Why is Sadiq Khan trying to restart his fight against Donald Trump?

London Mayor Sadiq Khan, whose official day job is to govern the capital, is in New York this week, where he has condemned Donald Trump and urged Americans to vote for Kamala Harris. Trump would set the wrong tone for the rest of the world, Khan explained. “What I would say to Americans in a respectful way is: I don't think you know that the rest of the world is watching, because we have an interest in it,” he said. Excuse me?

Americans are choosing a leader for their country. What the world thinks about it – least of all Khan, who hardly anyone in America has heard of – is really beside the point. No matter: Khan is determined to educate the American public in a way that he believes only he is qualified to do. The London mayor warned that a second Trump presidency would have more far-reaching consequences than Americans may realize:

“What happens in America is the metronome … that determines the best of what happens around the world,” he said. “It sets the pace for how other politicians conduct themselves on the campaign trail.” There's more like that. Trump's policies and positions – from withdrawing from the Paris climate agreement again to his refusal to accept the 2020 election results and his rhetoric about women and immigrants – are dangerous, Khan said. Yes, yes, but what does all this have to do with the London mayor?

The Trump campaign has not yet issued an official response to the mayor's comments. Perhaps Trump thinks it's not worth the effort. After all, Khan and Trump aren't exactly the best of friends. The mayor has called the former president a racist, sexist and homophobe, and “just one of the most egregious examples of a growing global threat.” Trump has said he doesn't think much of Khan, calling the mayor a “stone-cold loser” in 2019.

Nevertheless, it is politically foolish of Khan to unilaterally reignite this long-standing dispute. It is difficult to see what he wants to achieve by doing this. The disadvantages of such an approach are, however, obvious.

Khan's high-handed attack on Trump complicates matters for Prime Minister Sir Keir Starmer, who is also in New York this week to attend the United Nations General Assembly. He has already been forced to fend off unhelpful comments about Trump from Dame Angela Eagle, the immigration minister, who accused Trump of emboldening racists in Britain with his “scourge”.

When asked about this on the way to New York, Starmer said it was “absolutely clear” to him that full responsibility for the riots that rocked Britain this summer rested with those who took part in them. What Starmer says publicly about America and its would-be leaders has real political consequences, which is why he does not want to appear to be taking sides ahead of the US election in November. Khan, whatever he may think, is irrelevant on the international stage.

The London mayor simply made it clear that he is in America this week and not working in London. Khan is unlikely to attend the main UN event, but he is expected to appear at other fringe events on climate change and the economy.

The UN meeting, he claimed, would highlight issues and interests that the US, Britain and other countries have in common. Ordinary voters may disagree: the UN appears increasingly irrelevant at a time when the world is shaken by numerous crises and challenges, not least the conflict in the Middle East.

What Khan is doing in New York is anyone's guess: it is certainly – figuratively and literally – miles away from his actual duties and responsibilities in London. The priorities of the voters who elected him to his third term earlier this year are the core issues of crime, transport and housing. Nevertheless, Khan has cheerfully kept Londoners updated on his visit, writing on X: “It was so inspiring to visit the Billion Oyster Project in New York today and see how they are using nature to clean up the Hudson River and promote biodiversity. Reintroducing wildlife such as water rats, eels and otters will help us restore London's waterways.”

All of this begs the question: does Khan really have nothing more important to do in London at the moment? His little trip to New York, talking nonsense, meddling in US politics and needlessly upsetting the man who may become the next US president, is not a good example of the art of politics.

Just yesterday, the Prime Minister urged voters to start a “common fight” to save the country. Politicians like Khan, who hang around in cities like New York with no tangible benefits, hardly look like they are joining the fight – and voters know it.