In the months of introspection that followed their party’s dismal election performance last year, Conservative MPs bemoaned the lack of a Momentum of the right. Now, the prospect of an influx of new members is setting Tory teeth on edge.

Leave.EU, the hard Brexit campaign group run by Arron Banks, has urged its supporters to sign up as Conservative members in order to swing the result of the next leadership election towards Jacob Rees-Mogg or Boris Johnson.

They call it putting the spine back in the Conservative Party. “Fed up of how this weak ‘conservative’ government has handled Brexit?” it tweeted to its 181,000 followers last week. “A leadership contest is inevitable. Join the huge number of our members already flooding the Tory party, ready to elect a true Brexiteer.”

Pro-EU Tories like Anna Soubry, Nicky Morgan and Philip Lee are predictably worried, and warn of the risk of hard-right entryism. Well, one might say, as MPs who have all been threatened with deselection over Brexit, they would, wouldn’t they? But their anxiety is justified.

The Conservative Party is vulnerable to entryism precisely because it lacks a spine. An organisational one, that is. Its membership has dwindled to just 124,000 to Labour’s 570,000. Those who do remain are much older, whiter, and more Eurosceptic than the country at large.

No surprise, then, that some local associations are now barely functional: either organisationally or as cohesive campaigning forces. But perversely, some Tory MPs welcome the rot. “For me, an inactive constituency association is actually a blessing,” says one member of the government payroll. “It means you can control them.” Others are sanguine even as tempers flare over Brexit. “In a small association, your personal relationship with your members can mitigate most political disagreements,” reflects another May loyalist.

Less considered is the question this comfy logic inevitably throws up: what happens when new, ideologically-driven members make constituency parties active again? Those nonchalant MPs will not get to decide the answer.

While most discussion over entryism focuses on the influence of new members in a future leadership contest, an army of new recruits could effect change almost immediately. They could easily overrun the Potemkin structures of local parties and move to deselect recalcitrant MPs. Unlike Labour’s arcane system of trigger ballots, all that is needed to deselect a Tory MP is a majority of a local association’s executive and its members, and pro-EU MPs openly admit that parliamentary rebellions on Brexit are much smaller than they ought to be because their colleagues fear grassroots reprisals.

The intensity of feeling in the grassroots over Brexit and the inevitability of further compromise and infighting at Westminster means Soubry and her ilk are right to sound the alarm: Leave lunatics really could take over the Tory asylum. But they can at least console themselves with the fact that Arron Banks, a man famed for his attention to detail, is responsible for organising it.