I cannot recall a more critical week in British politics. It will decide whether parliament, the law and public opinion can hold the prime minister, Boris Johnson, to account, or whether a new poison has entered public life. We need constantly to remember that Britain faces no menace to its security or prosperity. It could by now be outside the EU with agreed terms of trade. This crisis is entirely the outcome of one man’s device to seize control of his party.

From the moment Johnson began his final climb to power, his appeal has been crudely populist. He has discarded the core Tory tradition of fiscal probity with a welter of spending pledges and tax cuts, plus plans for immigration control and toughness on crime. On Brexit, he has distorted a near trivial “freedom to trade with the rest of the world” with claptrap about vassalage, sovereignty and patriotism. The idea that Brexit will bring a new dawn of national wealth is absurd. It is simply how Johnson became prime minister.

Now I wonder if perhaps Johnson’s legacy may be just a bad dream, a sick and expensive Bullingdon brawl

So far, so familiar. What is novel about the current upheaval is the attempted disruption of political convention. This has little to do with Johnson but with his bizarre aide, Dominic Cummings, whose control over his seemingly disoriented boss appears total. Cummings’ tactic is brittle, divisive and implacable, though its effectiveness cannot be overrated. Polls showed scant support for Johnson over his supreme court defeat, but his party remains a full 13 points ahead of Labour.

Johnson’s theatrical belligerence – which he absurdly told the Andrew Marr Show on Sunday was “misunderstood … a model of restraint” – is a trope borrowed from US politics, seen most vigorously in the antics of Donald Trump. It goes back to the tactic adopted by the McCarthyites in the 1950s, when any victim of Joseph McCarthy and his young lawyer Roy Cohn would be assailed with wildly repetitive accusations of communism and homosexuality. Cohn’s tactic was “to bring out the worst in my enemies; that’s how I get them to defeat themselves”. He would “go after a man’s weakness, and never threaten unless you mean to follow through to the end”. Cohn went on to advise Richard Nixon and ended as personal lawyer to none other than the young Trump, clearly an avid pupil.

One Cohn method was the relentless recitation of a damaging falsehood. Trump incanted “crooked Hillary” at every turn and eventually wore her down. Cummings, as portrayed by Benedict Cumberbatch on screen, would order Brexit campaigners to ignore any facts and just keep saying “£350m for the NHS”. Likewise, Johnson, when pinned down by Marr over his dismissal of threats of violence against MPs as “humbug”, robotically repeated “get Brexit done” and accused opponents of surrender. The maxim is: never back down, never apologise.

Facebook Twitter Pinterest ‘An obsession with antagonism and belligerence, with the creative power of chaos.’ Dominic Cummings. Photograph: David Mirzoeff/PA

At his party’s conference in Manchester this week, Johnson’s audience is not parliament. It is a distant and exasperated electorate that he hopes despises parliament, the metropolis, remainers and judges alike. This is pure Cohn: “I don’t want to know what the law is; I want to know who the judge is.” A populist needs an identifiable elite against which to champion “the people”. The BBC plays along, with ridiculous vox pops each evening asserting the establishment is “out to stop Brexit”.

Britain’s present parliament, for all the insults being hurled at it, has spent three agonising years trying to implement the knife-edge 2016 Brexit referendum. Then, the nation did not speak, it mumbled. It left parliament to disentangle the meaning of how to seek a new relationship with the rest of Europe. With a nation clearly and evenly split, that search still continues. That is why the Johnson/Cummings talk of capitulation and surrender is so crass. A democracy means a 48% minority has some rights, and parliament’s duty is to honour them with compromise.

Last Wednesday Johnson’s response to the supreme court judgment suggested he had no interest in compromise. He shouted, offended, flannelled and played to gallery guffaws. He relished watching the hapless Jeremy Corbyn fall into his trap. Rather than rise to the occasion as a conciliator and national leader, Corbyn tried merely to out-rant Johnson. A meticulous dignity could have been lethal.

Until now, I had thought Johnson’s tactics were at least shrewd. I assumed that, once Manchester was over, he would change gear and become the embodiment of pragmatism and national unity. He would patch up the May deal, meet Dublin’s concerns over a future border and achieve EU withdrawal by 31 October. He would secure cross-party approval for the deal in parliament, leaving only a handful of diehard remainers and no-dealers in denial. He would have delivered. A Tory election victory would be near certain.

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I had reckoned without Cummings. Analysis of his limited writings in last week’s New Statesman shows an obsession with antagonism and belligerence, with the creative power of chaos. He seems a true revolutionary. When he and Michael Gove engineered the firing of David Davis as Tory chairman, they cited Al Capone: “Find the toughest guy in the room. Embrace him like a brother. And then slam his head against the wall.”

Johnson was in full Capone mode last week. He must have known he would need cross-party support for any deal. So why treat the Commons as he did? The answer is, Cummings sees his boss as the perfect bull in a china shop. Backed by an embittered and estranged electorate, he will smash institutions and conventions, and lead people into some new dawn, even if it is merely another hung parliament, a delayed Brexit and yet more rancour.

I am an incurable optimist. Just now I wonder if perhaps Johnson’s legacy may be just a bad dream, a sick and expensive Bullingdon brawl. Theresa May failed to leave the EU, and so perhaps might Johnson. After all the sound and fury, some new dawn will arrive, leaving nothing behind but a stale smell of the night before.

• Simon Jenkins is a Guardian columnist