Racism is not a specific illness but a general sickness. Display one symptom and you display them all. If you show me an anti-Muslim bigot, I will be able to guess his or her views on the European Union, welfare state, crime and “political correctness”. Show me a leftwing or Islamist antisemite and, once again, he will carry a suitcase full of prejudices, which have nothing to do with Jews, but somehow have everything to do with Jews.

The Labour party does not have a “problem with antisemitism” it can isolate and treat, like a patient asking a doctor for a course of antibiotics. The party and much of the wider liberal-left have a chronic condition.

As I have written about the darkness on the left before, I am not going to crow now that it has turned darker than even I predicted. (There is not much to crow about, after all.) I have nothing but respect for the Labour MPs who are trying to stop their party becoming a playpen for fanatics and cranks. It just appears to me that they face interlocking difficulties that are close to insoluble.

They must first pay the political price of confronting supporters from immigrant communities, which Labour MPs from all wings of the party have failed to do for decades. It may be high. While Ken Livingstone was forcing startled historians to explain that Adolf Hitler was not a Zionist, I was in Naz Shah’s Bradford. A politician who wants to win there cannot afford to be reasonable, I discovered. He or she cannot deplore the Israeli occupation of the West Bank and say that the Israelis and Palestinians should have their own states. They have to engage in extremist rhetoric of the “sweep all the Jews out” variety or risk their opponents denouncing them as “Zionists”.

George Galloway, who, never forget, was a demagogue from the race-card playing left rather than the far right, made the private prejudices of conservative Muslim voters respectable. Aisha Ali-Khan, who worked as Galloway’s assistant until his behaviour came to disgust her, realised how deep prejudice had sunk when she made a silly quip about David Miliband being more “fanciable” than Ed. Respect members accused her of being a “Jew lover” and, all of a sudden in Bradford politics, that did not seem an outrageous, or even an unusual, insult. Where Galloway led, others followed. David Ward, a now mercifully forgotten Liberal Democrat MP, tried and failed to save his seat by proclaiming his Jew obsession. Nothing, not even the murder of Jews, could restrain him. At one point, he told his constituents that the sight of the Israeli prime minister honouring the Parisian Jews whom Islamists had murdered made him “sick”. (He appeared to find the massacre itself easier to stomach.)

Naz Shah’s picture of Israel superimposed on to a map of the US to show her “solution” for the Israeli-Palestinian conflict was not a one-off but part of a race to the bottom. But Shah’s wider behaviour as an MP – a “progressive” MP, mark you – gives you a better idea of how deep the rot has sunk. She ignored a Bradford imam who declared that the terrorist who murdered a liberal Pakistani politician was a “great hero of Islam” and concentrated her energies on expressing her “loathing” of liberal and feminist British Muslims instead.

Shah is not alone, which is why I talk of a general sickness. Liberal Muslims make many profoundly uncomfortable. Writers in the left-wing press treat them as Uncle Toms, as Shah did, because they are willing to work with the government to stop young men and women joining Islamic State. While they are criticised, politically correct criticism rarely extends to clerics who celebrate religious assassins. As for the antisemitism that allows Labour MPs to fantasise about “transporting” Jews, consider how jeering and dishonest the debate around that has become.

When feminists talk about rape, they are not told as a matter of course “but women are always making false rape accusations”. If they were, they would suspect that their opponents wanted to deny the existence of sexual violence. Yet it is standard in polite society to hear that accusations of antisemitism are always made in bad faith to delegitimise justifiable criticism of Israel. I accept that there are Jews who say that all criticism of Israel is antisemitic. For her part, a feminist must accept that there are women who make false accusations of rape. But that does not mean that antisemitism does not exist, any more than it means that rape never happens.

Challenging prejudices on the left wing is going to be all the more difficult because, incredibly, the British left in the second decade of the 21st century is led by men steeped in the worst traditions of the 20th. When historians had to explain last week that if Montgomery had not defeated Rommel at El Alamein in Egypt then the German armies would have killed every Jew they could find in Palestine, they were dealing with the conspiracy theory that Hitler was a Zionist, developed by a half-educated American Trotskyist called Lenni Brenner in the 1980s.

When Jeremy Corbyn defended the Islamist likes of Raed Salah, who say that Jews dine on the blood of Christian children, he was continuing a tradition of communist accommodation with antisemitism that goes back to Stalin’s purges of Soviet Jews in the late 1940s.

It is astonishing that you have to, but you must learn the worst of leftwing history now. For Labour is not just led by dirty men but by dirty old men, with roots in the contaminated soil of Marxist totalitarianism. If it is to change, its leaders will either have to change their minds or be thrown out of office.

Put like this, the tasks facing Labour moderates seem impossible. They have to be attempted, however, for moral as much as electoral reasons.

Allow me to state the moral argument as baldly as I can. Not just in Paris, but in Marseille, Copenhagen and Brussels, fascistic reactionaries are murdering Jews – once again. Go to any British synagogue or Jewish school and you will see police officers and volunteers guarding them. I do not want to tempt fate, but if British Jews were murdered, the leader of the Labour party would not be welcome at their memorial. The mourners would point to the exit and ask him to leave.

If it is incredible that we have reached this pass, it is also intolerable. However hard the effort to overthrow it, the status quo cannot stand.