We may not notice when fascism creeps up on us: we may be too busy laughing. They say that clever people struggled to take the rise of the 1930s demagogues seriously. They found the strutting dictators in their silly uniforms just too ridiculous. And in some cases, derision was the right response. Britain’s own would-be Hitler, Oswald Mosley, was mocked into oblivion by PG Wodehouse’s fictional version, Roderick Spode.

Donald Trump similarly invites laughter. We are appalled but we are also amused. He is funny. The way he delivers a line, the way he repeats a phrase – “Not good, folks. Not good” – the way he tramples over every taboo. He has a comic’s gifts. But there’s a danger in this laughter. It can lower our guard.

Watch the video of Trump’s latest atrocity, his hint that if Hillary Clinton appoints judges committed to gun control, it may fall to gun rights activists to stop her. How? Well, Donald didn’t quite spell that out. He shrugged, said “I don’t know” and left it at that.

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But visible over his left shoulder was a middle-aged couple. Many have focused on the reaction of the man in the red shirt, whose jaw drops as he realises that the Republican nominee has all but issued an assassination threat to his opponent. But look at the woman next to him, apparently his wife. She smiles and then she laughs, a kind of “Ooh, you are awful” chuckle.

It helps Trump, this reaction. It enables him to step back from the brink, to say he was only joking. So last month he urged Vladimir Putin to hack Clinton’s emails, before insisting that he was just kidding.

Often he blames the media – for reporting what he said and for being stupid enough to presume he meant it. It’s a reliable line of defence for him. It’s never his fault. It’s your fault for not realising he was just messing with you.

Humour is only one of several Trump traits that make him, and those like him, maddeningly hard to tackle. Brazen dishonesty is another.

His lies are so legion, it can be impossible to keep up. The fact-checking site Politifact found that of the Trump statements it had assessed, 15% were mostly false, 36% were false and 19% were outright “pants on fire” lies.

He was at it again this week, calling Barack Obama the literal “founder” of Isis. He later said he was being sarcastic. But Trump lies all the time, on matters small and large. He lies about his poll numbers, he lies about the crime rate. He lies about his charitable donations, boasting of giving $1m to veterans but not actually giving it until the media demanded to see the money. And he lies about his relationship to Putin, first claiming that he had “got to know him very well” and that they had spoken “indirectly and directly”, then saying he had no relationship with him.

Donald Trump: I was being sarcastic about Obama and Isis Read more

There was a time when being caught out in a single deception could destroy a politician. But that relies on the politician implicitly accepting the usual rules of the game, which Trump does not. As Britons know well from bitter and recent experience – after the pro-Brexit camp cut through with the utterly bogus claim that Britain sent £350m a week to the EU – a willingness to lie can be a powerful asset. Trump enjoys a kind of freedom that his more conventional opponents lack. They have at least one hand tied behind their back, feeling an obligation to stick as closely as possible to the truth. Trump is unbound.

Which brings us to a sensitive point. Tony Schwartz, who as Trump’s ghostwriter had intimate and daily access to Trump over 18 months in the 1980s, witnessed the constant lies, the tiny attention span and inability to concentrate, the intense egotism – and concluded that Trump was “pathologically impulsive and self-centred”. In short, a “sociopath”.

The dean of Harvard Medical School has said Trump does not just have narcissistic personality disorder, “he defines it”. Indeed this view has become so widespread that the American Psychiatric Association felt compelled to remind its members this week of the Goldwater rule – named after an earlier Republican presidential nominee routinely branded as unhinged – which urges practitioners not to offer a diagnosis of an individual they have not treated.

The point is, someone who fits what we might politely call Trump’s psychological profile – someone as disinhibited, as willing to disregard social norms – is perilously difficult to confront. But there might just be a way.

For why was it that not just the liberal usual suspects, but almost all his fellow Republicans denounced Trump for attacking Khizr and Ghazala Khan, the Gold Star parents of Capt Humayun Khan, the Muslim-American killed while fighting for his country in 2004? The answer is the same one that explains the bipartisan outrage that greeted Trump’s suggestion that Judge Gonzalo Curiel could not be objective in deciding a case involving Trump because of his “Mexican heritage” – even though Curiel was born in Indiana.

There was a time when a single deception could ruin a politician. But Trump doesn't play by the usual rules

What Trump had done was violate a core American ideal: the notion – not always honoured, admittedly – that no matter where your family came from, if you were born in the US or had come there and subscribed to its founding principles, then you were as American as a direct descendant of those who landed on Plymouth Rock. This was what set the US apart, the belief that national identity did not reside in blood or soil, but in loyalty to the nation’s constitution and its bill of rights.

Or consider Trump’s proposal to ban Muslims from entering the US; it was again attacked by Republicans as well as Democrats because it contradicts America’s founding purpose, to be a haven from religious persecution, a purpose encapsulated in the constitution’s first amendment guaranteeing the “free exercise” of religion. Or reflect on Trump’s little joke this week, suggesting the way to deal with Clinton might be a bullet – at odds with America’s professed determination to resolve its differences through a constitution, the law and elections.

The common thread is that all these moves by Trump are not just reactionary or bigoted or dangerous. They contradict the ideals that all Americans are meant to regard as sacred. Perhaps this is the way to attack Trump: as truly un-American. He says he wants to make America great again. The truth is, he would stop America being America.