There is no deader horse in the kingdom than the deal brokered by Theresa May. Flogged and flogged again, it expired for a third time at 2.42pm today on the floor of the House of Commons. Something else lay dead alongside it: the illusion that the objection of MPs was not to the precise terms of Britain’s proposed exit from the European Union – contained in the withdrawal agreement – but rather to the sketched vision of our future relationship with the EU, the so-called political declaration. That fond thought was also bludgeoned to death, for MPs had the chance to vote on the withdrawal agreement alone – and they rejected it by 344 votes to 286.

Now we face a new cliff-edge, with 11pm on 12 April the new day of destiny. At that hour, Britain will crash out of the EU with no deal, in a scenario those charged with stewarding our economy, our industry and our safety warn will be catastrophic. The only way to avoid that fate is for MPs to agree on something else, fast, and, once they have, to impose their will on a reluctant prime minister. That is harder than it sounds.

Even if they somehow pull it off, MPs will have to seek an extension from the increasingly exasperated 27 countries of the EU that we’re leaving behind. The mood in those EU capitals is hardening, as the Brexit saga has drained any residual goodwill towards the UK. “They’ve had enough, they just want us out,” says one well-placed source. There will be a meeting of those EU heads of government on 10 April: it’s probably best not to assume their patience with Britain’s ongoing nervous breakdown will be infinite.

Since few thought today’s vote could be won, what was the point of it? Given that May was in charge, we should not be surprised by the answer: it was the same as it ever was, a pursuit of partisan advantage rather than the national interest. This was a stunt, a pre-emptive move in the blame game, so that if and when May has to announce a long delay to Brexit and the UK’s participation in European elections that comes with it, she can cast both as the fault of Labour.

It was Labour and its remainer allies who thwarted Brexit, she’ll say, contrasting them with the Tory MPs who were ready to swallow their reservations and vote to ensure the “will of the people” was done. If May fears that too many leave voters will regard a Brexit delay as a crime, today’s vote was about planting an alibi and framing an alternative suspect.

Of course, blaming Labour and the opposition parties for frustrating Brexit is laughably unjust, whether you’re talking about today or the last nearly three years. It may seem a minor injustice, but it matters more than the usual partisan bickering. For this blame game will extend far beyond Westminster. It will be a culture war, turning on the accusation at the heart of all culture wars: betrayal.

The outlines of the myth are already in place. Its victim is a noble people who voted as one on a summer’s day in 2016 for a simple, clear desire – leave – but who were thwarted by feckless and self-serving MPs, by the political class, by the liberal elite, by the BBC, by the universities, by Brussels bureaucrats, by the corporations, by the Germans, by the hidden forces of darkness who together conspired against the hardworking men and women of this country.

That’s the stab-in-the-back myth that will be fostered and lovingly nurtured, like all grievances. It will spread and mutate and poison our politics for many years to come. So it’s vital to do what can be done now to tackle it, to prevent it taking root and becoming immovable.

Start with today. Many of those MPs who voted no did so for the entirely legitimate reason that they were being asked to write a blank cheque, to endorse the withdrawal agreement shorn of the political declaration to which it had always been attached. But it was not just any old blank cheque. Now that May has announced her intention to quit Downing Street, it was a cheque that could have been cashed by Dominic Raab or Boris Johnson or any as-yet-undetermined future Tory prime minister.

That new PM could have happily banked the terms of exit, free thereafter to set their own destination, aiming blithely for Canada, Singapore or the Cayman Islands. The only leverage MPs have ever had over the direction of Brexit is their right of veto over the withdrawal agreement.

So Labour cannot be blamed for blocking a blindfold Brexit. But there is a greater dishonesty here, one which May wasted no time in advancing as soon as the vote was done: that somehow remainers have long been culpable for the UK’s failure to leave the EU today, as scheduled. The blame for that failure does not belong with those who opposed Brexit but, on the contrary, with those who were supposedly its greatest champions.

Just look through the list of names of those who defeated the first two meaningful votes. You won’t find arch-remainer Ken Clarke in there. Instead, it’s Johnson, Raab, Jacob Rees-Mogg and his comically misnamed European Research Group – it doesn’t want to be European, does no research, and is too divided to count as a group – along with the Democratic Unionist party. The ERG has been the stone in May’s shoe from the beginning, preventing her from reaching the Brexit they all promised in 2016 – namely one that was stable, agreed and orderly.

The hardcore leavers claim that it was principle that prevented them backing the agreement. This was Brexit in name only, they said. It was vassalage, it was slavery, it would take a very Moses to break its chains. But then the moment May dangled the prize of her own job should they switch sides, well, suddenly slavery and vassalage didn’t look so bad. It was proof, not that it were needed, that for many of the key players this remains what it was from the start: a Tory parlour game, driven by personal ambition. It’s been bad enough watching May put party before country; it’s worse to watch Johnson, backed by his acolytes, put himself before both.

Still, their guilt dates back earlier. It was partly thanks to their pressure that May made the series of fateful mistakes that led to this moment. The premature triggering of article 50; her painting of the reddest of red lines; the dismissal of the 48%, as if only the 52% were truly “the people” whose will had to be obeyed; the ceaseless attempts to win the support of the ERG crowd rather than even attempt to craft the softer Brexit that might have attracted a consensus in parliament. Whether those errors belong to May or to the Brexiters who cornered her is one for the historians, but one thing is clear: none of those mistakes can be blamed on remainers.

This, then, is the battle to come. Should Brexit either be softened into a Norwegian shape or become subject to a second referendum – and note that both options are jeopardised by their warring advocates now “taking lumps out of each other”, according to a shadow cabinet minister – then the Brexiters will need someone to blame. Their list of suspects will be long, but they should be told loudly and immediately that the fault is not in their opponents, nor even in their stars – but in themselves.

• Jonathan Freedland is a Guardian columnist