Garvan Walshe is a former national and international security policy adviser to the British Conservative Party. He runs Brexit Analytics.

To fight bigotry, it must be made unacceptable and unprofitable. Appeals to it, coded as well as open, need to be rejected and harsh consequences felt by people who would use it for political advantage. That’s what it means to uphold a norm.

At the core of all anti-semitism is the charge of dual loyalty: that Jews, from the time that, in the Christian telling, they chose to release Barabbas and condemn Jesus to the cross, put their fealty to the Jewish community ahead of justice. We, of course, associate anti-semitism with the right, because it comes easy to nationalist movements. Thus the question posed to a Jewish Conservative at a selection meeting in the late 1990s: “If Britain and Israel went to war, whose side would you be on?” (His reply “Against whom?” brought the house down, but he didn’t get the seat).

But it’s been just as easily accommodated by the Marxist left. Marxism is a nationalism of class, and it put Jews (though frequently Marxist themselves) under the same cloud. Would they show true class loyalty, or would it be safer to purge them, as Poland’s Communist government did in 1968?

The modern far left takes the class struggle global. The world is divided into the colonisers and oppressed, and Jews are twice damned by this division. Though an ethnic minority, they have prospered in Western society. Judged by socioeconomic indicators, their fortunes are as at least as good as those of the native white population. And if they were once stateless and deserving of outside sympathy and protection, their powerful and warlike nation-state has joined the oppressors’ ranks with élan.

So when British Jews raise accusations of anti-semitism levelled at them by sympathisers of the “good” class, even when, as with that mural in East London of hook-nosed men playing Monopoly on the backs of naked workers in front of a Masonic magic eye, which could have come straight from the frontispiece of The Protocols of the Elders of Zion, the far left doesn’t believe them.

Corbyn, who likened the mural to a painting by the Mexican artist Diego Rivera, had got used to overlooking anti-semitism in plain sight. He is a long-standing supporter of the Palestinians; and, like anti-Zionism, pro-Palestinian activism doesn’t have to be anti-semitic, but it often is. Yet, though anti-semitic themes have been part of opposition to Zionism since before Israel was created, but they’re not inherent in it. Corbyn could have chosen to support Palestinians who rejected anti-semitic language and tropes, or made his endorsement of them conditional on them giving up their anti-Jewish prejudice, but instead chose to overlook it. Certain that he himself wasn’t anti-semitic, and perhaps judging that while the Jews could defend themselves, the Palestinians needed people to speak up for them, he got into the habit of refusing to defend the norm.

The Labour MPs who signed his nomination papers “to encourage debate” also allowed the norm to crumble. And since he became Labour leader, Corbyn allowed the classic left-wing form of anti-semitism, of the Jews as beneficiaries of the unjust capitalist and colonising system, to resurface and spread on his watch.

Jewish community organisations’ complaints have been dismissed. They have not been treated as representing the real fears of a community that feels vulnerable, but as self-interested pleading. In this, Corbyn has the Labour membership behind him: in polling for The Times on 31st March, 61 per cent of Labour members thought Corbyn has handled the row well. They, too are refusing to defend the norm.

So when Corbyn decided to go to the seder (Passover celebration) thrown by the juvenile provocateurs of the ‘Jewdas’ Jewish community group, it could have been just another careless mis-step by a politician long known for his naiveté. Another, altogether more sinister, explanation cannot however lightly be dismissed: that he went to tell Jews that they’re on their own, and demonstrate that their complaints can safely be ignored. In Corbyn’s Labour, Jews have become citizens of nowhere.