The charm, the magic, the charisma were well polished – and this is as good as the man gets on best behaviour after weeks of behind-the-scenes training. If you puzzle as to how this man, unfit for every office he has held, can be about to sweep into Downing Street, his performance today is the answer. His snake oil of choice is optimism, so miserably lacking in politics now, radiating out of him like sunshine. All fake, all sun-ray lamp that turns off in private, but it outshines his rivals and dazzles anyone willing to ignore everything we know about his rotten-to-the-core character.

Cheering in the room erupted rapturously from the remarkable assembly of MPs backing him from all wings of the party. Many know him bitterly well from trying to work with him, yet there they were, shamefully prepared to subject their country to the whims of a man they know is unsafe at any speed. A man without qualities, devoid of public spirit or regard for anyone but himself, consumed by lifelong ambition, needy for acclaim and irritable when it’s denied, willing to swing dangerously in any direction to be loved, a man to shame the country as its figurehead.

The Conservative MPs in the room, starting with the bombastic Geoffrey Cox booming out accolades, are the ones to blame, crowding in to join the Boris bandwagon for his patronage of some 120 plum posts. Or else they came to save their seats and worship in his self-belief that only he can save them from the twin perils of Nigel Farage and Jeremy Corbyn – or rescue the Conservative party from its end of days. Yet he may be their downfall, and how well they will deserve it for turning a blind eye to his every unsuitability.

Barely a word he spoke was true, trustworthy or even faintly plausible, yet he brandishes his charlatanry with bravado. Does it matter that there was not one specific policy, no route map out of the thickets of Brexit beyond his own breezy confidence that with his “new mandate” and “new optimism” he will “hit the road running” to Brussels. Never mind those sour faces in Brussels who know him all too well.

Facebook Twitter Pinterest Boris Johnson at a fire safety training day in Essex in his first full week as London mayor in May 2008. Photograph: Shaun Curry/AFP/Getty Images

Unsurprisingly, he did not mention his two disastrous years in the Foreign Office, the chance he threw away to prove himself as a man with gravitas. His officials could get no plan, priority or strategy from him. What did he want to do? He just liked being there. He offended, mocked and idled, spouted Kipling in Myanmar. But worst of all, his blundering laziness helped to keep a distraught mother stricken in an Iranian jail, when any decent person would have devoted their life to her release. Let that one shocker stand for all his character faults.

Instead, Johnson kept returning to his mayor of London years, packed with factoids and untruths. No, Johnson, violent crime including knife crime rose in your last two years. No, you missed your affordable housing target. You praised firefighters today, but you closed 10 stations and fired 552 firefighters. You promised more apprenticeships, but they fell by 100,000.

Your vanity projects left a £43m bill for the garden bridge, a £40m overspend on your designer buses. What of the £320,000 wasted on water cannon: would you have used them on the climate protesters occupying central London? But only one truth mattered to these MPs: Johnson beat Ken Livingstone twice, when the Tories were behind in the national polls. A winner – that’s the only currency that rings their tills.

Today Johnson touched every base from business to the poor, calling himself a “one-nation” man – though he above all others is the one who split the nation with his referendum claims about 80 million Turks arriving and £350m a week for the NHS. Never mind that his tax plan gives £10bn to the rich, not the poor; or that refusing to pay the £39bn we owe the European Union for past debts will render us a pariah state.

Posing, for one day only, as the “sensible, modern, moderate Conservative”, he will be a nation-shamingly bad prime minister, but these Tories think that’s a price worth paying for their seats. The irony is their gross self-interest may wipe out their party.

More seriously, consider the clear and present danger in Ireland: what will he do to preserve the historic peace in the Good Friday agreement? The Brexit impasse hinges on that border, but Johnson said he’d not let the Irish “tail wag the dog”. What will he do, send in the water cannon? Or far worse?

The great destroyer now appears before us as the great nation healer – though pollsters find him the most divisive of them all. Policy-free, this launch was a touchy feely Borisorama, where all manner of things will be … well, just take it on trust.

• Polly Toynbee is a Guardian columnist