In his angry and undignified press conference on Tuesday night, Donald Trump deliberately and shockingly crossed the line that separates the acceptable and the unacceptable in the conduct of an elected democratic leader in a multiracial society. Mr Trump must now face the consequences of this momentous and inexcusable decision. Those consequences should include the way that the leaders of multiracial European nations, including Britain, conduct their dealings with the US president from this moment on.

On Saturday, Mr Trump had already equivocated between America’s white racists and its anti-racists, after clashes in Charlottesville in which an anti-racist protester was killed by a car driven by a neo-Nazi activist. Mr Trump’s evasions drew widespread and instant condemnation, not least from within his own party. On Monday, he then read out a statement, clearly written by others, that sought to repair the damage. But the very next day, speaking with his own voice, he trashed his own retraction.

Mr Trump not only reasserted his view that the white supremacists and their opponents in the Charlottesville clashes were morally equivalent. He went further. He said that there were good people on both sides, thus implicitly lending at least partial presidential approval to a far-right rally in which swastikas were displayed, Nazi salutes made and antisemitic chants shouted. He also went out of his way to side with the supremacists against the removal of a statue of the Confederate general Robert E Lee – the ostensible cause of last weekend’s clashes – suggesting this could presage similar action against memorials to George Washington and Thomas Jefferson since both were slave-owners. Mr Trump’s petulant and narcissistic demeanour made it clear that he is more outraged by criticism and with the American press than he is with his country’s racists and its neo-Nazis. He clearly cannot help himself. But that is no excuse.

This is therefore a moment at which America and the world need to display the moral clarity of which the US president is so embarrassingly incapable. There are not “many sides” to the arguments that came to the boil in Charlottesville and since. There is a right side and a wrong side. Racism, antisemitism, white supremacism and Nazism, new or old, are wrong. A leader who cannot bring himself to say this clearly and unequivocally is not just clueless. He also forfeits his claim to moral authority and much of his right to be respected as leader. Yet that is where Mr Trump has put himself.

The question facing America in the wake of these events is how to get through to 2020 with its values, institutions and social decencies intact. America has plenty of resources to show that it is a better country than Mr Trump makes it appear. It will surely succeed. The most important test is for moderate Republicans. They must find the right way to turn away from Mr Trump before the next election. If they do not, they will lose, and they will deserve to lose.

There are some signs that moderate and independent Republicans grasp this. The two former Presidents Bush issued a strong statement against racial bigotry, antisemitism and hatred on Wednesday. Senate majority leader Mitch McConnell did the same, stating “there are no good neo-Nazis”. House speaker Paul Ryan retweeted Mr McConnell’s remarks, and issued one of his own which said there can be no moral ambiguity about “repulsive” white supremacism and bigotry. Many other important Republicans made similar statements. Very few of them, however, called out Mr Trump by name. With all members of the house facing election in 15 months’ time – and a small cluster of off-year contests in November this year – Republicans will face many more demanding tests of their resolve well before 2020.

But Mr Trump’s behaviour poses questions for multiracial European nations as well. Politicians on this side of the Atlantic must show moral clarity themselves. British leaders responded well to Mr Trump’s remarks. Theresa May, Jeremy Corbyn and Vince Cable led the way on Wednesday. Others swelled the chorus, not least the communities secretary Sajid Javid with a “Neo-Nazis: bad. Anti-Nazis: good” tweet. Words, though, are not enough. Mr Trump is still US president, so governments must deal with him. But there is no place for special courtesies now. Mrs May was wrong to offer Mr Trump a state visit. This country does not want it. The Queen does not need it. It must not go ahead. We will all be better off without it, and Mrs May should now say so.