The far left controls the Labour leadership because the centre left did not take it seriously until it was too late. For a generation indeed, Labour and much of the rest of liberal-left Britain has lived with the comforting delusion that there was no far left to fight.
The left, on this reading, was one family. It may have had its troublesome teenagers. Their youthful high spirits may have made the little scallywags 'go too far' on occasion.
But everyone was still in one family, still on the same side. The old notion that the far left was the centre left’s enemy died away as the Labour party gave up on argument about what it was and what it wanted to achieve, and entered its long period of intellectual stagnation.
You can see the failure to think about basic principles, if you contrast how we talk about right and left-wing extremism. If a Tory politician shared a platform with a prominent member, or any member, of the BNP or Ku Klux Klan it would be a national news story. Journalists and political opponents would pile in, and the Conservative party would expel him or her without compunction. It may not even be possible now to become a Conservative politician if you were in a far-right party when you were young, even if you then rejected its dogmas in maturity. Certainly, critics would not dismiss your fascist past as 'youthful excess'. In other words, there is a border on the right of politics; a pale that conservatives cannot go beyond.
There have been no borders on the left for years. Labour politicians and left-wing activists can share platforms with Islamists, whose views are not so different from the views of neo-Nazis, and whose allies murder far more people than modern fascists do. Nothing happens. HOPE not Hate does not organise demonstrations outside their constituency officers. Broadcasters and journalists do not run stories asking what the politician’s behaviour says about Labour’s 'tolerance' of racism and misogyny.
The excuse that 'I can ally with clerical fascists because there are genuine concerns about Israel/ Western wars/ the treatment of Muslim immigrants' is accepted, whereas the equally derisory excuse 'I can ally with white fascists because there are genuine concerns about Muslim extremism/ Islamic State/ immigration' never is.
This double standard was once a huge cultural advantage to the left. It could appear virtuous, while its opponents appeared tainted. But all hypocrisies have their consequences. And what was once an advantage for the left has become its curse.
There are, as I and others have argued for years, left-wing arguments against Corbyn and his clique. Their support for Putin’s Russia, an imperialist kleptocracy, is as far away from the traditional liberal-left beliefs as their indulgence of religious tyrants. So widespread was the assumption that the far left was virtuous and could not be as malign as the far right, Corbyn’s rivals could not make a principled argument against him. They did not know what to say.
Now they must live with the consequences, and begin to put up what fight they can. The biggest fight will be over Andrew Fisher, Corbyn’s new head of policy, who infuriates Labour politicians more than any other figure the 'Jez we can!' revolution has elevated.
Even those shadow cabinet ministers, who said they had to work with Corbyn and embrace the enthusiasm of the movement behind him, are determined to get rid of Fisher. They may not yet be arguing on principle – perhaps that is asking too much of this generation of Labour politicians – but they are at least dispensing with the old lie that there is no difference between the far left and the centre left.
As my Observer colleague Toby Helm revealed, Emily Benn (granddaughter of Tony) reported Fisher to Labour’s national executive committee for breaking party rules. Benn was the official Labour candidate in Croydon South during the general election. Fisher tweeted that voters should support the Class War candidate instead of her. 'Actively advocating voting against the official Labour candidate, in favour of another party, contradicts Labour party rules,' Benn said, and rightly so.
Corbyn ought to have fired Fisher as soon as he saw Benn’s complaint. Instead, he and the far left are fighting to hang on to him because to their minds he has done nothing wrong.
For if you are on the far left, Fisher’s sentiments are unexceptional. Better to vote for Class War (and let the Tories in) than vote for Croydon South’s moderate Labour candidate. It feels as natural to say, as Fisher did, that Yvette Cooper is as bad as the BNP. This is what the far left has been saying about the centre left for years. Why should Corbyn treat it as a hanging offence?
Say what you like about him, but Fisher never pretended that the left was a happy family. He hated the mainstream Labour party, as indeed did Corbyn, Seumas Milne, John McDonnell and most of those around them. In the forthcoming London Mayoral election their honest sympathies will be with George Galloway rather than Sadiq Khan – a fact I don’t doubt Galloway will exploit in his campaign.
Now the far left that was contemptuous of most Labour MPs is in control of Labour MPs. Nowhere is its power over - and contempt for - the old Labour party better demonstrated than in its refusal to apply party rules to its supporters. Even the most docile Labour politicians realise it, and are learning at last that the left today is not a happy family. It has become a battlefield – and not before time.