Michael Kirk has been making documentary films for more than half a century. He has chronicled the peaks and troughs of US politics, winning every significant broadcast journalism award along the way. But nothing prepared him for the scale of the threat now facing American democracy.

“There’s never been a film I made where I was more anxious, unhappy to make it, unwilling to discover the things we were discovering,” Kirk, 74, says of his latest project for PBS’s investigative series Frontline. “‘Worried’ is not a strong enough word for how I feel about where we are as a country and I don’t think I’m alone.”

Lies, Politics and Democracy tells the story of how, like a colonial army of occupation, Donald Trump subdued the Republican party with a combination of brute force and manufactured consent. It is a chilling character study in how, one after another, party leaders ignored, acquiesced, collaborated and enabled a demagogue while fearing his fervent fanbase.

The film draws on more than 30 interviews with former government officials, political journalists and experts. Like the congressional January 6 committee hearings, it provides a compelling narrative of half-forgotten turning points that, viewed in totality, resemble a Greek tragedy hurtling towards the inevitable and deadly climax of 6 January 2021.

It is striking, for example, that three of Trump’s most oleaginous loyalists have also, at various stages, suffered the harshest blowback for defying him. Senator Ted Cruz of Texas was publicly humiliated at the 2016 Republican National Convention after pointedly refusing to endorse the party nominee. Senator Lindsey Graham of South Carolina was heckled as a “traitor” by furious Trump supporters after appearing to disown the president on January 6. Lesson learned, he was soon back on board. Vice-President Mike Pence, who for four long years remained unswervingly loyal, may have been hanged if the mob had their way after he refused to overturn the 2020 election.

Lies, Politics and Democracy – which will be broadcast on PBS on Tuesday at 9pm – complements a growing body of literature tracking the Republican party’s capitulation that includes It Was All a Lie by Stuart Stevens, American Carnage by Tim Alberta, Insurgency by Jeremy Peters, Why We Did It by Tim Miller and Thank You for Your Servitude by Mark Leibovich.

It has a heartbreaking opening for anyone who cherishes democracy: graceful concession speeches from Hillary Clinton, Mitt Romney, John McCain, John Kerry, Al Gore and other defeated presidential candidates going back decades. Cut to Donald Trump in 2020 falsely claiming, “Frankly, we did win this election.”

Kirk explains: “The one non-negotiable rule in American politics is peacefully transfer the thing. It’s just too tenuous otherwise. Around the inauguration, there’s always some news anchor who says, ‘Here it is again, the thing that makes us the strongest country in the world: the peaceful transfer of power.’ It says everything about where we are.”

The documentary recalls how, during his reality TV days, Trump claimed that the Emmy awards were rigged when his show The Apprentice was beaten by The Amazing Race. So it was hardly surprising that, when he lost the Republican caucuses in Iowa in February 2016, he tweeted that Cruz “cheated” and should be disqualified.

Speaking via Zoom from a book-lined room in Brookline, Massachusetts, Kirk reflects: “You see so obviously what his method is and that he is this kind of rich guy: ‘I didn’t get the table I wanted in the restaurant, I’m going to trash the restaurant. Or I didn’t win an Emmy for my show, The Apprentice; I’m going to trash the Emmys and all the competition.’”

In probably the nastiest primary in history, Trump insulted Cruz’s wife’s looks and implicated his father in the assassination of President John F Kennedy. The defeated Cruz was a speaker at the convention in Cleveland, Ohio, and agonised with his team for days over whether to support the strongman nominee.

Alberta, who along with fellow journalist Jelani Cobb worked with the film’s producers, says on camera: “He tells them his decision, that he’s not going to endorse Donald Trump in his speech, and they ask him why and Cruz looks at them and he says: ‘History isn’t kind to the man who holds Mussolini’s jacket.’”

The convention floor was not kind to Cruz, however. As he urged Republicans to vote their conscience, the crowd turned on him with boos, jeers and shouts of “Get out!”, “Pull the plug!” and “Get off the stage!” To view their snarling expressions with seven years’ hindsight is to witness the “Make America great again” base emerge screaming from the womb.

Kirk, who was in the arena that day, says: “It felt like a potent moment for sure when it happened, but to see it again now, all these years later, and to see the faces of the people – and I was really determined to try to show who’s complaining; it’s not Nazis; it’s Mr and Mrs Republican complaining about Ted Cruz of all people, booing him off stage – I said, there’s the Maga party right there.

“It’s growing right before our eyes because that was probably the purest manifestation even for Trump. He’s been seeing it in crowds out around America but right there among the Republican establishment, he had them. Boy, that sent a powerful message. I promise you it wasn’t lost on Mitch [McConnell] or Kevin [McCarthy] or any of the other establishment leaders.”

Cruz proved spineless and went on the campaign trail for Trump in an effort to stay politically relevant. Meanwhile Pence had agreed to be Trump’s running mate, a decision that did much to normalise and legitimise the nominee, giving Christian conservatives permission to support him.

Kirk continues: “This is the way authoritarians rise. They get the collaboration. Sometimes even the collaborator doesn’t really know that they’re collaborating. They also have their own agenda. They want to win. They want to get a heartbeat away from the presidency.

“Pence [then governor] was in deep electoral trouble in Indiana so sure, why not? ‘I’ll get on that train. I’ll get on TV 500 times over the next six months. I’ll stay there and wave and he’s not going to win anyway but it will rejuvenate me and I can run in four years.’

“There he is making a political calculation and not understanding that what he’s conferring on someone like Donald Trump to Pence’s own base, evangelical Christians and the right wing, is his power. Trump – no fool in any way about all of this stuff – knows how to use something like that and he sure did.”

Trump stunned the world by beating Hillary Clinton in the 2016 presidential election. Republican leaders thought that he would be a useful idiot, easy to manipulate, only to find themselves manipulated in turn. They enjoyed policy wins, for instance on tax cuts, and instinctively defended him when Democrats attacked. But under constant pressure from Fox News and other rightwing media, they willfully ignored Trump’s authoritarian impulses.

Bill Kristol, a conservative commentator and former White House official, tells the film: “Rationalisation is a very powerful force, it turns out, in human psychology. It was a funny kind of choice, though, because you read history books and it’s like, ‘this is the moment’ and you choose this or that. But there are also ways in which you choose gradually and incrementally and the choice is more of an accommodation and a rationalisation and an enabling.

“It’s not a sort of, ‘I’m standing up here and choosing this path’. Some did that but an awful lot went along and they kept on going along and then they had to rationalise where they were going along so they became sort of enthusiastic about going along. You can rationalise your way into a series of choices, which becomes a very damaging and dangerous choice.”

That said, Kirk highlights an inflection point when Republican leaders could have said enough is enough: a 2017 white supremacist march in Charlottesville, Virginia, that turned violent and resulted in the death of the civil rights activist Heather Heyer. Trump insisted there were “very fine people on both sides”. Paul Ryan, the House speaker, seemed ready to disavow the president but ultimately bit his tongue.

Kirk says: “That seemed like a real moment for Ryan and obviously a dramatic moment for the Republican party and what they were enabling in such an obvious racist act. It seems to fly in the face of who the Republicans used to be in Reconstruction in America after the civil war.

“They were the heroes in so many ways and, for the Republican party after Charlottesville to be where it found itself, either silence from somebody like Mitch, or shrinking back in a way like Ryan did, it’s just tragic to watch.”

If Charlottesville failed to break the fever, and if the heavy-handed clearance of protesters outside the White House for a photo op in 2020 also failed, then surely the big lie of a stolen election and the deadly insurrection at the US Capitol would do it? For a while, it seemed that way. Graham announced that he was done. McCarthy said Trump bore responsibility for the riot.

But as Lies, Politics and Democracy recounts, when Trump relinquished the presidency on 20 January, he had one more ace up his sleeve. From his last flight on Air Force One, he reportedly called Ronna McDaniel, chair of the Republican National Committee, and threatened to quit and form his own party – a potentially devastating split.

Kirk goes on: “That threat – that I’m going to create my own party, I’ve got the voting lists, I can wreck your Republican dream of a midterm in ’22 and a new president in ’24 – was Trump’s maximum moment of humiliation and loss and also his last great threat. And it worked.”

Sure enough, soon McCarthy was making a pilgrimage to Trump’s Mar-a-Lago estate in Florida to kiss the ring once more. Graham was playing golf with him again. Republican senators could have banned Trump from running for office in future but acquitted him at an impeachment trial. It was, despite everything, still Trump’s party.

He has always boasted that he can identify an opponent’s weakness and exploit it. Kirk’s take: “It’s possible that he’s not a master strategist but it is very possible that the guy is a street fighter at the highest magnitude. This is somebody who seems to know how to intimidate, how to strike fear, how to manipulate.

“It’s always an astonishing thing to me. How does he get people like [former attorney general] Bill Barr to do his bidding when there’s nothing about it that would make them do it if they wanted to keep their legacy? They go along. What is it about him? That may be at the heart of this film.

“It may just be that politicians are a different breed and for them the purity and the simplicity of right and wrong is very fungible. Their calculus about somebody is all about them. I was talking to somebody when we were making this film and they said, ‘The definition of a politician is somebody who wants to get reelected’. Say no more.”

Internal critics, such as Senator Jeff Flake of Arizona and congresswoman Liz Cheney of Wyoming, have been purged. Many received vile abuse and death threats from the angry Maga base seeking to intimidate them and influence their votes – another intimation of authoritarianism.

But not even Trump is immune. Last year he was booed for telling his supporters to get vaccinated against the coronavirus and again for revealing that he had received a booster shot. There have been moments during this year’s Republican primary elections when the base has appeared to take on a life of its own.

Alyssa Farah Griffin, former White House director of strategic communications, says in the film: “The biggest misunderstanding of the Trump era is that he leads the base and the base goes where he does. I actually think that he’s created a monster that he doesn’t even control and he is actually very much driven by the base, not other way around.”

That implies Trumpism will survive Trump; even when the man has gone, his dangerous anti-democratic movement will thrive and metastasise. With polls showing that a majority of Republicans believe that the 2020 election was illegitimate, is there any hope that the party of Abraham Lincoln and Dwight Eisenhower can be saved?

It was a question that Kirk put to many people in the party and many scholars who study it. “Something really fundamental has happened and all the old paradigms don’t fit,” he says. “The invasion of Donald Trump may have really, really changed the Republican party in a way that it’s hard to see how the current players remake it, if that’s what they try to do.”

Kirk and his team have made more than 15 documentaries seeking to understand Trump’s impact on American politics, including Trump’s American Carnage, The Choice 2020: Trump vs Biden and Trump’s Takeover. This latest film, showing days after Joe Biden gave a prime-time speech about the battle for the soul of the nation, is a portrait of a democracy more fragile than he ever imagined.

“Every single person we talked to, even among very conservative Republicans, you’re not finding any kind of, ‘Hey, it’s going to be OK, hey, it’s all right,” he says bleakly. “I am very, very concerned about where we find ourselves right now and I don’t know how it fixes itself. I don’t know what happens.”