Not often do MPs make great personal sacrifices. They can drift along obeying their whip, hoping some small ministerial plum rolls their way if they behave. The Commons is a comfortable place – those tea rooms, those bars. Mixing with pleasant pals of similar ideological ilk, assiduous work in safe constituencies – many MPs have barely troubled the national news in decades on the back benches. Until now.

Here comes a rare moment with a vote on a nation-changing question of such gravity that it takes their beliefs beyond the reach of whips. They may think ahead, to the time when post-Brexit national decline is beyond dispute in the history books, and that grandchild asks: “Why didn’t you stop it?” What can they say then? “I never thought Boris Johnson would really do it. He said it was just a negotiating ploy, but he drove us over the edge.”

Johnson challenged them outside No 10. They should ignore him. Tomorrow is the day that tests their worth. MPs have this one moment – no prevaricating, no side-stepping – because there will not be another day. Organisers of the attempt to force a Brexit delay calculate that there will be no time after MPs return on 14 October. This is it. Now or never. MPs Dominic Grieve, Oliver Letwin, Keir Starmer and others of all parties spent the summer gaming every legal option, every ploy and gambit – and this is the last chance. Tory whips tell their MPs there will still be time in October, by which time Boris Johnson might have struck his so far nonexistent new deal. Don’t believe a word of it, say Starmer and Grieve. It’s Tuesday or never.

If Johnson’s pitch is that he will strike a rebranded May deal with the EU, he’ll be eaten alive by Nigel Farage’s Brexit party

Most MPs know full well how a no-deal Brexit would deliver a deadly economic and social body blow. They know two thirds of voters oppose it. They know the prime minister, his chief adviser Dominic Cummings and his cultish cabinet have captured their party root and branch, monomaniacs usurping the traditions of Conservatism.

But Tory MPs are staring down the barrel of the Johnson bazooka – back his no deal or you’re out. Losing the whip, being ejected from their party and deselected from their seats will be a bitter end, but this one act of defiance will count for more than anything else they do. Don’t underestimate the emotional wrench of political schism – breaking the bond between an MP, their party and constituency, shunning the good people who have toiled for years running raffles, sitting outside cold polling stations, knocking on doors to sell their MP’s sterling qualities to local voters.

It’s touch and go whether there will be enough rebel Tory MPs to pass the bill that puts parliament in control. David Gauke and the “Gaukward squad” claim 20, maybe more. Five hardcore Labour Brexiters will back the government. Stephen Kinnock’s 30 Labour soft Brexiters will not vote with Johnson as the three-month extension asked is too short for a referendum. It’ll be tight, but probably the bill will pass.

Breathtaking is the notion that the government might ignore such a law: Michael Gove refused five times to rule it out on the BBC’s Andrew Marr show. Gauke, challenging the attorney general Geoffrey Cox, asks laconically that “the government clarify that they believe in the rule of law”. The response, it seems, is to threaten an election. Would he dare call an election for Guy Fawkes day, crossing over Brexit day, crashing out of the EU mid-campaign? These lawless wild men might – though mayhem at ports, in shops and pharmacies just before election day could be punishing. Johnson might call the election as soon as he loses tomorrow’s vote in the Commons, before there is time for it to pass into law in the Lords.

But to get the necessary two-thirds vote in the Commons for an election, he needs Labour’s backing. Jeremy Corbyn started today saying “any time”, bring it on. Fortunately that was later qualified: not if an election is used to block the Brexit delay.

Facebook Twitter Pinterest Shadow Brexit secretary Keir Starmer speaking at a London rally against Brexit. Photograph: Nicola Tree/Getty Images

The threat of a snap election is intended to deter wavering would-be Tory rebels. Should they be ready to sacrifice everything on Tuesday, only for an election to be called nullifying their gesture: deselected instantly, ejected for ever for one cavalier vote that led to nothing? The answer is yes. Defy this gang of ne’er-do-wells who dare evict Philip Hammond, their recent chancellor, and many distinguished others including Ken Clarke. What if Theresa May does as she should and votes to stop a no-deal disaster?

That mass expulsion of figures of far greater seriousness, brain and capability than the current shoddy lightweight cabinet would see the party break apart permanently. This would be the trigger for ordinary Tory voters to perceive that their old party no longer exists, overrun and ransacked by a gang of chancers and charlatans. The Liberal Democrats are waiting with open arms, or independent Conservatives might have the heft to make it on their own.

Tony Blair’s call for Labour to block an election – because Corbyn is too unpopular to win it – was unhelpful. True, Corbyn has plumbed unprecedented depths in the polls. True, Labour too has been captured by a separatist faction with its own slate. Consider this craziness: as Starmer, one of Labour’s heaviest hitters, fights in parliament to stop a no-deal Brexit, his local party (also mine) is holding ward meetings to decide whether to reselect him. Corbyn should tell the NEC, the party’s ruling body, to suspend all reselections immediately with the country in crisis and an election imminent.

Boris Johnson threatens to ignore MPs on no-deal Brexit Read more

The country is in a volatile mood, so no one can predict an election outcome. All parties are vulnerable to sharp swings. If Johnson’s pitch is that he will strike a rebranded May deal with the EU, he’ll be eaten alive by Nigel Farage’s Brexit party. If he pitches as a hard no-dealer, he may see off Farage but also moderate Tory voters – while he makes Labour’s task easier. Corbyn’s ambivalent Brexit stance has lost shedloads of Labour votes, but his campaign message would become crystal clear in simple opposition to a no-deal crash-out.

If, as in 2017, Labour can shift the electoral turf on to the threadbare state of the country and its stricken services, then shadow chancellor John McDonnell’s plans for breathing life back into a regenerated nation will always outshine the Tories. Sajid Javid’s spending review, to be presented on Wednesday as a glorious end to austerity, will be punctured, barely touching the gaping need in every department.

Johnson’s sojourn in No 10 may be too short for new curtains. Note how the pound plummeted on news that he might go to the polls. With no majority, no mandate, and paralysed in parliament, he would be fighting a high-risk election out of necessity. For all the bragging, the threatening and bullying, for all the wizard powers claimed by his unhinged puppet-master Cummings, Johnson is forced to face the electorate too soon with nothing but hot-air promises. Go before Brexit and Farage eats up his votes. Go after a no deal, and project fear will have become a terrible project truth, with results for all to see. He is no surefire winner.