Brexit is the reason the Conservatives are choosing a new leader, yet the competition has become a race to change the subject. Candidates are being judged not by any probability of success in taking the UK out of the EU, but by the ability to comfort fellow Tories that there is life for their party on the other side, once the deed is done.

How to get there is a side issue.

Boris Johnson is the frontrunner because he is a master of misdirection – the conjuring technique for steering an audience’s eyes away from sleight of hand, deception passed off as magic. The whole “Boris” persona – carefully careless hair and linguistic prestidigitation – is a vaudevillian trick that Johnson plays on British politics, manipulating debate away from his lying incompetence, idleness, philandering self-obsession and intellectual vacuity.

Johnson’s only credentials for the leadership are a charismatic energy that makes grassroots Tories feel good, and their belief that this power extends to other voters. It is a nebulous proposition (and an electorally questionable one) unrelated and ill-suited to the job of completing Brexit.

But it is also a pitch that Johnson’s rivals struggle to defeat, since none of them has a more credible plan for getting Britain out of the European Union before the 31 October deadline. Their manifestos are all based on the same rhetorical pivot – a gymnastic manoeuvre that launches off from the current intractable situation and lands with a flourish in a place where Brexit has already been “delivered” to a grateful nation. There is a missing middle section in the routine that involves contact with other countries and parliament.

This has been the most stubborn, parochial delusion in Tory thinking on Brexit: the belief that it is an internal party matter; that solutions can be cooked up on the backbenches and dictated to the EU. In that view, Theresa May’s failing was not her lack of fluency in the language of European interests but her refusal to explain British demands loudly enough at the concessions counter in Brussels.

That misperception sabotaged diplomacy during the article 50 negotiations, and is now feeding complacency about a no-deal Brexit. The Tory contenders who claim to be relaxed about that scenario trade in two vast falsehoods. One is that willingness to rip up draft treaties and renege on financial commitments somehow makes a country more credible in international negotiations. The other is that going through with such a threat hastens the day when Britain can sign free-trade deals elsewhere.

In reality, the first calls that any prime minister would make within hours of a no-deal Brexit are not to Washington but to Brussels. The topic would not be chlorinated chicken imports but averting a crisis at Dover and patching together ad-hoc legal arrangements to sustain the inward flow of vital goods. The balance of power in that conversation would not be on the British side. That is why some continental politicians are sadly coming round to the idea that no deal might be the only way to jolt UK politics into a clearer-eyed appraisal of the country’s needs regarding European markets, and what that costs.

The overwhelming EU preference is to avoid a messy breakdown in cross-Channel relations, but hope of a late realism surge inside the Conservative party is dwindling. Tory members might not mind that the ministerial careers of Johnson or Dominic Raab were marked by self-defeating bombast and ostentatious ignorance, but those who met them in the international arena are less indulgent.

While Brexit is still the driving force in British politics, it is no longer a living programme for government

“Brexit is dead,” one EU diplomat told me recently. Surprised by the bluntness of this assertion, I countered that it was very much alive. The clarification came back: yes, obviously it could still happen, and on the worst possible terms, but viewed from the outside, intellectually, as a proposition based on the original 2016 arguments for leaving the EU, the debate is over. Not even the most nationalistic parties in other member states contemplate taking Euroscepticism to the lengths taken in Britain.

It is true that the candidates to be our next prime minister are struggling to connect the imperative of getting out of the EU with any tangible benefits to follow. Even hardliners whose most cherished prize is a trade deal with the US looked awkward last week during Donald Trump’s visit, when the conversation turned to predatory private sector designs on the NHS. The economic case, once so confident, has shrivelled into a defensive ball: the pain can be minimised. In the very best-case scenario, there is a smooth transition, and the whole process feels benignly pointless, until the downsides become visible.

Meanwhile, none of the tax cuts or spending pledges spewing out of the leadership contest would be forbidden under EU rules. Esther McVey, the most radical Eurosceptic in the race, would cut foreign aid and give cash to schools and the police. She doesn’t need permission from Brussels to do that and never did, although it might be trickier in a fiscal emergency caused by the no-deal rupture she is relaxed about pursuing. McVey, like Raab, is prepared to dissolve parliament if it stood in the way.

That wild notion is symptomatic of the way Tory mania has mutated, from a belief that Brexit can work wonders, to a state of not really caring what it achieves as long as it is done. Then the search for alternative miracle cures to the nation’s ailments can begin. Deep down, the majority of Conservative MPs know that the whole enterprise is a warehouse full of snake oil. So there is a twisted logic in giving the sales job to Johnson, their most talented charlatan.

While Brexit is still the driving force in British politics, it is no longer a living programme for government. The Tories are trapped between pressure to complete it at any cost – a force applied with wrecking relish by Nigel Farage – and some residual understanding that to do it on Farage’s terms would be a surrender to madness. It is the same old Ukip agenda that has harried the Tories for years. Half of the leadership candidates are following it like Norman Bates, bullied by his mother in Hitchcock’s Psycho. They say it is an irresistible force, but it is a kind of sinister, internal derangement. The Brexit they crave, one that unites party and nation without ruin or rancour, is already dead.

That might not stop the next prime minister inflicting something called Brexit on the country, but it is getting harder. There is only so long that a government can parade a corpse and ask the public to admire it. Tories can dress it up in different costumes, stick a Boris-style wig on it, spray it with perfume, but the idea itself has started to putrefy. Its complexion has turned sallow.

None of the candidates acknowledges it, but there is a peculiar, nasty smell emanating from their contest. It is the project that has defined their party and British government for the last three years, rotting under our noses.

• Rafael Behr is a Guardian columnist