The hundreds of thousands demanding that Britain ban Donald Trump should ask themselves if they would be happy about America banning Jeremy Corbyn. The answer you give will show whether you understand how to fight the illiberal ideologies that are flourishing across the western world.

It’s easy enough to make the case for keeping Trump out. Whether you should call him a “fascist” is open to debate. But the fact that you can debate it shows you are dealing with an incontinent demagogue. His sectarian proposal to stop Muslims coming to America is one of the most un-American assaults on the constitution’s protection of freedom of religion in modern times. The casting of Mexicans as rapists and murderers is the most extreme racial profiling I have seen in years. Trump has the swagger of Mussolini and the sexism of a pimp. Leaving all other considerations aside, any man who can say of his daughter, “If I weren’t happily married, and ya know, her father…” should be stopped at the borders by social workers.

Equally, the doziest homeland security officer could make a case for refusing Corbyn entry to the US. He was a shamelessly useful idiot for the state propaganda channels of Russia and Iran. He repeated Kremlin justifications for the Russian invasion of Ukraine. If Trump is an out and loud anti-Muslim bigot, Corbyn is a quieter but no less insistent defender of antisemites. A US government that once banned Nigella Lawson, who has brought nothing but happiness to humanity, for snorting cocaine, should find banning Corbyn easy.

Easy, but hopeless. Attacks on freedom of speech are not effective attacks on your opponents. In principle, they are repugnant. In practice, they are disastrous. Conspiratorial thinking isn’t a part of the far right or far left. It is essential to both. Their leaders need to persuade their supporters that dark forces are scheming against them. They need to believe it themselves. Only a conspiracy can explain the failure of others to agree with them; the death of their hopes and waste of their lives.

George W Bush’s former speechwriter, David Frum, encapsulated how conservative America was barricading itself in a stockade when he said: “Republicans originally thought that Fox worked for us and now we’re discovering we work for Fox.” It’s not that Fox and the shock-jock stations have huge audiences. But they are listened to by the minority who vote for Trump in Republican primaries. Fox makes $1bn a year by painting every moderate republican as a sellout and every liberal opponent as a monster. There’s no evidence that it makes undecided voters turn into headbanging conservatives. On the contrary, it helps the Republicans lose by creating the atmosphere in which a bloated billionaire with a flabby tongue and rancid mind can pose as the defender of the “little guy”.

Even when other Republican candidates are trying to cling on to what remains of their party’s honour by criticising Trump, all Howard Kurtz of Fox News could manage was a sly whine about liberal journalists making “partisan” criticisms – as if political argument could ever or should ever be anything other than partisan. Stay in the tribe, he said in effect. Condemn Trump and you place yourself on the same side as the hated liberal elite.

Anyone who has tried to repeat the established facts about Corbyn will have seen the tactic deployed with depressing success here. First you’re told it’s a lie. Then you’re told it doesn’t matter. Then you’re told you’re a Tory.

Banning only heightens the conspiratorial delusion. The liberal elite, the new world order, the establishment, –THEM – want to silence truth and punish the bearers of bad news. Since the 1960s, liberal societies have replaced old laws censoring sex and violence with new laws censoring racism, sexism and homophobia. You cannot say that they have slowed the growth of far-right parties and movements across the developed world. On the contrary, censorship has made reactionary politics seem like honest dissent. France has more laws restricting free speech than any other western democracy. It also has Europe’s largest far-right party, which revels in its status as a victim of a coercive technocratic elite.

At the root of this self-inflicted liberal defeat is an inability to understand how argument works. You believe, with justice, that nothing you say will convince Trump or Marine le Pen that they are wrong. They are your enemy: unteachable and irreconcilable. You do not treat them as decent opponents, but use every power available at your disposal to defeat them.

Go too far down that hard road, however, and you lose sight of their lukewarm supporters and potential converts. They are spectators at the contest. They are waiting to see how the argument goes before making up their minds. The explosion of nationalist populism suggests that screams of “no platform” and calls to send in the police have not impressed.

To persuade them to break with leaders, you must go further than accept you don’t win debates by gagging them. You must address the real grievances of an extremist’s supporters, or at least find it in yourself to admit that not all their grievances are unreasonable. It is easy enough to denounce Trump; harder to admit that liberals on both sides of the Atlantic have treated criticism of Islamists as criticism of all Muslims and accused those fighting the most illiberal movements on Earth of racism.

By all means condemn Corbyn’s association with radical Islam and Russian imperialism, but if you forget to add when you bring up his closeness to antisemites that the Palestinians have a non-negotiable right to a state of their own, you will not deserve a hearing.

European politicians have every reason to warn against the far right, but will deserve to fail if they do not accept that it is driven by the misery that the single currency has brought and legitimate questions about how many immigrants European societies can take. And everywhere, whether in Europe or America, established politicians have yet to say how long their electorates should cope with falling incomes for the working and middle classes, or how they will offer hope of a future where children have better lives than their parents.

Condemning extremists is as easy a signing an online petition. Detaching wavering supporters from the hardcore is a long struggle. You cannot expect to enter the fight and emerge with every one of your old convictions intact.

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