If Boris Johnson does nothing more, his name in history is already inscribed as the man who lied so hard he nearly broke the constitution. He certainly broke the law, as definitively interpreted by the supreme court today. Any leader with respect for the responsibilities that come with high office, and capable of shame, would immediately resign. Nothing in Johnson’s record suggests he is such a person. He clambered to Downing Street over the wreckage of his own former beliefs, ruined friendships and betrayed relationships. He has never recognised the imperative of truth, so takes no instruction from it now.

The new pinnacle of Johnson’s career as a peddler of falsehood is his claim that parliament’s suspension earlier this month was procedural – a conventional legislative reboot ahead of a Queen’s speech to set out a new domestic agenda. No one believed him. His own ministers struggled to hold the line, blurting out the true motive, which was to limit MPs’ control over Brexit. By unanimous verdict, the country’s most senior judges ruled that prorogation did indeed have the effect of “preventing or frustrating” the legislature in pursuit of its constitutional duties – passing laws, holding ministers to account and, crucially, representing the public in Commons debates. The court noted that those functions were uniquely urgent, given the proximity of the Brexit deadline. The stymying deed had to be undone.

The supreme court judgment is a devastating blow for a failed prime minister | Martin Kettle Read more

The judges did not dwell on Johnson’s motives, noting only that the government did not give a reason – “let alone a good reason” – to justify “prolonged suspension” of the democratic process. In short, he abused his power. He raided the fancy-dress box of usually benign ceremonial features of parliamentary business, plucked out a potentially awesome ancient weapon – the ability to dismiss the legislature when it is being a nuisance – and wielded it as if he were himself the monarch. He snatched the crown from the Queen’s head and used it to try to bludgeon opponents into a Brexit they did not endorse.

Opinions on the wisdom of the court’s judgment have inevitably sorted themselves into trenches on the familiar partisan battlefield. Hardline Brexiteers see it as a land grab by an activist judiciary, impinging on terrain that should be contested by politics alone. That is then woven into the alleged conspiracy by a pro-EU ancien regime to thwart the Eurosceptic revolution. A dilute version of that view, outrage-lite, warns that the supreme court has mutated into something more like its US counterpart, with party politics likely to hijack future appointments.

That concern is not groundless, although it is tangential to the validity of the court’s verdict. Fear of future politicisation would be an argument for not bringing the case at all. But since it was brought – and if we presume the judges acted professionally – there should be no doubt that the exposure of an unlawful prorogation is a good thing. The alternative is wishing the prime minister could break the law.

That is the murky ground where Johnson’s supporters now stand. They know he is a liar. They also know that there is no purpose for his leadership of their party higher than the satisfaction of vanity. They have two reasons to stick with him. One is practical: their personal ambitions are folded into his. If he falls, they fall. The second is ideological: Johnson has mortgaged his career to hardliners who demand total rupture from the EU. He won the Conservative leadership with their support, using rhetoric pitched at an electorate groomed for European bridge-burning by Nigel Farage. For that faction, today’s court ruling hardly feels like a setback. On the contrary, it develops the myth of Johnson as a tribune of the masses, heading a national liberation struggle and opposed by Europhiliac elites in cahoots with the bureaucratic colonists of Brussels.

The prime minister’s decision to go native in the wilder corners of Brexitland limits his capacity to negotiate a new withdrawal agreement, if that is indeed what he wants. Even if he manages it, a reconvened parliament, feeling its sovereignty burnished anew, is unlikely to be indulgent with ratification. So Johnson needs an election where he runs as Eurosceptic Santa unable to deliver freedom to all the boys and girls because filthy remainers are blocking the chimney. Clear them out, he will say, and the goodies flow. Johnson’s reaction to the court ruling, locating the problem among “people who want to frustrate Brexit”, is a rehearsal for that campaign.

It is hard to know whether Johnson’s mishaps and defeats since taking office help or hinder that message. He is discredited in the eyes of people who had a low opinion of him already. They are not his target market. The audience he wants to reach is unfussed by constitutional niceties, and more likely to be impressed than appalled by a leader’s readiness to go a bit rogue to get a job done. Then again, the strongman shtick needs shows of strength to be credible. Johnson only has conspicuous weakness on his record.

The supreme court judgment may have electoral consequences, but those should not be the measure of its significance. At the core of the case is a dispute over the character of British democracy and the origins of a prime minister’s legitimacy. The Johnson proposition, never fully articulated but expressed in various ways by his supporters, is that the 2016 referendum result generated a single insuperable mandate – a “will of the people” – such that no institutional check or subsequent statute could qualify or restrain. He then offers himself as the incarnation of that idea, the super-mandate made flesh.

That is convenient, since the technical mandate that keeps him in Downing Street is flimsy as hell. He was chosen as Tory leader by 92,153 party members. He was automatically upgraded to the job of prime minister because Theresa May could just about scrape together a majority in the Commons. Johnson took that threadbare, hand-me-down legitimacy and shredded it. He lost control of parliament and so he tried to shut it down, using a royal prerogative power. His Brexiteer cheerleaders pray democracy in aid of their cause, but in confrontation with a sovereign parliament they reached for the tools of premodern monarchical caprice.

The hyperactive nature of Britain’s current political crisis makes it hard to keep up with the sequence of momentous events, let alone arrange them in order of magnitude. A relentless barrage of news is desensitising; numbness can be a symptom of shock. But when the highest court in the land rules unanimously that the prime minister tried to obstruct the basic functions of democracy, the frenzy has to stop for a moment.

Politics needs to catch its breath. Johnson holds his office by crown appointment on the basis that he governs with the consent of parliament, representing the people. There is no higher channel that somehow transmutes the popular will into the body of a supreme leader. That mystical power is claimed only by charlatans and autocrats. A healthy democracy might sometimes elevate such people to its highest offices. The vital test is how swiftly it expels them once their true character has been exposed.

• Rafael Behr is a Guardian columnist