Britain’s briefest prime minister was George Canning in the early 19th century who lasted just 119 days. He had the excuse that he died. A new record will be set if Boris Johnson is impelled to call an autumn election and the voters put the boot to his backside. The May premiership will have been followed by the mayfly premiership. He will be deposited in the dustbin of history as the fool who clawed all the way up to the pinnacle of the greasy pole, only then to almost instantly plunge off it. He has sufficient self-knowledge to grasp that this ignominy would be hugely satisfying not just to the great swaths of the country who would think that fate richly deserved, but to many of his colleagues as well. So when he swears that he has no intention of calling an election before the end of October, I almost believe him. My hunch is that he is extremely scared by the idea. Like many of his kind, outward bravado is the bluffer’s mask on a pulsating mass of insecurities.

Most of Westminster is increasingly convinced nevertheless that an election is coming and it is not hard to see why. The speeches introducing himself to the country as its prime minister have not been those of a man settling in for a long slog to 2022. They exude the sweaty aroma of a leader warming up for a dash to the country. He seeks to frame the argument as a contest between his blustery vision of a “golden age” for “the greatest place on Earth” and the negativity of opponents whom he casts as “the doubters, the doomsters, the gloomsters”. Deploying optimism can be effective as a campaign device and flamboyant pledges can seduce the gullible, but only during the shortish interval before reality catches up with the rhetoric.

Another electoral rune is importing veterans of the Vote Leave campaign, including its mastermind, Dominic Cummings, into Number 10. “Twenty thousand more police officers” is sloganising from the same playbook that brought you “£350 million more for the NHS every week” during the 2016 referendum. The bones of a Tory manifesto targeted at swing voters can be seen in the promises to splash more cash on law and order, education, the railways and housing. His forays to the West Midlands on Friday and the north-west yesterday, regions packed with marginal, must-win seats, also had the hallmarks of a leader limbering up for an election.

Then there is the composition of his cabinet. The brutal culling of so many senior ministers looked like a re-enactment of the multiple revenge killings at the climax of The Godfather, a cinematic bloodbath that he recently revealed to be his favourite movie scene. This was not just a display of the cold-eyed ruthlessness that lurks just below the clowning surface of this prime minister. It also displayed a desire for his government to be viewed as an entirely new one. An election this year would be a request to the country to extend Tory rule from nine years to 14, a big ask. It will suit him if voters can be persuaded to forget the May government and memories of the Cameron era are also obliviated.

‘A consequence of the Midsummer Massacre of ministers is to swell the ranks of potential Brexit rebels on the Tory backbenches.’ Photograph: POOL/Reuters

A consequence of the Midsummer Massacre of ministers is to swell the ranks of potential Brexit rebels on the Tory backbenches. The prime minister’s friends are predicting that Philip Hammond and other ex-members of the May government will be “a nightmare”. He has created another tribe of resentful backbench Tories by removing Jeremy Hunt from the Foreign Office and purging all his supporters, even the Brexiters among them, from the top of government. Having this many angry colleagues seething on the backbenches would be a headache for a prime minister with a plump cushion of support in the Commons. He is squatting on a tiny and evaporating majority. The only way to change that is to try to alter the maths in parliament.

The most compelling reason to think that he will choose, or be forced into, an election is the trajectory he is taking on Brexit. During the leadership contest, some Tory moderates tried to reconcile themselves to the spectre of a Johnson government, or to rationalise their desire to get a job in it, by telling themselves that he would soften his approach once he had blagged his way into Number 10. There’s been absolutely no indication that he is tempering his position; rather, he is hardening his red lines. Brexit ultras now occupy every senior ministerial position engaged with the enterprise and he has doubled down on his demands that the withdrawal agreement be ripped up.

The supposed logic of ramping up preparations for a no-deal outcome is that this will induce the EU to blink and drop the repeated insistence of its leaders that the agreement can’t be reopened. They will then buckle and give him concessions that they wouldn’t offer to Mrs May. Now ask yourself this: how likely are Europe’s leaders to make themselves look very stupid in order to make Boris Johnson look very clever? Especially when most of them continue to think that no deal is a bluff, because they believe Britain’s parliament won’t allow it?

I suspect that, deep down, he knows that there is a vanishingly slight chance of securing a radically recast agreement. So the best educated guess about where he will get to in the autumn is this. There will be no better deal with the EU and parliament will prevent him from leaving without one. There would then be three avenues forward. One: he could seek another extension to the deadline for withdrawal. This would be a humiliating betrayal of his solemn pledge to his party that Britain is leaving “come what may” on Halloween and that would put rocket boosters on support for Nigel Farage’s Brexit party. Two: he could seek to resolve the deadlock by calling a fresh referendum. Mr Cummings used to argue for a two-stage plebiscite, but trying to take that course now would shatter the cabinet and detonate the Tory party. In these circumstances, option three – calling an election to seek a mandate for a no-deal Brexit – looks like his only viable choice. It could anyway be forced upon him by a successful no-confidence vote in the Commons.

We know how he would wage the campaign. I did my best, he would say, but I was thwarted by those perfidious Europeans and Remoaner MPs. Give me the mandate to finally liberate us from the yoke of Brussels. This would not unite and heal Britain – it would divide the country even more deeply and nastily. But he doesn’t have to unite Britain to win an election this way. He doesn’t even have to get a majority of Britons to agree with him. He just has to get enough voters in enough places to put him back in Number 10.

An early election would seek to exploit the honeymoon that most new prime ministers enjoy for a while. The Opinium poll we publish this weekend indicates some evidence of a “Boris bounce”, as it will inevitably be called, as Leave voters migrate back to the Conservatives. The Tories are up seven points and the Brexit party is down seven. He has a substantial 21-point lead over Jeremy Corbyn on the “best prime minister” question. The dire condition of the Labour party is obviously an encouragement to Tory thinking that a swift election could hit what some of the prime minister’s allies call “a sweet spot”.

Faced with the threat of a Johnson election campaign designed to mobilise the Leave vote behind him, rational folk argue that the opposition parties must put together some kind of Remainer alliance and get working on it urgently. It is possible to envisage the Greens, the Lib Dems and the Welsh nationalists forming a common front. The Greens and Plaid Cymru stood down their candidates for Thursday’s Brecon and Radnorshire byelection to allow the Lib Dems to be the anti-Brexit standard bearer. It is very much harder to see an alliance involving the Scottish Nationalists and impossible to see one involving Labour, certainly so long as Labour is under a leadership that repels other parties and doesn’t want to work with them anyway.

It is impossible for anyone to be confident about how an autumn election would play out and it may not be all that much surer at the point when Mr Johnson will have to make a decision or it is forced upon him. Even for a man with a well-documented appetite for risk, this would be an epic gamble. Get it wrong and history will record him not just as the briefest of prime ministers, but the most risible. What’s the phrase, Mr Johnson? Do or die.

• Andrew Rawnsley is Chief Political Commentator of the Observer