By Harry Siegel

Spoiler alert: Roger Stone, the body-building dandy and sex-club habitué with Nixon tattooed on his back, is a bad guy, a tumor in our democracy.

The tactician who’s been degrading our politics since Watergate is also the visionary who foresaw President Trump 28 years beforehand — when the Donald himself saw the idea as at best a publicity hustle — and who created the conditions that finally made it happen.

His tunnel vision is the subject of “Get Me Roger Stone,” debuting Sunday at the Tribeca Film Festival. The legendary rodent copulator, as Ted Cruz once put it — who says he has two tickets, one for him and one for his libel lawyer — will see then for the first time what co-directors Dylan Bank, Daniel DiMauro and Morgan Pehme have made of his life’s work over five-and-a-half years following him.

(The documentary, streaming on Netflix May 12, also includes 90 seconds or so of my made-for-radio face.)

When the filmmakers first connected with him in 2011, Stone and “Stone’s Rules” — including “Attack, attack, attack. Never defend,” “Admit nothing, deny everything, launch counterattack” and “Nothing is on the level” — seemed like yesterday’s news.

The two-faced hustler, whispering frontpage stories to reporters at lamestream newspapers out of one side of his mouth while back-writing himself into starring roles in every infamous incident in Republican history out of the other, got his start as a nickel-and-dime trickster for Nixon and then Reagan.

He and Paul Manafort — a friend who served as Trump’s campaign CEO at the recommendation of Stone (who Trump had supposedly “fired”) — then revolutionized Washington with a lucrative business model: lobbying the guy they’d helped elect on behalf of foreign dictators and other nasty clients.

Then the, err, Pulitzer-worthy National Enquirer outed him as a swinger in 1996, forcing him out of Bob Dole’s geriatric presidential campaign back when debauchery still troubled Republicans. So Stone was reduced to working for the likes of Democrat Al Sharpton in his 2004 presidential run, self-identified Eliot Spitzer madam Kristin Davis in her stunt run against Client Nine, and libertarian Gary Johnson in his 2012 presidential bid. He even did dirty work for corporate clients to make a buck.

With his influence diminished, Roger Stone, still playing by Stone’s rules, pushed the envelope on “Roger Stone,” kayfabe creep — smearing and insulting his ever-growing list of enemies and then partnering with sweaty race-war conspiracist and tinfoil-hat salesman Alex Jones.

But like fellow Roy Cohn protégé Donald Trump, Stone is a dangerous one to underestimate. Cast from the Republican club, he kept grinding, calling the same old tunes that people keep dancing to and gnawing relentlessly at the institutions of American democracy to open what holes he could and suck money out of them.

The guy who made a nice buck from Ross Perot’s Reform Party while hollowing it out to the benefit of the GOP ended up hollowing out the GOP, too, when Trump was finally ready to launch the run Stone had been encouraging him to take since 1988.

So after years following Stone, says Pehme, the filmmakers went from a story about an also-ran evil genius to a story about one making what would surely prove his futile last hurrah to “the most important story in the world — a timeline to the rise of Trump.”

It’s one that goes back decades, as Stone has hewed to a vision of a strong GOP with a show-biz sheriff pitching “law and order” to “the silent majority” — a tradition that flows through him from Nixon to Reagan to Trump, each reading from the same script.

Which is one of the two reasons Trump, who doesn’t like to see names other than TRUMP in big print, fired Stone after he’d quietly helmed the campaign for its first year. The other reason was so that Donald could keep using Roger — who, among other things, was the campaign’s evident conduit to Russian sock puppet Guccifer 2.0 and Wikileaks.

And who beta-tested seemingly wild lines of attack, like calling Bill Clinton a rapist, that Trump later picked up word for word. “No, you’re the puppet.”

As Stone’s first wife told the filmmakers, he and Donald have been politically wed on and off for decades, and always find their way back to each other.

The documentary is dedicated to Wayne Barrett, the dogged investigative reporter who spent decades exposing both Trump and Stone.The muckraker lived long enough to see his work finally get its proper due — and then to see that it wasn’t enough to keep these creeps from winning.

Perhaps the only thing Barrett and Stone agreed on was that the fight between their kinds is as enduring as human nature. To win the rounds to come, we need to understand how and why Stone and Trump triumphed this time. To do that, watch “Get Me Roger Stone.”