It falls to Michael Moore’s empty stomach to help explain to me the difference between hope and optimism. “Right now, I’m hoping that somebody will feed me today,” he says. But that hope is passive. It may whet his appetite, but the disappointment will be all the more crushing if it isn’t satisfied. On the other hand, he explains, optimism is constructive, strategic. “I’m in a first world country, and somewhere I have a wallet with a credit card and some money in it. So the optimist in me has credibility, because it’s safe to say I will eat. Does that make any sense?” I mull it over, and I think so. While hope is passive, optimism determines the actions we will take.

Throughout our conversation – mostly in a London hotel as he munches on vegetarian dumplings – he scrambles around for these illustrations: some work, some don’t, sometimes he gets lost down a rabbit hole. But it is an insight into the mind of one of the western left’s great communicators: an almost obsessive desire to popularise political issues and causes, to trigger an emotional reaction among audiences that spurs them into action.

For young leftists, myself included, Moore’s work was something of a political life-raft back in an era when the traditional left was all but sunk. Fahrenheit 9/11 – his indictment of George W Bush’s so-called war on terror – courageously advanced otherwise marginalised ideas: it suggested that a proposed gas pipeline through Afghanistan may have played a role in the war, and noted links between the Bush administration and the Saudi regime. Moore has called for Bush administration officials to be put on trial. All this strays from the respectable centrist critique of the invasion, that it was simply the wrong war at the wrong time, or a “dumb war”, as Obama put it, rather than a crime.

His documentaries are designed not simply to inform, but to mobilise people. “Yeah, I was hoping to stop the Iraq war, hoping to end gun violence, to ensure every American has health insurance,” he explains. But he is keen to emphasise that his new film, Fahrenheit 11/9 (9 November 2016 being the day Donald Trump was declared US president-elect), is different. “It’s not some single issue, it’s not just about Donald Trump,” he says. “There’s nothing I can tell you about him that you don’t already know. You’d be wasting time and money to watch that.”

Play Video 1:45 Watch the trailer for Michael Moore's new film Fahrenheit 11/9 – video

Moore is right: that would be a tediously by-numbers film. Trump’s average disapproval rating among Americans has not been below 50% since March 2017; in the UK, more than three-quarters have a negative view of the self-described “very stable genius”. They don’t need a film to tell them all the bad things about Trump. When I last met Moore, on the eve of the EU referendum, he predicted that Trump had every chance of winning the presidency. For much of the pundit class and Democratic politicians, such an event was less likely than an asteroid slamming into the Earth: their cocky predictions that, no, don’t be silly, Trump is going to lose, are mercilessly paraded on the screen in the film. Why did he see it coming and they didn’t?

For me, one of the strongest elements in the film was his J’accuse against the Democratic party establishment. He seethes with profane rage when he talks about their failures. He makes a parallel with his documentary – he’s down a rabbit hole again: if it’s a great film, if the critics like it, then he’s done his bit as a director, he hands his raw materials to Vertigo, the UK company in charge of distributing the film, it’s on them to make it a success.

It’s an allegory for the Democrats, he suggests. The polling shows that on the key issues, such as progressive taxation, or abortion rights, or healthcare, or gun control, most Americans side with the progressive side of the argument. In the past six out of seven presidential elections, he notes, the Democrats won the popular vote. “So the Democratic party are handed a population which agrees with their entire platform,” he says; they even have more voters. “Yet they’re still unable to put themselves and us in power.” If his film distributor kept doing that, it would go out of business. But here is what happens, his film posits, if the Democrats become too much like the Republicans, too in hock to a corporate agenda.

Here’s what I wanted to find out about the absurd, frightening antagonist in Fahrenheit 11/9: what Trump could be capable of in certain circumstances, how he could take advantage of a crisis to concentrate power in his hands. What would happen if there was a major terrorist attack? That should worry everyone, Moore says. “He’ll immediately propose militarising the local police. He’ll give police and prosecutors wide latitude to make sweeping arrests. He’ll, say, temporarily suspend habeas corpus, things like that.” All will be justified on the basis of the need to protect the US, but things would not go back where they were, these would not be temporary measures, and he would keep piling on authoritarian measures.

I mention countries such as Turkey, Poland and Hungary, where authoritarian leaders keep the formal trappings of democracy – there are still elections and opposition parties – but its substance is hollowed out. “Yes, I think that’s a better model for democracy: they’ll keep up the appearance of democracy, but their leader becomes more and more autocratic.” And here’s a chilling, under-discussed scenario. What if Trump is just the starter, the foreshadow? What if his role is to shift the terms of what is deemed an acceptable Republican candidate in favour of a more sophisticated authoritarian leader? “I think they now know the formula of what they need to win an election,” is how Moore puts it. “You need someone who people are familiar with, someone comfortable being on television.”

But there is a source of hope – or indeed optimism – in the film. To paraphrase George Orwell: “If there is hope, it lies in the young.” The survivors of the Parkland massacre confront not just the US gun lobby, but an older generation they believe has failed them. “We appreciate that you are willing to let us rebuild the world that you fucked up,” one survivor told US host Bill Maher.

The old trope is that the young begin as naive leftists then drift rightwards with age. This is something Moore refutes – “As I get older, I become more angry, not pacified, in my political thinking” – and it is not backed up by the data. In the 1984 presidential election, under-25s were more pro-Reagan than Americans in their 30s and 40s, and only marginally less so than pensioners.

And Trump is not popular among today’s young. In the 2016 election, among Americans under 30, he lagged about 20 points behind. Why? “When we were that age, my generation, if we went to college, we’d graduate debt-free. That world was our oyster: we could do whatever job we wanted to do, or not want to do any job – a lot of people took off, put on their backpack, went to Europe, got a Eurorail pass. The so-called ‘American dream’ seemed like a reality.” But what has happened in the US mirrors Britain: a neoliberal system promised freedom but instead delivered insecurity and stagnating living standards.

“The reason they’re more active and more aware is because they saw the writing on the wall, probably from middle school, definitely from high school. That there were not going to be good jobs for them, that they were going to be in a debtors’ prison for the first 20 or 30 years of their life. They’re angry – but not angry enough, in my opinion.”

Facebook Twitter Pinterest A still from Fahrenheit 11/9. Photograph: Dog Eat Dog Films

In Moore’s film, the Democratic former speaker of the House Nancy Pelosi slaps down a young, leftwing voter with: “I have to say, we’re capitalists, that’s just the way it is.” But it is striking that in 2018 – nearly three decades after the end of the cold war, in the nation of red scares and McCarthyism, in the very citadel of free-market capitalism – polls show most younger Americans prefer socialism to capitalism, “whether Pelosi likes it or not”, says Moore. He thinks back to how the socialist senator Bernie Sanders – whom he backed – casually launched his presidential bid in 2015. “He didn’t even realise how vast what was about to take off was. If he’d started a couple of months earlier, infrastructure in place, who knows what would have happened.” Will he run run again? “I believe he will,” Moore says, optimistic that Sanders can win both the nomination and defeat Trump. He tells me of an unreleased poll in West Virginia – which Trump won by a 42-point margin in 2016 – that has Sanders beating Trump in a head to head. “Even if people don’t agree with Bernie, they know he’s honest – what you see is what you get.”

But the Democratic establishment is on the run, he believes: he points to the rise of Alexandria Ocasio-Cortez, a 29-year-old New Yorker who defeated one of the Democrats’ big wigs in a primary election in June. “We covered her when nobody even knew she was running.” At the root of the Democratic civil war are fundamentally different interpretations of what happened in the 2016 election. The Democratic left believes it represented the nemesis of centrist orthodoxy, failing to capitalise on the anti-establishment mood against a system that is fundamentally broken, while the Democratic right believes Trump supporters need to be won over, and blame so-called “identity politics” for the loss.

Moore gives winning over Trumpists short shrift, spluttering: “A waste of time! Oh my God! If you’re still for Trump [after] two years, [with] everything you’ve seen, you’re gone, no one can convince you of anything!” What is pejoratively labelled identity politics – civil rights for black people, LGBTQ rights, women’s rights – has “reinvigorated politics”, Moore suggests. “It’s getting more people out to vote. You’re never going to convince people who hate gay people – that’s a waste of time. Our energy has to be getting our own people to polls.”

He keeps drifting back to the November midterm elections, and it’s clear that his film is, in part, a get-out-the-vote operation. It’s desperately needed: young voters who favour the Democrats are least likely to vote. Why? “They don’t feel that the political system is going to help them – that’s the biggest problem.” He keeps meeting US voters on his travels, “filled with a lot of despair. I could see some had given up.” They would vote, he suggests, but they wouldn’t end up bringing 10 other voters with them. He will be campaigning in swing districts, and has been using the money he has saved as a beneficiary of the Trump tax cuts to back Democratic candidates.

It’s here that Moore comes into his own, full of righteous fury. “I’ve seen films where you’re so reinvigorated that at the end of the film, you can’t wait to get to get out of the theatre and go out and do something. I love those kinds of films … A film like the one we’ve made asking people, not to give up or give in, but to realise how much fucking power and strength we have! We have a power on 6 November to fucking crush Trump, the uber rich who are thrilled with his performance, the old white male establishment that thinks they’re going to keep running the show, when their show was long, long over.”

Firing up what he sees as America’s progressive majority is why Moore is so dismissive of accusations he preaches to the choir. “The choir need a song to sing – that’s why they’re the choir! They need a song to pull them out of despair, and need to light a fire underneath themselves.”

Fahrenheit 11/9 review: Michael Moore v Donald Trump = stalemate Read more

He sees the Republicans’ nemesis as what he calls “the avengers” – women, young people, “who’ve had the future ripped from them”, people of colour – who history would record “got together and crushed the forces of evil”. But he is very clear: there cannot be a return to the corporate Democratic agenda of the past, and it is clear he sees himself having a big role to play. “People like Sanders and myself and Alexandria [Ocasio-Cortez] and Rashida [Tlaib, a socialist Democrat set to be the first Muslim congresswoman], we will be the ones steering the ship, and we’ll do thing differently than the Democrats in the past. We’ll essentially give the people what they want: equal pay for women; ending mass incarceration of black people; protecting women’s reproductive rights; creating a living wage for everyone.”

But Moore’s optimism is not delusion. Earlier, he soberly told a London cinema audience that he cannot promise them a happy ending. He’s right not to. Whether Trumpism can be defeated will surely, in part, depend on who wins the battle for the soul of the Democratic party: its rootless, corporate-backed wing or a new insurgent left offering an alternative to a broken system. If the latter triumphs, history will surely record that Moore, now in what he wistfully calls “the final third of my life”, played a significant role.

Fahrenheit 11/9 is released on 19 October in the UK.