A Tory government making our country more unjust; a harsh chaotic Brexit that threatens devastating consequences for jobs, living standards and the economy; a resurgent populist right, headed by president-elect Donald Trump, sweeping the western world.

These are the things I want to write about, not the internal woes of the left. The left has had something of a reputation for turning infighting into an art form, immortalised by that People’s Front of Judea sketch in Monty Python’s Life of Brian. But an emergent crisis in Britain’s left is so serious that, sadly, it cannot be ignored.

Momentum – the grassroots movement set up in the aftermath of Jeremy Corbyn’s leadership victory – is currently facing a takeover bid by Trotskyist sectarians. If they succeed, Momentum will be destroyed. The most prominent sectarian figures are embittered veterans of struggles from the 1970s and 80s, people who have only experienced defeat, and who won’t let an unexpected opportunity afforded by the seismic political developments of the last two years slip through their fingers. This is their last chance.

They jump from organisation to organisation, and are adept at manipulating internal structures for their own advantage: sitting out long boring meetings, coordinating interventions, playing victim when it suits. They’re not interested in say, door-to-door campaigning, but rather in debating their obscure pet issues with long-winded interventions at meetings on a Thursday evening.

Their opponents are younger, idealistic, campaign-oriented and pluralistic, lacking Machiavellian strategic ability – all of which the sectarians exploit. The sectarians smear their opponents as rightwingers, Stalinists, bureaucrats, as having ulterior and sinister motives (this article will be dismissed as the work of a rightwing establishment careerist in the service of a Guardian conspiracy to destroy the left). Everything goes wrong, they believe, not because of their own almost farcical strategic ineptitude, but because of the betrayal of others. Momentum offers hope to young people who have long been demoralised by politics. Those wrecking Momentum – if they succeed – could destroy that hope, and that is unforgivable.

When Corbyn stood, few believed he could win: the function of his candidacy was to shift the terms of debate leftward

Momentum was originally set up by a grouping led by Jon Lansman – a veteran Labour leftist – to harness the energy behind the Jeremy Corbyn leadership campaign. When Corbyn originally stood, few believed he could win: the function of his candidacy was to shift the terms of political debate leftward and galvanise a grassroots movement. But he did, so Momentum sought to turn the excitement behind the Corbyn surge into a community-directed movement, organising the sorts of people who find the traditional, bureaucratic structures of the Labour party alienating. A pluralistic movement that would run local campaigns and start ambitious community initiatives, such as food banks, for instance. The highlight of Labour’s otherwise drab party conference this year was Momentum’s the World Transformed, a series of events that won accolades even from hostile media commentators. When it came to Corbyn, many of us were more interested in the wave than the surfer. Momentum was a key part of the wave.

But the sectarian groups had other ideas. If you’re a British Trotskyist, the last few years have been miserable. The biggest cuts for generations, the longest period of falling wages since the 19th century, huge demonstrations and strikes: but the Trotskyist groups haven’t benefited at all. Their memberships have stagnated or even fallen. They offer no appeal, even at a time of social and political upheaval. They spend years finger-wagging about how the Labour party is dead, denouncing any leftwinger who says otherwise, and then suddenly there is a massive upsurge of the left. Where? In the Labour party. Take the barrister Nick Wrack, one of the sectarian leaders. Last year he stood for the catchily named Trade Union and Socialist Coalition in Camberwell and Peckham, and secured 0.6% of the vote. There’s no life for the left in the Labour party, he told me in 2014.“Time for [a] new party that stands for socialism,” he lectured me before the general election. I was right to call for more working-class representation, he tells me, “but it won’t come from Labour,” he tells me, after Labour’s defeat.

‘Momentum was originally set up by a grouping led by Jon Lansman – a veteran Labour leftist – to harness the energy behind the Jeremy Corbyn leadership campaign.’ Photograph: Brayan Lopez/Rex/Shutterstock

The younger Momentum protagonists aligned to Lansman – who himself has gone on a political journey away from top-down structures – are known as “movementists”: those who dislike hierarchies and who are attracted by social movements.

Their proposal for Momentum was this: rather than Momentum’s direction being decided by delegates sent by local groups, it should be decided democratically online. Direct democracy: all of Momentum’s 21,000 members get an equal say. Yet such a movement would destroy the influence of the sectarians. Yes, they can dominate sparsely attended boring branch meetings on rainy evenings and secure the election of delegates. But they represent a tiny proportion of the membership. The more the members are empowered, the more the sectarians are disempowered.

The proposal was debated at Momentum’s national committee, events well described by committee member Laura Catriona Murray and others: a fraught meeting where, other than Lansman, the split was broadly on generational lines. Some older sectarians used aggressive tactics against the younger movementists, reducing at least one to tears. But they won – by one vote – defeating direct democracy.

If Momentum’s plight is baffling, consider it like this. You have a birthday party and you have to invite all your workmates, and that includes Jim: you’re a nice inclusive person, and it’ll only cause a scene if you don’t. But Jim is a problem. Jim turns up to your party with a couple of obnoxious friends and they start taking it over, aggressively ranting and raving about their pet subjects. People start to trickle away from the party, including the new ones you wanted to befriend: they won’t come next time you invite them. You tell Jim you’re not asking him and his friends to leave, but could they calm down a bit and let others speak and stop being so domineering? Jim gets angry. They’re just having their say! What’s your problem with them! Don’t you allow free speech at your party?

Unlike Jim, the sectarians are highly disciplined, highly organised, and highly experienced. The interests of their own sects are far more important than any movement. Only their sect, they believe, has the correct politics: everybody else’s are fatally flawed. They have no faith in the Labour party. Momentum, for them, is an embryonic political party. The prize is Momentum’s contact data, containing the details of tens of thousands of people. At an opportune time, they will walk away from Labour and found a new party, which will get 300 votes in a byelection. They will triumphantly hail these as 300 votes for socialism.

These sectarians must be stopped. They are throttling the enthusiasm and excitement of the young people who have been inspired in the last 18 months. One man is uniquely placed to save Momentum from the sectarians who would throttle the enthusiasm and excitement of the young people who have been inspired in the last 18 months. That’s Jeremy Corbyn. An intervention by him would stop Momentum being taken over, allowing its rebirth as an open, campaign-focused movement. Without that, the sectarians will win. They must be stopped in their tracks. So much hope, so much optimism. We can’t let it end in rancour and betrayal.