Two and a half years after Theresa May rushed to become the first world leader to meet the newly inaugurated President Trump in Washington, she has chosen to make a state visit that should not be taking place the final act of her premiership. While the prime minister’s poor political judgment and obstinacy have been hallmarks of her three years in office, the spectacle of the next three days will make a particularly awful ending. Mr Trump is only the third US president ever to be honoured with a state visit, the others being George W Bush and Barack Obama. Inviting him in the first place was a crass error. Following through in the midst of the UK’s current political crisis is an act of gross irresponsibility.

That’s because, though such visits are symbolic occasions, there is more at stake here than pomp and circumstance. Mr Trump is a demagogue who represents a threat to peace, democracy and the climate of our planet. As elected leader of the UK’s closest ally, he can’t be ignored. But making him, his wife and four adult children the honoured guests of the Queen risks legitimising his destructive policies, his cronyism and his leanings towards autocracy.

Mr Trump’s vanity is a joke to his detractors – and the target of the Trump Baby balloon that is set to fly over London again this week. His power makes his personality a legitimate source of fascination. But the greatest danger of the visit, which will see the president meet royals at Buckingham Palace, St James’s Palace and Clarence House as well as at the D-day 75th anniversary commemorations in Portsmouth, is not that it will boost his ego. The more serious threat to the host nation is that Mr Trump’s presence and public statements will boost anti-democratic and rightwing populist elements here.

Indeed, this has already happened. Mr Trump’s suggestion that Nigel Farage should be involved in future negotiations with the EU was widely reported. So were his declaration of support for no-deal-supporting Boris Johnson in the Conservative leadership contest, and his advice against paying the EU’s divorce bill. Before the official visit has even begun, and four days prior to a byelection in which the Brexit party aims to win its first parliamentary seat, Mr Farage has been gifted an endorsement from the most powerful man in the world – while diplomatic norms have once again been trashed.

For the government of a nation in the throes of what threatens to become a full-blown constitutional crisis to invite such irresponsible interference is a form of national self-harm. But the harm caused by the choice to indulge Mr Trump will ripple beyond our shores too. Last week’s comments by special counsel Robert Mueller reignited debate over whether the president should face impeachment. House leader Nancy Pelosi has so far opposed this. Some have interpreted Mr Mueller’s remarks as a message that she should reconsider. Meanwhile, Mr Trump’s widening of his trade war from China to Mexico – with the threat of new tariffs – set international nerves jangling, and Louisiana became the latest state to dramatically restrict abortion, ramping up the threat to women’s reproductive rights that Mr Trump has enabled.

It is incumbent upon Mrs May and others to challenge him directly – or risk appearing to give the assault on women’s rights, and bullying of neighbouring states, a seal of approval. The climate emergency should be on every agenda – including Prince Charles’s. The protesters, whom police have wrongly barred from the entrance to Downing Street, are justifiably angry. So is London’s mayor, Sadiq Khan, whom Mr Trump has previously insulted, and who hit back on Saturday in the strongest terms. Others will rightly add their voices in the coming days to the chorus of disapproval against this costly and destructive jamboree.