Last month, a gunman walked into a supermarket in the East Side neighbourhood of Buffalo in New York state and opened fire on the predominantly black shoppers. Eighteen-year-old Payton Gendron, who has been charged with first-degree murder, was wearing body armour and a military grade helmet, and carrying a modified Bushmaster XM-15 rifle. Five minutes after the attack began, the gunman surrendered to police: 13 people, 11 of them black, lay wounded, 10 of them fatally.

It doesn’t require an investigative journalist to discern the suspect’s motives. An 180-page manifesto he allegedly posted online drips with antisemitism, racism and white nationalism. Declaring his support for neo-Nazism, the author parroted the key tenets of the “Great Replacement” conspiracy theory which holds that “global elites” are using immigration to supplant the white race. Although he had deliberately targeted black Americans, the suspected gunman wrote, Jews were “the biggest problem” and could be “dealt with in time”.

Curiously, despite spending more than three decades in journalism, the host of America’s most popular cable news show chose to skate over these facts. Instead, Tucker Carlson, the host of Fox News’ nightly political talk show, said the Buffalo gunman’s manifesto was “not really political at all” and dismissed talk of his motives by suggesting the teenager was “mentally ill”.

This omission is even more strange considering Mr Carlson’s familiarity with the various iterations of the replacement theory. As the New York Times reported a month before the Buffalo shooting, the right-wing commentator “amplified the idea that a cabal of elites want to force demographic change through immigration” in more than 400 episodes of his show.

Mr Carlson’s propagandising is certainly working. Fellow Fox News hosts and leading congressional Republicans are singing from the replacement song sheet. Polling suggests that one in three Americans – including nearly half of Republicans – are humming along, too.

The Buffalo shooter is not believed to have watched, or been influenced by, Mr Carlson’s show; the author of the published manifesto himself pointed to chatrooms like 4chan, and the neo-Nazi Daily Stormer, website for his conversion to the far right.

But with his huge ratings, the Fox News commentator has helped to bring the thinking swirling around the conspiracy theory – with the rougher, antisemitic and anti-black edges smoothed off – from the fascist fringes of the internet into more mainstream political discussion. “No public figure has promoted replacement theory more loudly or relentlessly,” suggested the New York Times last month.

But while Mr Carlson has condemned political violence and suggested that his concerns are rooted in worries about voting, not race, such talk is undoubtedly playing with fire. Heidi Beirich, co-founder of the Global Project Against Hate and Extremism, calls the Great Replacement “the most mass-violence-inspiring idea in white supremacist circles right now”. This particular idea, she believes, “has superseded almost everything else in white supremacist circles to become the unifying idea across borders.”

Numerous recent killing sprees – in 2018 at the Tree of Life synagogue in Pittsburgh; early the following year at two mosques in Christchurch, New Zealand; and at a Walmart store in El Paso, Texas, barely six months later – have all been laid at the door of far-right extremists influenced by the Great Replacement theory. It was also the notion which led the those “very fine people” to chant “Jews will not replace us” as they marched through Charlottesville nearly five years ago.

Experts worry that Mr Carlson’s obsessive chatter about the subject could act as a gateway drug to the harder stuff. As Nicole Hemmer, a historian at Columbia University, argued last month: “Someone like Carlson can introduce viewers to ideas that they then explore more fully online, searches that lead them into far-right spaces that either reinforce their existing views or radicalise them. But someone like Carlson is also important because he legitimates those ideas, making them seem less radical when viewers see them.”

Fox News has been repeatedly warned of the perils of such nonsense being peddled on air. Last year, for instance, the ADL’s Jonathan Greenblatt urged the network to fire Mr Carlson after he argued that the Democrats are “trying to replace the current electorate, the voters now casting ballots, with new people, more obedient voters from the Third World”. This rhetoric, charged Mr Greenblatt, was “was not just a dog whistle to racists – it was a bullhorn”.

While further clashes with the ADL have followed, Fox has stood by its star presenter. Unsurprisingly, all this replacement theory talk has left America’s white supremacists giddy with excitement.

But beyond winning plaudits from neo-Nazi keyboard warriors, Mr Carlson also has some fans with real power – foremost among them, the Hungarian prime minister, Viktor Orbán.

When the American Conservative Political Action Conference was held in Budapest last month, Mr Orbán singled out the Fox News commentator for lavish praise as he railed against “the rule of the liberal media”. “His programme is the most watched,” the prime minister declared of Mr Carlson’s show. “Programmes like his should be broadcast day and night. Or as you say, 24/7.” Of course, in Hungary – where Mr Orbán’s decade in power has seen an assault on minorities, the independent media and judiciary, and democracy itself – radical right, populist claptrap is now the staple diet of the country’s neutered broadcasters and newspapers.

The admiration between Mr Carlson and the prime minister is mutual and so are their enemies. Last summer, the Fox News host spent a week broadcasting his programme from Budapest, lauding Orbán’s Hungary as a “small country with a lot of lessons for the rest of us”. “If you care about western civilisation and democracy and families and the ferocious assault on all three of those things by the leaders of our global institutions,” he continued, “you should know what is happening here right now.” Mr Carlson’s paean to Orbánism was capped with an interview with its gracious founding father.

Aside from a shared dislike of what Mr Orbán terms the “international left” and “gender madness” – his government last year launched a further clampdown on LGBT rights – the Hungarian prime minister and Mr Carlson are united in their hostility to immigration. After taking the oath of office after his handy re-election in April, for instance, Mr Orbán once again parroted the Great Replacement lie, warning of “recurring waves of suicidal policy” in the west to implement a “population replacement programme” which aims to “replace the missing European Christian children with migrants”.

The two men are also in agreement that one of the principal threats to “Christian Europe” is George Soros. The supposed danger posed by Jewish billionaire philanthropist – a generous benefactor of liberal and humanitarian causes and a convenient and long-time bogeyman for the Orbán regime – was set out by Mr Carlson in a documentary for the Fox Nation streaming service in January. Entitled “Hungary vs Soros: The Fight for Civilisation”, it first aired on the eve of Holocaust Memorial Day and praised Mr Orbán and damned Mr Soros in equal measure.

Despite his 30 years in politics, the prime minister is “surprisingly normal” – he apparently drives himself to work and eats out in Budapest without security, suggested Mr Carlson – while Mr Soros was presented as anything but. The philanthropist is, claimed the Fox presenter, “waging a kind of war — political, social and demographic war — on the west”, while working with the EU to “undermine democracy in Hungary”. His Open Society Foundation, moreover, is “trying to eliminate national borders, to oust democratically elected leaders, and install ideologically aligned puppets into positions of power”.

As US academic Cas Mudde told the Vox website, such chuntering about governments being overthrown and “puppets” imposed in their place, is “not just factually wrong but also … very much in line with classic antisemitic conspiracy theories”.

In a Fox News interview previewing the documentary, Mr Carlson was blunt about Mr Soros’ “programme” to make the societies “he focuses on more dangerous, dirtier, less democratic, more disorganised, more at war with themselves, less cohesive”. In other words, he concluded, “it’s a programme of destruction aimed at the west!”

The “conservative Disneyland” Mr Carlson believes Orbánism is building in Hungary is, of course, a sham. Mr Orbán’s populism – like that of Mr Carlson and their great hero, Donald Trump – has no time for the principles – respect for the rule of law, a belief in constitutionalism and an understanding of the importance of strong, healthy civic society institutions – which decent conservatives have long rightly advocated and upheld.