For Labour members, losing eight MPs who have spent months gaslighting, slandering and sabotaging us, should be no big deal. They’ve spent months hopping on and off the couches of the talk shows, systematically undoing hours of activism on the doorstep. But that is where the upside of the Independent Group’s departure stops. To understand why, let’s do a thought experiment.

Suppose, instead of another eight or ten MPs from the Labour and Conservative parties, half of all MPs in parliament joined Chuka Umunna’s group. They would become the government without an election. Suppose they then said to the public, as Umunna and co. have: “send us your ideas for what policies we should pursue,” adding “but don’t rush”.

You would then have the ideal form of neoliberal governance in the UK – and not far off what you have in the French National Assembly under Emmanuel Macron. A single centrist party with no programme other than to manage the failing system, deploying riot cops on the streets against people who don’t like it, and manipulating the constitution to disconnect the executive from democratic control. Their task would be – as Macron’s is – to delay as long as possible the open fight between the two rising forces: radical leftism and authoritarian racism.

There is no chance, I hope, of this thought experiment becoming real. But in a less extreme form it is exactly what is threatened by Tom Watson’s extraordinary solo video.

Watson was signalling: I agree with what the seven renegades say, but not with what they’ve done; and if Corbyn refuses to change, I could do the same. Nobody could accuse Corbyn of poor party management over the loss of Joan Ryan MP. But preventing a cold split with Labour’s political centre is a critically important task.

There may be some parts of the Corbyn support base that fantasise about removing all MPs to the right of Ian Lavery, taking organisational control of all Constituency Labour Parties, council groups and socialist societies, and subjecting all who waver to hit-jobs by Skwawkbox – but I am not one of them. For me, the left political project is about hegemony: mobilising a wide electorate that wants progressive change around a radical programme of social justice; and building the political alliance necessary to get it past parliament, the civil service, the courts, international trade obligations and the British state.

It is clear – and has been clear for years – that the purpose of the Blairite stay-behind group inside Labour was to prevent a left government taking power. A left government, with the working poor mobilised behind it, would be in position to take the first major country out of the neoliberal experiment that began 40 years ago, and dismantle the power of the financial elite that has installed itself at the heart of the British system. Plus, it would weaken the UK’s support for repressive regimes in the Middle East. Their job is to stop that – and to do it they’ve been prepared to stab the People’s Vote campaign in the back as well as their own constituents.

But there is another group within Labour which, though swamped by the mass influx of leftish members, remains genuinely attached to the party’s traditions. For them, the problem is not the radicalism of the economic programme, but what a Labour government might do to Britain’s pivotal role in the global power system.

Wider than that, is a much bigger group of MPs, members and voters who rightly see Brexit as a project of the xenophobic right, a threat to jobs and growth, and an act of vandalism against an open and tolerant British culture – and who view Corbyn’s triangulation with Leave voters with dismay.

For Corbyn, the art of statecraft during the next three months should be to prevent the project of the Umunna group from synergising in any way with the discontents of these two wider groups.

But Umunna’s strategy is tailor-made for exactly this effect. It is aimed at stirring up so much bitterness among members against the splitters that its spills over into hostility against perfectly loyal but critical Labour members. When Chris Leslie says Corbyn is a “threat to national security” he means “cannot be trusted to unleash NATO’s lethal firepower against unarmed civilians”. But the argument is also designed to resonate with those critical of Corbyn’s nitpicking response to the Skripal attack, or his position on Syria, or his professed dislike of the nuclear deterrent.

The Umunna split was long planned and is a long game. Its aim is to destroy the cohesion of the Labour Party, wear down its membership and prevent it winning the next election. But Watson and his followers – as far as I can tell - want something else. They want an end to fractiousness, probably an end to the further democratisation of the party at a local level, and a different tone when it comes to terrorism, defence, national security and policing. They want a government that is left on economics but fairly traditional on national security - and which gives short shrift to the economic nationalism and Putin-propagandism of Britain’s tiny Stalinist movement.

I think, given the scale of the resources being lined up to destroy the Corbyn project, I would be inclined to listen carefully to what Watson is asking for.

Politics is not about what you believe: it’s about how much of your beliefs can you turn into action given the electoral and social forces at your disposal. With the present balance of forces, a radical left government would have to be formed through a de facto coalition with the SNP. We will have to win over a growing cohort of young voters who exhibit no loyalty at all to parties – and are as likely to waste their votes by the million for the Lib Dems and the Greens as they are to vote Labour, should the party tarnish its reputation by facilitating a Tory Brexit.

Watson’s demands seem to be “a programme that delivers beyond our traditional labour base”; a reshuffle to bring senior centrist MPs into the shadow cabinet; demonstrative measures to pro-actively discipline and deter members accused of anti-Semitism and reassure all the party’s Jewish members, not just the anti-Zionists, that they have a voice.

All these things are both desirable and deliverable – and it will be the deciding moment in Corbyn’s leadership if he can bring Watson and others from the old guard onside. For Watson, Yvette Cooper et al, their task is to make clear they are not trying to unravel the Corbyn project – and to make clear what this alternative programme, which Watson alluded to in the video, involves.

The way forward is to recast the project: around the core goal of major nationalisations, a state-led reindustrialisation project, a green new deal, £50bn worth of redistributive tax rises and the democratisation of Britain. The parliamentary arithmetic may be against stopping Brexit, but delaying Article 50 and forcing a second referendum are not impossible – and the frontbench should throw itself enthusiastically into making this political argument positively.

When it comes to defence, policing, and security, Labour has the right policies: it’s a question of showing we take the issues themselves seriously, and understand the concerns that drive voters’ distrust of the party.

To achieve this, the senior MPs from the Brown era should be invited back into the shadow cabinet – and awarded posts commensurate with with the signal that, if this is Corbynism, it is very much the 2.0. version.

Resurrecting true cabinet diversity and collectivity should be a goal for Labour irrespective of the factional balance: it was after all Tony Blair who destroyed it, in favour of the sofa-plus-Cabinet Office system. So Corbyn needs to reform his office, bringing senior politicians in alongside the small group of party employees who are said to call the shots now.

There’s a mood among Labour activists on social media that says, “screw the centrists, let’s go all out for ideological purity”. If so, they are exchanging the chance to run a major state for the chance to run a party.

What emerges will be an altered project but no less radical. The bottom line is: now we know we are fighting not just Theresa May but Umunna and his millionaire backers, and face a snap election if the Brexit deal is signed, we need everybody on our side to have skin in the game of victory.