforcement officials are party to a virtual coup attempt. Clearly designed to muddy the waters of special counsel Robert S. Mueller III's investigation, the memo is said to implicate a number of FBI agents, as well as Deputy Attorney General Rod J. Rosenstein. The decision to publish it — without disclosing the sources from which it is derived — has destroyed whatever comity might have existed on the House Intelligence Committee at a time when members ought to be cooperating against a long list of cyberthreats.

"Extraordinarily reckless" is Assistant Attorney General Stephen Boyd's official characterization of this radical act. I'd say that's too gentle. Moscow and Beijing ought to close their propaganda shops; they can't outdo Trump and his supporters when it comes to impugning American institutions.

Still, "Reckless" could have been Roy Cohn's middle name. Trump's mentor in the dark arts of power burst on the scene as a legal prodigy in 1951, prosecuting the most sensational case in America — the atomic espionage trial of Julius and Ethel Rosenberg — at an age when most would-be lawyers are studying first-year torts. From there, his career path charted one of the steepest descents since Lucifer was cast out of heaven.

As the chief counsel to Sen. Joseph McCarthy (R-Wis.), Cohn used selective leaks and exaggerated charges to convince millions of Americans that the State Department and Pentagon were chock-full of treasonous enemy sympathizers and spies. When his excesses helped to discredit McCarthy, Cohn set up in New York as an all-purpose fixer for clients ranging from Trump to mob boss Anthony "Fat Tony" Salerno. "When Roy Cohn was at the height of his power," Harvard law professor Alan Dershowitz told The Post in 2016, "nobody did anything in New York politics, in New York real estate, without going through Roy Cohn." A fixture at the celebrity nightclub Studio 54, Cohn died in August 1986 of AIDS-related illness, six weeks after being disbarred for legal chicanery.

If Cohn had been around in May, when Trump's bumbled firing of FBI Director James B. Comey led to the Mueller inquiry, he would surely have advised exactly the sort of tactics Trump and Co. are now deploying. Attack, leak, distort, impugn. Magnify the offenses and mistakes of opponents, and use individual flaws to tar entire agencies. Cohn would not scruple over the smirching of reputations. After all, alongside McCarthy, Cohn threw dirt at the likes of Gen. George C. Marshall, one of America's greatest soldiers and diplomats. Nor would he fret over lasting damage that might be done to national unity and confidence in government.

Unlike a lot of people who share their opinions for a living, I don't purport to know where Mueller will end up with his examination of Russian meddling in the 2016 election and Trump's relationship to it, if any. But I have thought that our best chance, as a country, to put the questions to rest was to allow Mueller to collect his evidence and present his conclusions. There would then be time to judge whether the exercise was a "witch hunt," as Trump has complained.

Now, I am skeptical that we will ever reach a reasoned conclusion. Aided by such men as Rep. Devin Nunes (R-Calif.), his lackey on the Intelligence Committee, and bully-boy Sean Hannity, his mouthpiece on Fox News, Trump is convincing millions of Americans that there can be no justice from the Justice Department and that there is no agency in all the land honest enough to establish the truth.

Yet he asks us to believe that the state of the union is strong.

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