s on many subjects, Boris Johnson sounds different when talking to different audiences about Donald Trump. In 2015, as London mayor, he accused the then Republican candidate of “quite stupefying ignorance”, for saying that the city was blighted by Islamist-controlled “no-go areas”. Trump was “frankly unfit to hold the office of president of the United States”. Two years later, as foreign secretary, Johnson’s position had evolved somewhat. He told the US ambassador that President Trump was doing “fantastic stuff” and “making America great again”. In a meeting of British business leaders in July 2018, Johnson recognised that Trump showed signs of madness, but admired the method in it. He pondered how effective the approach might be if applied to Brexit: “There’d be all sorts of breakdowns, all sorts of chaos … But actually you might get somewhere.” That influence is now on display in the proposals to renege on parts of last year’s Brexit withdrawal agreement, which by the government’s own admission involve breaches of international law. Some ministers revel in the image of Britain flinging shreds of a deal they never loved into European faces. Others, though, still care about the country’s reputation as a law-abiding member of the treaty-honouring community. The threat to that status triggered the resignation of Jonathan Jones, the most senior official in the government’s legal department. The full extent of Johnson’s appetite for Trumpian chaos is hard to know because he believes many opposing things at the same time and is congenitally indecisive. Inside City Hall, he had a reputation for agreeing with people who gave contradictory advice in successive meetings. The trick was to be the last person in the room with him before a decision had to be taken. Some of the fiercest battles inside No 10 are for that privileged position. Johnson admires the effectiveness of Trump’s mania, but copies it in spasms, lacking the temperament to emulate it full-time. People who have known the Tory leader for years insist that the persona he adopted to win elections as mayor of London – the metropolitan liberal at ease in the multicultural capital – is closer to his true identity than the chest-beating populist who trawls for votes with Nigel Farage. That may be so, but there is no electoral bridge back to the London-friendly version of “Boris”. That guy would have been a remainer. The government’s majority was won by his hardline Brexiteer incarnation. The legacy of Farage is instructive when it comes to the relationship with Trump. In both cases, the issue is as much stylistic as ideological. Conservatives were jealous of support that the former Ukip leader kept poaching from them. Mostly they wanted the same destination – total separation from the EU. But they didn’t want to have to sound like Farage, or keep the company he kept, in order to get there. A man such as Jacob Rees-Mogg, for example, does not wish to be seen as a common nationalist, but something more high-minded: a sovereigntist. He likes the votes that Farage’s methods bring in, but he wants them delivered to the tradesman’s entrance. Johnson’s liberal friends claim he was uncomfortable with some of the anti-immigration rhetoric deployed by leavers in the referendum. Not too uncomfortable, though. He rode Farage’s tiger all the way to No 10. Vanity and snobbery prevent him from acknowledging the debt. Likewise, Downing Street is coy about praise from Trump, who hails Johnson as a fellow traveller and sees Brexit as the twin project to his own presidency. The cultural affinity is strong enough. As campaigns they mined similar grievances, probed parallel divisions. In government they have a common ethos of ripping up rules, ignoring constitutions and burning old alliances. But to Tory eyes the Trump model is vulgar: too brash, too American. Britain prefers its nationalism dressed in genteel hypocrisy. There is also a practical problem in Britain’s proximity to the rest of Europe. No amount of Brexit hot air can inflate the UK into a free-floating continent, equal in stature to the US or China. In terms of economy, population and geography, our strategic peers are France and Germany. The alliance we had with our neighbours, embedded in a dominant trio within the EU, had the effect of enhancing Britain’s global power. When the US president does something outlandish, his office exerts the gravitational field of US power. The rest of the world has to bend. That is not true of Downing Street. Johnson is still at the stage of quitting his European band and dreaming of a glittering solo career. But the international scene is a tough gig for a bumbling English Trump tribute act. And the bottom falls out of that market if the original Trump fails to get re-elected in November. President Joe Biden would be in the business of restoring the US’s credentials as a responsible actor on the global stage. He would discontinue the current White House policy of spite towards the EU. His victory would signal a recovery for the grownup way of doing things, leaving Britain isolated as the only democracy practising tantrum diplomacy. Johnson would be forced to moderate his temper. The opposite pertains if Trump defies current polling and wins a second term. A lesson that Downing Street would take from such a comeback is that doubling down on incendiary rhetoric works; that the limit of decency defined by constitutional lawyers is a laughable fiction; that the anguished cries of liberals herald success; that elections are won through perpetual culture-war provocation. Any doubts Johnson might have about that approach would be discarded as casually as every other scruple he claims to have known. It is a truism of American elections that the rest of the world has a stake but not a vote. This time, the stakes are highest for Britain. The ballot cannot effect regime change in Westminster, but it can confirm or spoil the calculations on which that regime operates. The personality of the next US president will dictate the direction of the UK government and the character of the prime minister. • Rafael Behr is a Guardian columnist • Join the Guardian live event Are we heading for a no-deal Brexit? As talks reach a crucial stage we’ll be discussing this with Prof Anand Menon and Guardian journalists Sonia Sodha, Lisa O’Carroll and Jennifer Rankin. On Wednesday 9 September at 7pm