Donald Trump, who may well have attempted to obstruct justice within just a few weeks of taking his oath of office, came to the Presidency with a wealth of experience in the art of deceit. He may know little of domestic or foreign policy, he may be accustomed to running an office of satraps and cronies, and he may be unable to harness an institution as complex as the executive branch, but experience told him early on that he could dodge any accusation and deny any aggression against the truth.

As Trump’s biographers Marc Fisher and Michael Kranish tell the story, Roy Cohn, who lived for decades under various indictments for bribery, extortion, and other sins, and yet always managed to escape conviction, first instructed Trump more than forty years ago in the dark arts of counterattack and an over-all “go to hell” philosophy. Cohn, as a devious young lawyer, had been the protégé of Joe McCarthy, during the anti-Communist witch hunts of the fifties. He met Trump at a club called—seriously—Le Club, and began to tutor this eager young scion of an outer-borough real-estate family in the art of what’s what. Nothing delighted Trump more than to learn that prosecution did not necessarily follow from wrongdoing.

“When Cohn boasted that he had spent much of his life under indictment, Trump asked whether Cohn had really done what was alleged,” Fisher and Kranish write. “ ‘What the hell do you think?’ Cohn responded with a smile. Trump said he ‘never really knew’ what that meant, but he liked Cohn’s toughness and loyalty.”

Trump knew very well what Cohn was telling him, and he lived by that lesson. As a businessman, he distinguished himself as a disreputable con; he was spurned by the New York business community less for his cartoonish flamboyance than for his essential dishonesty, his meanness of character. He routinely stiffed contractors and workers. He screwed creditors. He violated casino regulations. He bragged of charitable contributions that he never made. He promoted scams such as Trump University. In the nineties, as his bankruptcies mounted, he lost the ability to obtain credit from the largest and most reputable American banks. In foreign deals, brandishing an inexplicably attractive marketing name, he ignored his legal obligations to carry out due diligence and did deals with flagrantly corrupt business partners. In Azerbaijan, he was party to a deal whose only real enterprise might have been the laundering of money. And yet he always avoided serious legal peril, not least because he played by the lessons imbibed from Roy Cohn. And all the while he lived it up, acquiring the life-style decorations of a third-world dictator or a second-world oligarch. His excess was his brand.

As a politician, Trump has had little reason to discover the qualities of modesty, scrupulousness, or seriousness. Throughout the primary and Presidential campaigns, he succeeded in no small measure because of his defiance of convention. Emboldened by his astonishing early exposure on cable television and his first wins in the primaries, he came to see himself as invulnerable.

Nothing could hurt Trump. Even he seemed stunned by this stubborn fact. “I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose voters,” Trump said, while campaigning in early 2016. And he was, for a long time, right.

Until now. In the past two weeks, a Presidency of ideological meanness and unsurpassing incompetence has moved into another, more recognizable realm. The usual comparison is with the Watergate era.

“I actually think the comparisons at this point obscure more than they reveal. Nixon was just so shrewd, so strategic: it’s simply inconceivable he would get caught with his pants down implicating himself on the record, like Trump now does almost daily,” Rick Perlstein, the author of “Nixonland,” told me. “My favorite Nixon maxim was ‘Never get mad unless it’s on purpose.’ But the words ‘on purpose’ and ‘Donald Trump’ now feel like matter and antimatter; with him, it’s all impulse. Nixon was so obsessed with preparation he used to memorize answers to likely press conference questions, questions he’d delegate to staffers like Pat Buchanan to dream up. Can you imagine!? And, look, when Nixon fired Archibald Cox, he was truly backed into a corner, his king in check: that was the only move he had before the world discovered, via the tapes, that everything he’d been saying about the scandal since June, 1972, was a lie. But, even then, he managed to keep moving pieces around the board for ten more months!

“Both, of course, were authors of their own predicaments,” Perlstein went on. “But Nixon was so much the smoother criminal: everything was buffered through intermediaries and cutouts. An example that comes to mind: on the famous meeting with John Dean of March 21, 1973, Nixon, realizing he’d said too much, maneuvering Dean near the microphones to say something along the lines of ‘. . . But that would be wrong.’ Can you imagine Trump with that kind of situational awareness?”

Despite the shrewdness gap, Nixon once paid Trump an encouraging compliment. In 1987, when Trump was thinking about politics for the first time, the disgraced ex-President heard from his wife, Pat, that Trump had put in an entertaining performance on “The Phil Donahue Show.” Nixon wrote to Trump, “Dear Donald, I did not see the program, but Mrs. Nixon told me you were great. As you can imagine, she is an expert on politics and she predicts that whenever you decide to run for office, you will be a winner!”

Nixon himself was never a political mentor to Trump, but one of his aides, the original dirty trickster Roger Stone, was. Stone was as instrumental in creating Trump’s political career as Roy Cohn had been in forming Trump’s moral behavior in business. (Watch the new documentary “Get Me Roger Stone,” starring my colleagues Jeffrey Toobin and Jane Mayer, and you will understand the current craziness more deeply.) It was Roy Cohn who introduced Stone to Trump, and Stone was instantly enamored.

“I was like a jockey looking for a horse,” Stone says in the film. “And he’s a prime piece of political horse flesh in my view.”

Stone helped Trump see the political advantage in many sleazy tactics and alliances. He pushed him on birtherism (which was for Trump what the Southern strategy was for Nixon); he led him toward conspiracy mongers like Alex Jones and Infowars, and operatives like Paul Manafort, who led the campaign for a while and is now a source of intense investigation for his associations in Russia and Ukraine.

Over the years, Trump has been the focus of investigations on housing discrimination, bribery, corruption, dealings with the mob, misleading earnings reports, fraud, and improper campaign contributions. (Of his behavior with women we shall not speak.) But that was nothing compared to the hard light that is on him now from the F.B.I., Congress, the press, the public, and various other realms of civil society. Discussion of Trump’s Presidency ending before his four-year term is up is no longer an oppositional fantasy. The events of these recent days­­—the Comey firing; the opera-buffa intel giveaway with the Russian delegation to the Oval Office; and now the news of the Comey memos—just may be the point of no return for a Presidency that has been a kind of emergency of chaos, incompetence, injustice, and deception from its first days.