When Daniel Ragussis was looking for funding for his first feature-length film, people suggested that the American white supremacists in his thriller script Imperium were a bygone phenomenon. "The thing I always heard," the Jewish writer-director tells me on the phone from New York, "was, 'C'mon, maybe like 30 years ago. This doesn't really exist, does it? This isn't real.' Now nobody says that."

Far from looking backwards, Imperium, co-written by Ragussis with former FBI undercover agent Michael German, and starring Daniel Radcliffe, couldn't be more topical. Rather than just a distant memory, far-right ideologies are in fact spreading, not least via social media. On Twitter, the number of American white nationalists and self-identified Nazi sympathisers (the UK has its own home-grown equivalents) has multiplied more than 600 per cent since 2012, according to researchers at George Washington University. Meanwhile attitudes that were largely confined to fringe groups have gone mainstream.

"What's shocking to me," German told Ragussis, "is what people were talking about in dark garages and dirty basements in the '90s, they're now talking about on national television."

During what is becoming an increasingly disturbing contest for the White House, the Republican presidential nominee Donald Trump's demonisation of Mexicans and Muslims has attracted support from the antisemitic white power movement known as the Alt-Right, giving them a media presence they'd previously lacked. Instead of pushing the extremists away, Trump's eldest son, Donald Trump Jr, posted an image that included Pepe the Frog, a popular Alt-Right symbol, on Instagram, on 9/11, and then provoked an antisemitism storm a few days later, by invoking Holocaust imagery while chastising journalists.

"It's no surprise to say that some of the positions that Trump is taking are ones that a mainstream presidential candidate has never taken before," says Ragussis.

"And there are also candidates, in smaller elections in local areas in the United States, running on openly racist platforms." He believes that, since 2008, conditions have grown more conducive for hateful ideologies "to spread and prosper," adding: "That's certainly been happening with the immigration crisis in Europe, and it's certainly been happening with the way that the [American] economy has gotten the shock that it has."

One of the messages of Imperium is that when people feel like victims, fascism finds fertile soil in which to take root.

The son of an English professor, Ragussis first "stumbled" across the thriving white supremacist and neo-Nazi movement in the United States when researching a short film about the controversial Nobel Prize-winning Jewish chemist, Fritz Haber.

"It's not something that I really was aware existed," he admits. "And yet the moment I became aware of the community, its scope and size and breadth and depth was stunning to me."

He learned then what many people are just realising now: the movement cuts across class and economic divisions. "It's populated not only by people that are uneducated and poor, but also middle- and upper-class, and affluent. There's a whole scholarly community within it that's filled with Holocaust deniers and people writing about eugenics, all sorts of things."

In Imperium, Radcliffe plays an FBI agent inspired by German, who infiltrates a group of skinheads in a bid to foil a suspected bomb attack. Along the way, he encounters the KKK, Swastika-wearing Aryans, a cynical radio host, and a "sensitive" fanatic.

The Jewish former Harry Potter star was drawn to the film by its script, and the challenge of the role. "But he also said he thought that it was valuable to do something that was important and that was about issues that mattered," says Ragussis, "and that had the potential to strike conversations about something that we all need to know about and become aware of."

Although it hasn't been making headlines as much, Ragussis says that, in the United States, far right terrorism, "far out-paces Islamic-based terrorism".

If people don't hear it reported in the same terms that's partly because of the way the violence is categorised. "If three extremist Nazis murder two immigrants, is that considered terrorism? Many times it's not."

Also, "it's politically expedient to focus on the Islamic terrorism," Ragussis argues, "because that's something that's very much in the news and very much galvanises people. So the people in power, what is the most beneficial group for them to target?"

Imperium's power lies in its detail. We see how the groups operate, how they talk, and learn about some of the key texts that inform followers' thinking, such as The Turner Diaries, an antisemitic novel the FBI calls "the bible of the racist right", and the book that gives the movie its title, Imperium: The Philosophy of History and Politics written by Francis Parker Yockey, an American attorney, antisemite and fascist, published under the pen name Ulick Varange in 1948.

I ask Ragussis if he ever thought about whether he might be risking unwittingly introducing the books to people susceptible to their ideas.

"Rightly or wrongly, that was something that I was never worried about," he insists.

"To my mind, the virtue of the kind of society that we live in, and the virtue of a liberal democracy, is that there are no forbidden ideas… In my mind, it's better for people to be aware of everything, be exposed to all the different influences, and then be led to the right ones through debate and through discussion."

He is keenly aware of the power of words, and "that the only power that Hitler really had was the power of speech, and everything that followed came from that."

The film's subject matter felt very dark to him because of the unshakable conviction of many white nationalists. However, there is hope, as "people leave this movement all the time. They change their mind all the time."

"There's no easy solutions, but we have to have absolute freedom of expression and we have to engage in dialogue about these things. So I guess I just have to believe that the better words, and the words that conform to our values, are going to win out. But that's something, I think, we always have to fight for."

Imperium opens this weekend