The clown is no longer funny; the joke is now a sinister threat. Donald Trump has long been treated as a laughable absurdity, the generator of the occasional Twitter storm, a “only in America” caricature who simply could not be taken seriously. Yes, polling makes him the decisive Republican frontrunner with only two months until the first primary; in Iowa – one of the first states to choose – he has a 13-point lead.

But surely a campaign fuelled by idiocy, chauvinism and bigotry is doomed. Surely, as some speculate, he is a spoiler (consciously or otherwise) to help his friend Hillary Clinton. It would be like uber-troll Katie Hopkins becoming the Tory party’s frontrunner for leader.

You can stop laughing now. Donald Trump’s call for a “complete shutdown” of Muslims coming into the United States – including US Muslims abroad – matters, and not just in the United States, for three reasons.

Donald Trump: ban all Muslims entering US Read more

Firstly, any complacency about anti-Muslim prejudice must now be discarded. Yes, in the dark days of the 1930s, there were US demagogues like the priest Charles Coughlin who used a national radio show to incite hatred against Jews: but even then, the Republican frontrunner didn’t call for Jews to be barred from entering the US.

Secondly, his grim intervention shifts the terms of acceptable political debate. Anti-Muslim prejudice that is more sophisticated and subtle (which isn’t hard) suddenly seems less extreme, and somehow instantly more palatable.

Thirdly, Trump demonstrates that Islamic State is winning. One of the death cult’s strategic aims is to divide western societies from western Muslim communities: to fuel a sense among Muslims that they are rejected, unwanted, even despised, driving them into the camp of global jihad.

Trump’s comments do not exist in a vacuum. Anti-Muslim prejudice has been mounting across the western world ever since al-Qaida’s atrocities in New York, Washington and Pennsylvania back in 2001. Polling in June found that, if presented with a “well-qualified” Muslim candidate for president, 38% of Americans would not vote for them. Even among more accepting younger Britons, a 2013 poll suggested that 44% believed Muslims did not have the same values as other Britons, and 28% thought the country would be “better off” with fewer Muslims. Another poll the same year – taken in the aftermath of Lee Rigby’s murder – found that 34% believed Muslims represented a serious threat to democracy. A resurgent European far-right targets Muslims, just as their forerunners scapegoated Jewish people: whether it be the Front National, thriving in France’s weekend regional polls, the Swedish Democrats, the Danish People’s party or the Austrian Freedom party.

Obama condemned Islamophobia in America. It's time Republicans did, too | Nihad Awad Read more

The “Overton Window” is a concept that was born in rightwing American circles. It is a simple idea: everything within the window is seen as acceptable and mainstream, and everything outside is seen as extreme and fringe, but it can be shifted. With one ignominious intervention, the window has now been moved, and various anti-Muslim bigots can say: “Well Donald Trump has gone too far, but here’s what I would say instead.” They suddenly become the more moderate alternatives where once they would have been seen as themselves extreme. Mehdi Hasan has admitted to missing George W Bush because of the anti-Muslim shift that has taken place within the Republican fold since.

It wasn’t just what Trump said; it’s the fact his fans instantly cheered. This is no clown, no pitiful absurdity that can simply be laughed off. Trump is a menace, an inciter of bigotry and a recruiting sergeant for terrorism – and this must be taken seriously.