Boris Johnson rode into power on a wave of populism akin to the tide that propelled Donald Trump in the United States. Trump once dubbed Johnson “Britain Trump”, and the Prime Minister, like the former president, emerged immune from each scandal, gaffe and political firefight, basking in undimmed ratings from his base. Then suddenly, last week, something changed. Now Johnson’s 2½-year premiership appears to be entering its endgame.

The reversal has been swift. Unscathed by scandal before, the Prime Minister seems pursued by it now, his characteristic smirk transformed into the uncomfortable grimace of a man who has run clean out of goodwill. Yet nothing true about Johnson himself has changed. As revelations surface about parties at No 10 Downing Street at the height of the coronavirus lockdown, what has shifted is the public mood.

That sudden darkening exposes the critical differences in how our two countries view politics, power, wealth and persona — and how our approaches to our respective blustering leaders are really nothing alike after all. Britons are baffled that someone such as Trump can be possibly regarded as anything but a shame and a disaster. His undeniable dangerousness notwithstanding, Trump’s whining, churning demand to feed his toxic self-regard amounts to the worst of crimes for us Brits: he is devoid of humour.

Yet it is our own high regard for that quality that has tripped us up with the Prime Minister. Johnson might have succumbed before to the sort of scandal and criticism that would have doomed anyone more moderate, more politic, perhaps more emotionally intelligent were it not for his seeming to have had a sense of his own ridiculousness.

His rise is the stuff of rich anecdote and sobering record. As a young journalist, he was recorded agreeing to supply an old Oxford chum with another reporter’s address so the friend could send over heavies to beat him up. As Mayor of London, he allegedly had an affair with an American business executive, for which he was later the subject of a police investigation centring on her access to tens of thousands of pounds of public money. As prime minister, faced with dire warnings from business leaders about the damage Brexit would do, his reply was simply to shrug and fire back an expletive. Even at the outset of the pandemic, he declared he would continue shaking hands with people, especially in hospitals, even as the oversight group Sage warned the public not to.

And yet with every infraction, falsehood or dereliction of duty, he seemed not to fall but to rise. He was “BoJo”" or just “Boris” — a national joke. And he could do anything, because he smirked at the cameras, looked dishevelled, mugged and mumbled. He cut you in on that joke.

Make no mistake, we were ready. After a decade of punishing austerity in which we saw bankers bailed out and an expenses scandal that exposed politicians who had billed taxpayers for everything from duck ornaments for ponds on their estates to non-existent mortgages, Johnson’s very existence seemed to be a sly comment on just how venal, idiotic and self-centred politicians are. His flapping, untucked shirt and blond thatch became a walking set of quotation marks, placing every meeting, every photoshoot, every summit, inside their irony-field.

He would issue a word salad, its lack of coherence or preparation seeming only to satirise the uncannily slick cadence of most career politicos. He would mutter that the councillors of the London Assembly were “great supine protoplasmic invertebrate jellies”, and everybody could hear that it was nonsense. So when he said gay men were “tank-topped bum boys”, Muslim women in burkas “letterboxes” or Black people “piccaninnies” with “watermelon smiles”, his enablers took it not for bigotry and imbecility but an Edward Lear-style comment on the language of politics. He cultivated a reputation as a master of irony. A scholar. Hadn’t he been to Oxford, where none but the highly intelligent and the overprivileged dim get to study? Surely he couldn’t be the latter. No. He was a wit, and we were laughing along with him.

This strange, meta quality didn’t save him from censure. But it did place that very censure inside those weird, absurdifying quotation marks. Johnson’s premiership might be one without substance — the public record of his media appearances and speeches in Parliament reads more like the sequence of scrawled caricatures, rude words and doodles in his homework book, but he was showing it to us every day, with a laugh and a wink. Critics were like sour, disapproving teachers. They just didn’t get the joke.

His poor performance in government became that most prized of things in Britain, “a bit of a laugh”. The Prime Minister himself was “harmless fun”.

One of the things Americans often fail to understand about the British people’s relationship to the “ruling class” of the privileged and wealthy is that it is not one of dominance but one of indulgence. And that indulgence is ours, not theirs.

We are exquisitely aware of their absurdity. We had our own civil war in the 1640s — we love to remind you that we deposed a King of England before you did, and what’s more we chopped off his head — the upshot of which was that monarchs and nobles could stay, and cosplay at being the head of state or whatever, but only because they made frankly hilarious ornaments. In modern Britain, the posh are our hopelessly unadaptable birds: already extinct in anything like a natural ecosystem, yet we let them live on in gilded cages, at our expense. We do this because they are deeply weird, uproarious, gloriously impractical, sentimentally quite interesting — and they provide some genuinely powerful bogeymen, from Lord Lucan to Prince Andrew. A package of ready-made cautionary tales.

This is at the heart of our love of eccentrics, oddballs and twits. To know it is to suddenly understand the edge of Roald Dahl and the high camp of James Bond. But beneath it all is an unspoken social contract. We will indulge and allow them much, just so long as they are aware of their own absurdity, and do not take advantage. Prince Andrew seems to have forgotten this, and on the heels of his hubris — and perhaps much worse — nemesis comes complete, swift and sure.

It is they who must know their place.

Until last week, Johnson seemed to fulfil both criteria. He seemed self-aware, and at least as far as conservative Middle Britain was concerned, he didn’t do much of anything really, bar mugging for cameras, so how could he be taking advantage? The enchantment seemed unbreakable. Until suddenly, it wasn’t.

Last week’s revelations about more involved parties held by the Prime Minister and his close team inside No 10 Downing Street — at the height of the 2020 lockdown — have changed, soured and darkened the atmosphere around him in ways it is impossible to overstate.

While Britain, too, has its refuseniks and conspiracy nuts, most of the country had buckled down, sacrificed and worked hard to protect the public. We had shuttered our businesses. Lost our jobs. Kept our children home from school for months. We observed curfews and practised social distancing. Londoners were arrested by the Metropolitan Police for so much as sitting on a park bench the very month that Johnson and his team were quaffing wine and partying into the night.

A group of university students just north of London held a party at home and were fined £10,000 (about $13,600) by police. The Home Secretary issued stern warnings on television to “rulebreakers”. Johnson himself mocked opposition calls for more clarity on the rules by thundering ominously that “everybody knows” what the rules meant. Just 55 minutes before a clandestine party kicked off at No 10 — “Bring your own booze!” said the invite sent by Johnson’s personal secretary — his own cabinet minister, Oliver Dowden, put out a warning to people not to gather or there would be consequences.

The evidence that there was one rule for us and another for the Prime Minister, who attended this party with his wife and mingled among the 30 to 40 guests for almost a half-hour, did not sit well. And now, there is new evidence that his team were holding more parties, sending colleagues out of the Prime Minister’s home with a suitcase to buy bottles of wine at the supermarket and smuggle the booze back in past the police on the door. One of those occasions occurred on the eve of Prince Philip’s funeral, which the Queen herself had to attend in a socially distanced manner.

People had been told they could not go to their bereaved mothers’ sides. People had been kept apart. Had done as they were told by this man. Had he been laughing at us?

For once, the Prime Minister had nowhere to go. Characteristically, he smirked and mumbled and shuffled. He failed to show up for Prime Minister’s Questions in Parliament. The next day, he arrived with a lawyered statement. Donors and MPs from his own side joined calls for him to resign. One MP cried in Parliament while recalling his mother-in-law, dying alone because the family had to obey the rules while Johnson flouted them in secret.

For supporters and opponents alike, this was Johnson’s manner recontextualised.

We had been had. Not by any wiseass clown or grinning scamp, but by an idiot. He never had been witty, or commenting on venality at all. He was just getting away with what he could. We let him take a mile when we offered an inch. To let him impose. And suddenly, just as we continue to hear about Prince Andrew, we all felt rather gross. Everything wealthy playboys or aristocratic scamps had told us, they were depended on their being harmless and self-aware.

Like Trump, Johnson insinuated, nudged, winked and babbled his way to our indulgence. His fatal mistake was to reveal, as he has here, that he was not laughing with us at all. Not now, and not ever.

He was laughing at us.

• Matt Potter, a British journalist and broadcaster, is the author of Outlaws Inc.: Flying With the World's Most Dangerous Smugglers and The Last Goodbye: A History of the World in Resignation Letters