Depending on who you listen to, Momentum is either the cutting edge of 21st century progressive activism, or a Corbynite conspiracy to take over the Labour party, eject any errant MPs, and commence the age of copper-bottomed British socialism, following the new leader’s example.

According to its online blurb, the group is intended to be “a network of people and organisations that will continue the energy and enthusiasm of Jeremy’s campaign”. There are 150,000 names on its email list. It has four full-time staff and pledges of funding from trade unions. Branches are forming all over the country; exactly what they will do, and how they may or may not connect with the Labour party, are currently matters of discussion and debate. But after the torrid events of recent weeks – the Labour split on Syria, endless explosions of nastiness, and all that talk of deselecting “pro-war” MPs – certain people’s understanding of what Momentum is up to now drips with disdain and fear.

Tom Watson, the Labour deputy leader, calls the group “a bit of a rabble”. The former minister Caroline Flint warns it may yet “destroy our party”. Wayne David, the hitherto obscure MP for Caerphilly, wants Momentum wound up – because, he says, “it is clearly acting as a party within a party”.

Listening to all this fuss, it feels like a bit of humility might be in order. Among other things, Momentum is providing a focus for the twentysomethings brought into Labour politics by Corbyn’s leadership campaign. In a lot of places, young idealists have already collided with staid local party branches and wondered what to do with themselves. Momentum apparently aims at something much more in tune with not just the political appetites of Labour’s new members, but the early 21st century.

There is talk of setting up food banks, a new private-sector tenants’ union and childcare cooperatives. To its credit, the organisation’s first high-profile announcement was of a massed voter registration drive, aimed at some of the 1.9 million people who may be about to be excluded from the electoral roll. Moreover, its activists say the benefits Momentum might bring to the Labour party are clear: witness, for instance, the five Momentum coaches that took hundreds of its people to Oldham West and Royton the weekend before that unexpected Labour triumph.

This is the kind of stuff Momentum’s young movers think is being overlooked. But some of them also acknowledge the more troublesome bits of what’s going on. One centres on an array of splinter groups and micro-parties, among them the Socialist party (formerly known as Militant, before they decided to leave Labour alone), Ken Loach’s less than successful Left Unity, and what remains of the Socialist Workers party.

So far, these elements have taken advantage of the organisation’s openness to non-Labour people by piling into its events. But on Tuesday, Momentum announced they would not be allowed into its decision-making gatherings, though its “public meetings” will remain open to all comers.

‘Its activists say that the benefits Momentum might bring to the Labour party are clear: witness the five Momentum that took hundreds of its people to Oldham West and Royton the weekend before that unexpected Labour triumph.’

What this ruling leaves untouched is an associated element within Momentum that is causing no end of trouble, made up of people who have come back to Labour after a long time in self-imposed exile. One party insider characterises them as being “white, male and over 55”. A lot of them, he says, seem to have left Labour in the early 1990s, furious at even Neil Kinnock’s supposed betrayals. They tend to have a deep attachment to the writings of dead Russians, and remain well versed in the kind of bureaucratic manoeuvring that certain left activists tend to mistake for changing the world (“resolutionary socialism”, as one of my Labour friends puts it).

In some places, they seem to be engaging in a cold, calculated nastiness aimed at ensuring the people they see as adversaries simply give up on Labour, leaving them in control. Those of us who were driven to despair by Militant in the 1980s will instantly recognise the basic script: one Labour MP I recently spoke to talked about party meetings suddenly being packed with new (if old) faces, and apparently orchestrated attempts to “wind people up so much that they just leave the room – heckling, proposing resolution after resolution, and not allowing any kind of debate to unfold”. A lot of younger Corbynites, he went on, view these old-school operators as passionate idealists in the Jeremy mould rather than hard-left wreckers, which compounds other members’ increasing sense of desperation.

This is where the issue of deselection – and not just of MPs but also local councillors – comes into sharp focus. For Momentum’s veteran element, it is as totemic now as it was in the early 1980s. From a sceptical perspective, it betrays their comically limited horizons, and a politics in which the electorate – remember them? – recedes into the distance as “representation” becomes a tightly maintained client relationship between MPs and party activists. More generally, it speaks of an insane quest to remould an organisation as diverse as the Labour party into a homogeneous monolith. Those involved ought to learn from their enemies: on that score, even the equally manipulative people behind New Labour were miserable failures – ironically enough, the fact that Corbyn is now in charge proves it.

Which brings us to an obvious question: while all this stuff bubbles away, where is the current leadership? On a bad day, it is difficult to shake off the sense that Labour’s older troublemakers remain a big part of Corbyn and John McDonnell’s essential tribe. Amid speculation about a “revenge reshuffle”, Corbyn talks about MPs who voted for airstrikes on Syria having “no hiding place”. McDonnell seems to pour cold water on the prospect of organised deselections, and yet tells Momentum activists they will “eventually be able to ensure that the people selected represent you”. (As ever, exactly where MPs’ constituents fit in is unclear.) Meanwhile, the leadership’s surreally verbose outrider Ken Livingstone is characteristically upfront: “People” – and, obviously, he means his people – “have got a right to a candidate they agree with,” he says. Note also that Momentum’s founder is Jon Lansman, a veteran Labour activist who cut his teeth campaigning for the old hard-left shibboleths of mandatory reselection and MPs controlled by the Labour membership, and remains a devout believer in both.

Momentum’s forward-thinking activists sound both more emollient and a bit more imaginative. They say they have a battle on with their group’s old-school elements but they are trying to change Labour’s culture and practices, not single out particular MPs, and fixating on the party rule book. They talk about their fear of being dragged into a calamitous civil war, and what that would do for the party’s electoral chances. And though they seem just as enthusiastic about the party’s new turn as they were over the summer, some of what they say suggests two overlooked facts: not just that things inside Labour are much more complicated than some people think, but that even at the heart of what Corbyn supporters call “the new politics” there are tensions and contradictions that may yet explode.