A thread twitches across time: I was only nine when the “battle of Lewisham” was fought on 13 August 1977, but four decades later I can still remember the frightening chants of the National Front marchers, audible from our street, and the charred bins and strewn rocks and glass left behind after the three-way clash between white nationalist demonstrators, police and antiracist protesters.

There were 111 injuries that day. The riots in Charlottesville, Virginia, provoked once again by neo-Nazi agitators, have already claimed one life and endangered many others. Ostensibly objecting to the planned removal of a statue of the Confederate general Robert E Lee, a rabble of white supremacists has brought ugliness, violence and shame to a city of great beauty.

Such circumstances nurture statesmanship – or disclose its absence. Even by his own tawdry standards, Donald Trump’s response to these disturbances has been breathtakingly feeble. His initial tweet was a feast of blandness:“We ALL must be united & condemn all that hate stands for. There is no place for this kind of violence in America. Let’s come together as one!” Don’t you just love those exclamation marks?

Play Video 2:27 President condemns violence 'on many sides' – video

Later he condemned “this egregious display of hatred, bigotry and violence on many sides – on many sides”. The repetition is a Trumpian tic, and often, as in this case, psychologically revealing. Instead of denouncing explicitly the white nationalists responsible for the bloodshed, the president took craven refuge in moral equivalence and evasion. Only last week he threatened a distant dictator with “fire and fury”. Yet, confronted with homegrown bigotry and its violent consequences, he dons the kid gloves of a politician anxious not to upset the militant wing of his base.

Compare and contrast the splendid outrage of Virginia’s governor, Terry McAuliffe, who did not hesitate to distinguish between right and wrong. “You are not patriots,” he said. “You came here today to hurt people and that is not patriotic … My message is clear: we are stronger than you. You will not succeed. There is no place for you here and there is no place for you in America.”

During the battle for civil rights in the 1960s, it was the governors of the southern states who defended racist protesters against the intrusions of Washington. In 1961, John Patterson of Alabama clashed with the Kennedy administration over mob resistance to the Freedom Riders. In 1963, Patterson’s successor, George Wallace, was overruled by JFK over the admission of two African American students to Alabama University. Now, to the shame of the presidency, it is a southern governor who calls for decency, while the commander-in-chief prevaricates. By temperament, Trump is clear to the point of brutality in his use of language. On this occasion, however, he has resorted to vapid ambiguity. This is no accident.

How Charlottesville became the symbolic prize of the far right | Edward Helmore and Lois Beckett Read more

Though he insisted in a February press conference that he was “the least racist person”, his personal history suggests otherwise. Discrimination against African Americans in housing rentals; his full-page newspaper ad in 1989 calling for the death penalty for five black and Latino teenagers (later exonerated) in the Central Park jogger case; his championing of the conspiracy theory that Barack Obama was born in Kenya; his retweeting of messages from white supremacists; his outrageous statements about Mexicans and Muslims on the campaign trail: there is an ugly pattern of behaviour.

More important, however, is the conduct of others; the vile culture of nervous hatred that Trump has encouraged by acts of commission and omission. It is hard to say how far he shares the alt-right convictions of his chief strategist, Steve Bannon, who believes that the essence of American identity is Judaeo-Christian nationalism and that this identity is in mortal peril. But, as candidate and president, he has been quite happy to surf on a tide of white resentment, careful not to disavow strongly the support of extremists and bigots.

Baffled by the institutions of government and their constraining continuities, Trump is very much at home in the world of social media, and understands that white nationalists are capable of a digital ferocity disproportionate to their numbers. The thugs who imported violence to Charlottesville are the street fighters of a movement whose online stormtroopers have been at the heart of the Trump phenomenon.

It is often lazily asserted that this presidency is a reaction against “political correctness”. But that characterisation does not begin to capture the true nature of the Faustian pact between this president and the white nationalists – of all classes – who support him. Charlottesville is an embarrassment to Trump, not a challenge. It lifts the lid on the forces he has stoked up, allowed to thrive, and fostered – sometimes directly, sometimes at arm’s length. He squirms precisely because he cannot afford to single out those responsible for the bloodshed.

Those excited by the rightwing populism that has been unleashed around the world should confront what they have spawned

Imagine if a phalanx of British Movement skinheads was captured on video chanting “Sieg Heil!” beneath an image of Theresa May. Whatever you think of the prime minister, concede this at least: she would waste no time in condemning their actions unequivocally and with cold fury. But the president is trapped by the origins of his own unmerited success, as well as by his innate moral cowardice.

When masked inadequates give the Nazi salute and yell “Heil Trump!”, he dare not disown them. Instead, he disgraces himself and the presidency with a stream of banalities that only emphasise his aversion to stating the truth – a very obvious truth in this case. Against stiff competition, it is the clearest evidence to date that he is unfit for the great office he holds.

Was Trump responsible for the death of Heather Heyer, the 32-year-old paralegal killed on Saturday by a car ploughing into a crowd of antiracist protesters? Not directly, of course. But we cannot acquit him entirely, either. Since announcing his candidacy, he has conjured into being a permissive phenomenon: a series of signals, nods and winks that has led, finally, to the horrors of this weekend.

In his inaugural address, the president spoke of “American carnage”. Well, this is what that carnage looks like. Those who are excited by the rightwing populism that has been unleashed around the world should reflect more deeply upon the animal spirits that they so blithely salute, and the havoc those spirits may yet sow. Let them confront what they have spawned. Let them own the shame of Charlottesville.

• Matthew d’Ancona is a Guardian columnist