Nothing corrodes like victory. Ukip’s triumph in the referendum has triggered its near-complete breakdown. The success of Vote Leave resulted in its biggest brain, Michael Gove, immolating both his career and the prime ministerial hopes of the campaign’s biggest star, Boris Johnson. A similar fate has befallen Momentum, the organisation born of Jeremy Corbyn’s first successful bid for the Labour leadership and the motor for his second landslide in succession. Victory over Corbyn’s rivals in September has been swiftly followed by infighting.

The seeds of the discord were planted at Momentum’s birth. Its founder, Jon Lansman, entered politics as Tony Benn’s back-room fixer. He knew that Corbyn’s leadership would be challenged in parliament by sceptical MPs, and at the grass roots by more centrist activists. Momentum’s early guarantors in the labour movement – the transport union TSSA and the Unite mega-union – also saw its purpose as being to rebuff such efforts. And so Momentum came into being as a cudgel for the party’s left. It was designed to be a power base for the leadership, just as Labour First and Progress had been for the party’s right.

Momentum, however, had other archi­tects with other motivations. Some of them hoped that it would fulfil the promise of “a new kind of politics” that Corbyn made during his leadership campaign in 2015. They looked not to the struggles of the 1980s but across the Channel to the emerging parties of the European radical left, such as Podemos in Spain. The highest-profile advocate for that approach was James Schneider, one of Momentum’s national organisers and the group’s public face. Schneider, who joined Labour after the 2015 election, had impressed some of the left’s veterans as a volunteer on the Corbyn campaign. He also proved to be a natural in front of the camera, able to stick to the party line with verve and conviction.

When the cameras weren’t rolling, Schneider was a less biddable presence. Along with Emma Rees and Adam Klug, two other young staffers, he favoured a more open and democratic approach than the one that Lansman pushed.

The split was generational, though not in the way that might immediately be supposed. Yes, Lansman is pushing 60 and Schneider is not yet 30, but mostly it was the number of years spent as a Labour member that was the fault line, rather than age.

That divide was felt in Momentum’s back rooms and at meetings, but it rarely threatened to reach the public eye. From time to time, a Momentum official might complain about “the munchkins”, as Schneider and his allies were nicknamed, or newer activists might fret that the organisation was far too much dominated by old veterans fighting yesterday’s wars.

The generational split was manageable, but now there is a new schism over the group’s structures – and specifically how much power Lansman holds. This became public because, of all things, he scheduled a meeting of the steering committee at short notice. Those present on 28 October voted 6-3 to cancel another meeting, this time of Momentum’s national committee, at which Lansman’s opponents had hoped to remove some of his power.

Two days later, a group called Labour Party Marxists issued a statement asking: “How could we ever again gripe about the bowdlerising of Labour party conference democracy if we acquiesce to the travesty that Jon Lansman and his cohorts are attempting to finagle us into?” Matt Wrack of the Fire Brigades Union sent an email to the rest of the steering committee describing the situation as “a major crisis”.

For Lansman’s allies, what is at stake is the future and purpose of Momentum. They believe that their opponents want not to transform the Labour Party but to leave it – and use Momentum to found a new party to Labour’s left. It is telling that the biggest beneficiary of the revolt against Lansman has been the Alliance for Workers’ Liberty (AWL), a small Trotskyist organisation. One AWL member, Jill Mountford, sits on Momentum’s steering committee.

The issue that preoccupies Lansman’s allies – the group’s relationship with the Labour Party – also motivates his opponents. They are divided between those who deem him too sympathetic to Labour and those who view him as a would-be Bonaparte, desperate to control Momentum as his own creation. This faction notes that he is in sole control of the valuable contact data collected during Corbyn’s leadership campaign.

Lansman’s reaction to the crisis has been to take a bold gamble. He has long believed in delegate-based structures (not least because whoever controls the delegates controls the outcome). But he has now come round to the idea of online votes by the grass roots on important issues. Under new proposals, Momentum’s 20,000 members will decide its structures and its fate.

Michael Chessum, another steering committee member, has attacked this plan on Facebook, arguing that it would deprive Momentum’s conference in February of power and turn it into merely “a live-streamed national gathering”, with the group’s “structures decided by e-ballot”.

Who will win? It’s hard to call. Lansman’s strategy is risky because Momentum’s members could yet vote to adopt a delegate system, making a gift of the organisation to his internal critics and ultimately, perhaps, to the AWL.

Before this crisis, Schneider and others were talking confidently about Momentum’s expansion, with plans to train community organisers and test new campaign methods. Yet it seems that before moving forward, the group must first decide what it is – and what it wants.

George Eaton is away