Earlier this month the frontrunner for the leadership of the Conservative Party, Boris Johnson, told the One Nation group of Tory MPs that they face extinction if they do not deliver Brexit by the end of October. The irony of this high priest of Brexit using his own version of Project Fear may have been lost on his audience, but the thinking behind his extraordinary statement still needs unpicking. After all, amid the vapid plethora of unsupported assumptions which seems to pass for political debate in Britain nowadays, this is a choice example.

We should perhaps leave to one side what Johnson meant by delivering Brexit. Given that the EU has made it crystal clear that negotiations will not be reopened, whatever the fond illusions of various Tory contenders, the obvious implication is that the Johnson version of Brexit means no-deal. So is Johnson is claiming that if Britain does not leave without a deal by 31 October the Conservatives face extinction? If so, why? And how?

Clearly Johnson’s project fear is that the Conservatives will be punished at the ballot box for such a failure. His rhetoric is designed to encourage his listeners to imagine that delivering Brexit will instead render Nigel Farage’s latest political franchise a flash in the pan, corralling his supporters instead back to their natural political home with the Tories. Some of the One Nation MPs seem to have been seduced by this comforting scenario. Yet it rests on a number of dubious assumptions.

One is that the no-dealers, now polling at around 25% of the electorate, will reward the Tories for delivering “the Brexit they voted for”. However, since Brexit means so many different things to this group and is more often about cultural issues than European ones, simply leaving the EU will not necessarily satisfy them. Many of them do not always vote anyway. A fair proportion are socially conservative Labour voters who are unlikely to go to the Tories permanently over the one issue of Brexit. Others may decide they will stick with Farage.

The border issue is not the only lesson from Northern Ireland that the Tories appear to have overlooked as they blundered into Brexit. There voters have steadily abandoned the Conservatives’ erstwhile allies, the Ulster Unionists, in favour of the more hardline offer of the DUP. A no-deal Brexit may simply help to legitimise the more hardline offer from Farage, rather than put him back in his box. Why will this section of voters vote for the compromises a Johnson government would have to make in pursuit of rebuilding bridges and trade with the EU after the rupture of a no-deal, when they can have the emotional satisfaction of voting for hardliners instead?

In any case, 25% of the electorate is not sufficient to sustain the Tories as the party of government they have been for the past century. So Johnson also appears to be presuming that the business interests he has insulted in the recent past will still stick with the Tories in a no-deal scenario. This might be plausible as long as Corbyn the bogeyman is in charge of the Labour Party. Evidence suggests that many fear Corbyn more than they fear the disruption of a no-deal Brexit. Yet an outright Corbyn victory seems highly unlikely given the swing required at the next election. Accordingly, middle-class Tory voters, disgruntled in a no-deal setting, might safely go elsewhere. These are 25-30% of the party’s current supporters, and it would not take the defection of many of them to ensure a Tory electoral defeat.

Johnson, with Etonian insouciance, may think he can keep those voters onside. This, rather than his project fear that the Tories must do Brexit by the end of October or face extinction, may prove to be an even bigger, and even more unwarranted, assumption.

Pippa Catterall is Professor of History and Policy at the University of Westminster