Before Donald Trump came along, Roy Cohn was widely regarded as one of the most repellent, unscrupulous and unrepetently divisive figures in American political history.

A virulently ruthless right wing lawyer, extortionist and political operator, a self-loathing Jew who crowed about “frying” the Rosenbergs, a self-hating homosexual who denied his queerness to his death — from AIDS — the gnomish Cohn was among the most feared political figures in America for much of his not-short-enough life.

So it’s no surprise that he spent some of his peak years mentoring an uncouth, unsophisticated New York developer and would-be playboy into someone who could bully his way out of many a fix simply by the public act of bullying.

Jeremy Strong, who played Jerry Rubin in “The Trial of the Chicago 7,” masters Cohn’s gimlet-eyed stare, his blunt bluster and poker-faced shamelessness in “The Apprentice,” the bracing, eye-opening new film from Iranian expat Ali Abbssi (“Holy Spider,” “Border”).

And Sebastian Stan (“A Different Man,” “Bucky” in all the “Avengers/Captain America” movies) reaches beyond comic impersonation to show us a naive Trump learning confidence and tactics for underhandedly “winning” from the master impersonator Cohn, building on the racism, abusive sexism, con artistry and other shortcuts that were his family birthright in faking “success” and exaggerating wealth.

The drug abuse? That he probably learned on his own. Or not.

It’s a fascinating portrait in bareknuckle venality and a movie that, whatever its shortcomings, does a pretty good job at conveying the monster who begat another monster. An alleged “patriot” who “loved this country” and used that as his cover for much of his hedonistic, closeted, take-no-prisoners ultra-conservative life mentored a faithless narcisist and lifelong self-dealer who avoided taxes like a mob boss, stole classified government documents, fomented an insurrection to overturn a free and fair election, cozied up to dictators and curried favor with foreign powers to help him steal elections.

If nothing else, we get a pretty good picture of why Trump was able to avoid any sort of reckoning for so long, and how Cohn blackmailed legal authorities and pulled mob strings to help him do it.

We meet Trump as a vain young loner, reduced to collecting rent from his father’s (Martin Donovan) rental properties door to door. It’s the early 70s, the Nixon administration is collapsing under its own corruption and Trump catches the eye of Cohn and his retinue at an exclusive club Trump had just managed to join.

We sense a hint of idealism in Trump, bridling at his father’s control of the family company, fretting at the “one step from a Depression” state of the NYC and how his dream is to “bring it back” by building a big hotel in the downmarket streets just below Times Square.

Cohn catches the naivete, tries not to recoil at how uncouth and unworldly this striver is, and takes him under his wing. That little government/NAACP assault on Trump business practices? Cohn will take him to school on that.

“File a lawsuit,” he says, setting the course for every future Trump battle against those who would hold him accountable for his crimes. “Always file a lawuit.”

Roy shares Roy’s “rules” for “winning.

“Attack attack attack.” “Admit nothing, deny EVERYthing.” And “no matter what happens, you claim victory. Never admit defeat.”

“The Apprentice” follows their relationship over the next dozen years, Trump clumsily trying to convince Hyatt to get in business with him (they do, with Cohn’s help), developing a public life and injecting himself into politics when he faces a mayor — Ed Koch — who doesn’t want to give him tax abatements to build his monument to himself, Trump Tower.

We hear Cohn’s lessons on ruthlessness — “You have to be willing to do anything to win.”

We see Cohn’s methods — networking, blackmail, as he knows the sexual proclivities of the elite and the governmental and the extension of “favors.”

“You ‘ll pay me back with your friendship,” he oozes. “Quid pro quo.”

And over the years, we see Trump fumble some lessons, ignore others — including advice that he not marry the Czech model Ivana (Maria Baklova of the last “Borat” film).

Trump is taught to seek the limelight, so he curries favor with the press. That first New York Times profile is arranged by Cohn, setting up a lifetime of mutual interest that the newspaper has been loath to abandon.

He grows famous, publishes a memoir and grows bored with Ivana and with Roy.

And so on.

“The Apprentice” serves up two indelible characters and assorted lowlights and highlights of their shared history — Cohn’s infamous orgies, his belligerent, bullying defiance right through his final interview, Trump’s pigheaded blunders into affairs and the casino business, Cohn recoiling at Trump’s gauche, proletarian appetites.

Having Trump run into Warhol, and not know who he is, at a Cohn party in the late ’70s — when the artist was one of the most famous figures in the world — is a chef’s kiss of a moment.

Vile political dirty operators like Roger Stone have cameos as other Cohn proteges.

Even if Gabriel Sherman’s script underplays the mob connections — Cohn introdues Trump to Fat Tony and the boys at lunch — even if he hits the “Make America Great Again” borrowing from Reagan hard and doesn’t hit the infuriating hypocrisy that the “patriotic” poseur Cohn passed-on to his equally anti-American protege hard enough, “The Apprentice” makes for a pretty good primer on how Trump became Trump.

Cohn beat the naivete and any hint of scruples he had out of him. And the Trump we see today — still unsophisticated, unworldly, arrogantly stupid and unfailingly cruel — because empathy is a trait that ties to higher intelligence — could very well have been created by just the circumstances depicted here, almost all of them coming from the many books reporting the sins and crimes of both men, including rape.

Strong finds his character and stays focused, first scene to last, a brilliant performance even if it never quite matches Ron Leibman’s ferocious turn as the man-as-dying-monster in the stage version of “Angels in America.”

Bakalova gives Ivana agency and the smarts to be repelled by the ruthless operator who pushes a pre-nup on her as Trump begs her to marry him.

And Stan is terrific, getting across the externals — the obsession with his hair, the pouty look, dismissive bluster and defensive posture he fights with naked displays of private school boy “aggression” — even if he can’t match the best vocal impersonators in terms of Trumpspeak.

You’d like to think Trump’s fanbase would appreciate this portrait. Many seem to aspire to be ignorant, rich, America-hating bullies, too. The rest of us can at least appreciate the irony of one last “fair” depiction of the guy before the rubes take another stab at voting us all into a doddering dimwit’s dictatorship.

Rating: R, sexual assault, sex, nudity, drug abuse and profanity

Cast: Sebastian Stan, Jeremy Strong, Maria Bakalova, Charlie Carrick and Martin Donovan

Credits: Directed by Ali Abbasi, scripted by Gabriel Sherman. A Briarcliffe release.

Running time: 2:00