“Where’s my Roy Cohn?” is a quote attributed to Donald Trump, who asked this of his aides in a moment of exasperation when he was unable to make the Russia investigation disappear. Nowhere is this explained or mentioned in Matt Tyrnauer’s documentary, giving its title a weirdly philosophical air. Is the implication that everyone has a Roy Cohn? I should hope not!

Roy Cohn was a corrupt lawyer, political dirty trickster, mafia associate and scumbag. He was a self-hating Jew who powered the engine of one of the worst antisemitic moments in American history, the demonization and execution of Julius and Ethel Rosenberg. He was a closeted man who refused to publicly identify as gay even as he was dying of Aids. He was so famous for being a mean bastard that there were not one but two jokes at his expense on the Simpsons. There are not too many lawyers that can make such a claim.

Cohn, born in New York in 1927, was heir to a number of fortunes on his mother’s side. She was considered ugly, and had trouble finding a husband. Cohn’s father agreed to an arranged marriage so long as her powerful family made him a judge. This sort of blatant, unfeeling corruption would be a hallmark of Cohn’s life.

He graduated from Columbia Law School at the age of 20 and quickly found himself as one of the leading “red-baiters”, allegedly rooting out communists in government positions and the US army for the good of democracy. He was the Gríma Wormtongue to Senator Joseph McCarthy, whose last name is a now a synonym for political witch-hunting.

McCarthy and Cohn’s harassment of presumed communists and sympathizers has overshadowed a subsequent “lavender scare” in which the pair harassed and exposed homosexuals. (It is rumored that McCarthy, like Cohn, was also secretly gay, and the same is often said of the FBI head J Edgar Hoover, who encouraged these practices.) A series of hearings in 1954 suggested that much of McCarthy’s pressure on the US army was led by Cohn’s desire to secure a better position for a man named G David Schine, who was either Cohn’s boyfriend or someone he was infatuated with.

One would think that such a blatant lack of ethics would put a cinch on Cohn’s career, but that is not, alas, the world we live in. He fueled himself off accusations and fighting. His strategy was always to deny then lie even louder. As a personal attorney he would win high-profile cases by deflection, misdirection and fear-mongering. It made for great tabloid copy, earned him powerful friends and attracted wealthy clients in New York, most notably the heads of organized crime families and the flashy, young real estate mogul Donald J Trump.

Tyrnauer’s film is very standard: a collection of talking heads (including former protege Roger Stone) and news clips. What this by-the-numbers approach lacks in artistry it makes up for in an avalanche of facts. If there is a thesis to be found (other than “boy, wasn’t Roy Cohn a jerk?”) it’s in finding the seeds to Trump’s garden nurtured by Cohn’s odious work. Despite a 20-year age difference, the two were close for many years. They first bonded over a shared love of denying African Americans their civil rights. This blossomed into corruption and kickbacks during the erection of Trump Tower. Cohn loved to see his picture in the paper, and was known for his must-attend parties, so there are ample images in this documentary to turn your stomach. Barbara Walters, at long last, what is your deal?

I can’t call this the most explosive film (especially for theaters) but I predict it is part of a forthcoming wave from film-makers trying to grapple with just how in the hell we got to where we are. To that end, it is an important one. Donald Trump was, for many years, a joke (though never a harmless one) but the damage he’s currently doing shames all of us for ever laughing. This film connects a direct line between Roy Cohn’s belligerent, boorish and obstructionist ways and our current, less eloquent nightmare. To answer the question “where’s my Roy Cohn?” he is, unfortunately, in the White House.