It is not Jeremy Corbyn’s stated ambition to scrub ideological impurity from the Labour ranks. He calls for tolerance of different opinions on the left in a spirit of democratic renewal. He opposes rule changes that would make it easier for trigger-happy activists to unseat MPs of doubtful loyalty to the new regime. No purges, is the line from the top; natural attrition is another matter.

Few on the left mourned the departure of Norman Warner, a New Labour-era health minister who relinquished the whip on Monday, citing “calamitous decline” in the quality of the party’s leadership and warning of certain election defeat. For Corbyn supporters, Warner’s fate was a felicitous self-scalping. Here is an unelected peer who voted with the Tories on hated NHS reforms, and once suggested that patients be charged by the night for hospital stays: a perfectly despicable heretic.

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That puts moderate Labourites in a bind. Many share Warner’s assessment of the party’s prospects, but not his views. They cannot cheer the defection because, quaint though it seems in this climate, they still think the goal should be to gain numbers in parliament, not to shed them. But nor are they minded to adorn beyond-the-pale policy with the colours of heroic resistance.

This expresses a deeper tension for the sceptics, between respect for Corbyn’s mandate among party members and dread of the country’s verdict on Labour as it digs a moat of venom between itself and anyone with a shred of Conservative sympathy. There isn’t any agreement among moderates about what the alternative looks like, only vague solidarity in gloom. There is also a shared conviction that, regardless of the leader’s platitudes about pluralism, moves are afoot to foment cultural revolution in the party, making it a hostile environment for the right.

That suspicion is currently focused on Momentum – an embryonic activist network that describes itself as the successor to Corbyn’s leadership campaign, a grassroots organisation to energise activists in pursuit of noble causes. Its inaugural mission is a voter-registration drive: laudable and anodyne. Clearly a majority of people signing up to Momentum, like most who voted for Corbyn in the first place, will be motivated by nothing more sinister than civic duty and social conscience.

But the question MPs want answered is why Corbyn needs an organisation that isn’t Labour. Surely the successor project to a successful leadership campaign is leading the actual party. It doesn’t help that Momentum is covered in the organisational fingerprints of Jon Lansman, a hard-left agitator of Tony Benn-era vintage and, at least until very recently, a supporter of mandatory reselection ballots for MPs.

It would be a mistake to see the new organisation as part of a well-oiled conspiracy. Factional politics anywhere on the spectrum is always debilitated by the narcissism of small differences – the fringe left’s boundless appetite for betrayal means it is constantly biting itself in the back. Corbyn’s impeccable courtesy has protected him on that front, but John McDonnell is trailed by a cloud of ancient grudges, especially in the unions, the nuance of which is impenetrable to all but the aggrieved parties.

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The opposing camp isn’t exactly unified. The first line of defence, guarding the party apparatus against colonisation by militants, is Labour First, an activist network chaired by John Spellar, MP for Warley. Its politics are, broadly speaking, old Labour right; its instruction manual is Hammer of the Left, John Golding’s memoir of pragmatic trade unionism and the battle against the Bennites in the 80s. Then there is Progress, once a Blairism powerhouse, now struggling for legitimacy in a party that is airbrushing Tony Blair from the record.

Relations between the two groups are cordial in acknowledging a common foe, but they stand culturally apart. Progress sees itself as a proper organisation with subscribing members, conferences and magazines, while Labour First is a mere faction with little to offer by way of policy renewal. Labour First sees Progress as a talking shop for technocrats and wimps, the likes of Tristram Hunt and Chuka Umunna, whose cushy careers to date offer no preparation for the bare-knuckle politics to come. When your enemies come at you with baseball bats, you won’t beat them back with pamphlets about paradigms, warn the streetfighters. There isn’t much point picking a fight before you’ve come up with a vision to fight for, reply the (post-)Blairites.

Such differences are entirely academic when a majority of members think Corbyn is the solution and recalcitrant MPs the problem. The right expects that evidence of the country rejecting Labour will be sobering enough for some milder Corbynites to start heeding calls for better leadership. The fear is that hardline supporters of the current leader are not measuring success by the standards normally applied in electoral politics. They are unswayed by grim polls or lost council seats because they see the party, not the country, as their field of combat. Victories are denominated in well-attended rallies, control of committees and conference resolutions.

That is partly why the leadership can afford to sound magnanimous on differences of opinion within the shadow cabinet. With over four years to go before the next election, most public-facing policy is disposable; what matters is internal power. To Corbyn and McDonnell, a commitment demanded by Hilary Benn that Labour campaign to stay in the EU, for example, is an easy concession to make. The bigger prize was the shadow foreign secretary’s seat on the national executive committee, snatched from under him and awarded to Rebecca Long Bailey, a Corbyn loyalist.

Viewed from the other side, it is reasonable for the elected leader of the party to embed supporters in key posts. It would be perverse if he didn’t, just as it is reasonable to build a network of volunteers who will spread the good news about the leader’s vision, especially if his MPs won’t share it. Thus are the parliamentary refuseniks snookered. The permission they have to disagree with Corbyn on policy makes their resistance to a parallel Corbynite organisation stalking Labour structures look reactionary – typical of an embittered old guard. Yet they are hardly going to rejoice at measures designed to hasten their irrelevance. They can only welcome the leader’s insistence on civil debate, yet the messages in their inboxes and the lessons from history point to the rise of a militant fringe baying for their blood.

They cannot fight back without evidence that Corbyn is leading Labour over an abyss. And even then, the abyss holds no fear for people who shut their eyes to evidence, believing they can fly.