Yes, it’s belated. Yes, it’s just one issue among a host of others. Yes, they helped to push him to prominence in the first place. But beneath all the caveats, Momentum was at least right to distance itself from Peter Willsman this week, after the Corbynite candidate for Labour’s NEC was recorded claiming that Jewish “Trump fanatics” were “making up” claims of anti-semitism in the Labour Party “without any evidence at all”.

Whatever the exact reasoning behind deciding Willsman was bad news at this particular point, it’s a welcome decision on Momentum’s part.

However, the response within the Labour grassroots has been both alarming and instructive. Since announcing that they were withdrawing support for the candidate, Momentum’s social media outlets have been deluged with complaints and criticism from the group’s members, supporters and sympathisers. It is a “Blairite” decision, apparently; an indulgence of “smears” by Jeremy Corbyn’s enemies; a cowardly retreat in the face of conspiratorial criticism. Some are even quitting the group in protest.

Viewed from the outside, the stereotype of Momentum is often that it is carefully and authoritatively controlled by Jon Lansman and his lieutenants, the ranks of loyal Corbynites following orders on what to say and where to campaign. Perhaps some of those inside the machine sometimes believe that to be true, too.

If so, this backlash is a rude awakening.

The monster which Lansman and others have created is now refusing to obey them. The tactics used to forge ‘the movement’ have worked rather too well. In the face of any criticism, no matter how serious or true, they soaked Corbynism’s grassroots with the language of excuses and whataboutery; they encouraged the view that critics must be complicit in counter-revolutionary plots against the Dear Leader; in short, they inculcated a siege mentality, and their audience evidently listened closely.

That might have seemed fine when the targets were Blairites and Tories, but it is probably less comfortable for Momentum’s leadership to be on the receiving end. Having made a relatively sensible decision, they now find a chunk of their own base have turned their vitriol on them. Prominent Corbyn-supporting commentators are experiencing the same attacks in response to their own objections to Willsman.

This is familiar stuff to anyone who has ever expressed public doubts about Corbyn, his views or his allies. Criticism is not to be accepted, comrades are to be defended, and anyone attacking a comrade must be an enemy. Even on the serious issue of anti-semitism, apparently the interests of the movement win out above any other consideration.

Aside from the rather poetic irony, there is a serious implication. It’s no use relying on Momentum, or other supposedly influential Corbynite commentators, organisers, groups or politicians, to rein in the sickening extremes of their movement. Even when they are willing to do so – too rarely, even now – it turns out that they are not able to. Controlling their own creation is no longer in their power, if it ever was.

Worse, that power to put a stop to this obscenity lies with one man and one man only: Jeremy Corbyn himself. And he still shows no sign of realising the nature, scale or severity of the problem.