THERESA May must be delirious with relief that President Trump will not, after all, be taking up her ill-advised offer of a state visit. Imagine: Donald Trump at Buckingham Palace, complaining about the food and the décor – “so tired”. Comparing it unfavourably to Trump Tower – “now that's got real class”.

He would have enjoyed mixing it with the “losers” and “nut jobs” in Parliament who have criticised him. “Number Ten is a slum and Westminster is a zoo, believe me”. He would surely have been unable to resist calling Scotland a “failing country”, with the “worst weather”, led by “ball-breaking feminists who can't play golf”. Well, he's managed to antagonise just about every race, country and religion in the world, so why not the Scots?

“We hold these truths to be self-evident,” declares the US constitution, “that all men are created equal and are endowed by their Creator with certain inalienable rights.” Not in Trumpland, they aren't. We thought that the puerile POTUS couldn't do any more to shock us. But the casual dismissal of immigrants from half the southern hemisphere as coming from “s**thole countries” really moves Trumpshock to a higher level.

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This scatological outburst, which senators insist happened despite the President's denials, revived the hurt caused by his depiction of Mexican immigrants as “rapists” and “criminals”, his attempt to ban all Muslim immigration and his equivocation over the racist riots in Charlottesville. Trump’s racism must now be beyond dispute – remember how he promoted the absurd theory that Barack Obama wasn’t actually an American.

Even the language is debased. Newspaper editors across the world last week agonised over how to translate “s**thole”. Literal translation was too much for many, and Japan made it an “outdoor toilet”, which is scarcely less offensive. In Germany they used the work “Drecksloch”, or dirt hole, which actually sounds like a rather fitting description of Trump's speaking style.

This kind of talk may go down well with the Trump base – apparently that is the view in the West Wing where they initially made little attempt to deny his offensive remarks about Latin American nations, Haiti, El Salvador, and all of Africa. It was in a meeting with congressmen which was supposed to be about securing a future for migrant children. “Why do we let them in?” he complained. "We should have more people from Norway.” (“Thanks, but no thanks,” replied one prominent Norwegian politician.) Trump's message was clear: if you're white you're all right, but if your black, get back.

Hitherto, the Republican party has been "managing" the toddler-in-chief – humouring him and using him to push through legislation like the recent tax cut to the corporate rich. Trump has delivered cherished right-wing policies like withdrawing from the Paris climate change accords, opening up the Arctic to oil drilling and recognising Jerusalem as the capital of Israel. Trump may have arrived on a wave of anti-establishment populism, but almost every act has been to reward the very corporations and bankers that he promised to clear out. But even the Republicans must now be thinking of an endgame. It just can't go on.

If I were them I'd be making overtures to Oprah Winfrey to run as a liberal Republican, while the Democrats are dithering about inviting the chat show host. She may be a plutocrat celebrity with no fixed political views – but after her powerful speech at the Golden Globes, she is clearly a contender. In many ways Winfrey is a better fit with the Republicans than the Democratic Party, which is thought to be her natural home. If American politics is now effectively a branch of the entertainment industry, it could do worse. It would certainly send a message to the rest of the world that Trump was an aberration.

It's not just that his dismissal of countries in Africa and Latin America is racist and has been condemned as such by the UN Human Rights spokesman Rupert Colville. It is that sensible international relations cannot be conducted by a world leader who uses the language of a street-fighter and is an incontinent user of social media. Imagine what could happen if there were a serious confrontation with China over the future of disputed Taiwan? Trump may be able to get away with trading abuse with a tin-pot dictator like Kim Jong-un, but it would be unthinkable with a super-power.

China is due to overtake America as the largest economy on the planet in 2018 or shortly thereafter, and Chinese money is pouring into Africa, Latin America and South East Asia. The centre of gravity of the world is shifting. It is geopolitical insanity for America to alienate half the planet by harbouring a man who brings racism to the White House in the very week he signed a proclamation celebrating Martin Luther King. This is not just a question of dysfunctional diplomacy but of the damage Trump is doing to American “soft power”.

The United States has dominated the world for the last half-century, not through conquest, but through example. The US is the first global empire to rule the world by consent rather than military might. Developing nations have broadly accepted American hegemony because they subscribe to what they thought were American values of freedom – giving everyone a fair chance, whatever their background, race, colour or creed. This idealism may sometimes have been misplaced: certainly America has not been shy of resorting to force in countries like Vietnam and Iraq. But the failure of these military misadventures only confirmed that America's real strength has never been in hard power.

America defeated the Soviet Union, not on the battlefield, but on the air-waves, as citizens in the Eastern Bloc countries demanded the American way of life they saw celebrated in the TV shows beamed across the Iron Curtain. Hollywood did more to dismantle communism than Trident. Intellectuals may have sneered at those Coca-Cola adverts of the 1970s, bringing people of all colours together over a soft drink, but in a world riven with religious, ideological and racial divisions, it offered a kind of hazy multicultural dream. Trump is capable of doing severe damage to American soft power, because he stands as a living contradiction of all those much-vaunted American values.

Michael Wolff's much-hyped insider account of life in the White House, Fire And Fury, has been criticised for exaggerating and even inventing stories about the behaviour of Donald Trump. But almost every action of the President since its publication reads like a vindication of Wolff's journalism. Trump's petulant and self-congratulatory tweets responding to the book, in which he insisted he was “like really smart” and “a stable genius”, confirmed Trump's vanity, illiteracy and self-delusion. His s**thole remarks revived speculation about Trump's mental health.

His outbursts are such a bizarre counterpoint to everything else that is happening in America. Last week saw a historic rejection of sexism by millions of American women. After four decades of culture wars between black and white, there have been signs that a racial reconciliation is possible – in the media at least. Trump may be a temporary head of state, but like a child with a box of matches, he can do a lot of damage before he's sent to bed.