Brexit has produced few scenes as absurd as the race in Westminster to count up backers of Boris Johnson’s “deal”. The DUP were on board! Tory hardliners too! Maybe disaffected Labour leavers as well. Just one hitch: the EU was never in on this deal. There are complex technical reasons why the Downing Street plan was a non-starter in Brussels, but the broad outline is simple enough. It would impose a new customs border in Ireland that would be both intrusive and unenforceable. It would give a veto over every aspect of Northern Ireland’s future to the DUP, making a fragile peace settlement hostage to a party that rejected the Good Friday agreement.

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Was this plan even meant to be the basis for a negotiation, or was it pure provocation – a phoney offer designed for rejection, setting the Europeans up as the Brexit bad guys in an election? There are certainly Tory MPs who are stupid or uninterested enough in the detail to believe that the government offer was serious. There is also a pro-sanity faction inside Downing Street, led by Eddie Lister, Johnson’s chief of staff from his London mayoralty years, that believes in complying with laws and maintaining relationships with continental allies. But that side of the operation competes with the revolutionary set around Dominic Cummings, who has imported the single-minded fanaticism of the Vote Leave referendum campaign to No 10.

In a race to set the terms of debate, there is an advantage to the side that has no interest in diplomacy and no hesitation in setting fire to things, even when one of those things is Britain’s reputation as a rational actor on the international stage.

Westminster fizzed today at anonymous briefings – vindictive threats of retaliation against EU states and a colourfully spun conversation between the prime minister and Angela Merkel, in which the famously cautious and temperate German chancellor appeared to demand EU annexation of Northern Ireland for ever. That all indicates that the Cummings method is prevailing.

Sanity might intrude far enough up Downing Street to allow grudging submission to the law that obliges the government to seek an article 50 extension in the absence of a deal. It isn’t clear how that can be choreographed to spare Johnson the shame of breaking his “do or die” pledge to meet the 31 October Brexit deadline, but his electoral fortunes need not be ruined by the breach. He would present it as a Dunkirk moment for the forces of Brexit – heroic retreat before a Churchillian finest hour standing alone, unbowed against the foreign aggressor. Johnson can count on most of Fleet Street to help narrate events in those terms. Newspapers that gleefully boosted Nigel Farage to torment David Cameron and Theresa May will turn that dial back down for a Tory leader who shows suitably Faragist zeal. The Brexit party is bound to get a boost from article 50 extension, but not perhaps on the scale that Johnson’s enemies would like.

But the wellspring of Tory electoral confidence is Jeremy Corbyn. His personal ratings probe historically unfathomed depths. His supporters are in denial about this problem partly because a cult of infallibility has built up around the Labour leader, and partly because he has overturned a commanding Tory poll lead once before, in 2017.

But a repeat of that performance is not easy to engineer. Johnson cannot be relied upon to run a campaign as inept as May’s. The Corbyn brand has also lost much of its shine in the past two years. The whiff of antisemitism has become a stench. The latest target is Liverpool MP Louise Ellman, threatened with a no-confidence debate in her constituency. It was originally scheduled for the eve of Yom Kippur, Judaism’s holiest night, which suggests some commitment to making Merseyside Labour a hostile environment for Jewish women. Luciana Berger has already been driven out of the local party. Even if voters do not follow every detail, they sense that the opposition is crawling with cranks and bullies.

Luckily for Corbyn, Johnson is striving to make the Tories look even more unhinged and thuggish. Labour has three props holding it up in an election. One is Corbyn’s dynamism in a campaign spotlight, which belies his peevish diffidence the rest of the time. The second is a manifesto in tune with voters’ austerity fatigue – an all-you-can-eat menu of public goods. Third, and most vital, is horror at the prospect of a Johnson victory, compelling some Liberal Democrat supporters to shuffle over to Labour as the only way to stop the Tories. But many Lib Dems refuse to make that journey because they find Corbyn’s Brexit position opaque and dishonest. The Labour faithful present their leader’s EU ambivalence as Solomonic wisdom, as if he has sagely risen above the superficial division between leavers and remainers. To less indulgent eyes, it looks like refusal to take a view on the biggest issue facing the country for generations, which suggests a failure of courage and judgment.

Many anti-Brexit voters have no more patience with Labour and no respect for its leader. But Johnson’s Trumpian excesses make Labour government seem sensible by comparison, even to people who once recoiled at the thought of Corbyn in Downing Street. It is hard to know which of those factors weighs more in a ballot box.

Both main English parties hope that traditional loyalties will overcome promiscuous voting patterns when the choice is presented in binary, red-blue terms. Both also think they can mine seams of new voters, either those who never used to turn out before the referendum (and may rally to Johnson’s Brexit banner) or those who were too young last time (and may be amenable to Corbynism).

Labour can afford to wait for an election, but Johnson desperately needs one, not because he can be sure of victory but because he has no idea what else to do. Downing Street is already a campaign headquarters, not the centre of a serious government. Brexit was only ever really a campaign. It is an idea born in protest and raised on wild, unrealistic promises. It refuses to be domesticated as government policy. May tried and failed. Johnson hasn’t seriously bothered trying. He just wants to get the issue back into the realm of empty slogans, its natural habitat.

The past three years have proved the impossibility of turning the fantasy of easy, heroic release from EU membership into a practical policy. There is no alchemy that satisfies leave voters without inflicting harm on the country and diminishing its standing in the world. But for their different reasons neither Johnson nor Corbyn wants to admit that.

So they’ll fight an election, not to solve Brexit, but to sustain the fiction that a solution lies just around the next bend down the same infernal road.

• Rafael Behr is a Guardian columnist