Donald Trump and Boris Johnson both have a capacity to provoke torrents of abuse from otherwise moderate, well-behaved people. An article this week for The New York Times raises the question of whether, given the failure of the most vicious insults to have any visible effect on the President’s poll ratings, “the search for a killer line on Mr Trump is a fool’s errand”.

He has been called “a pathological liar”, “a race-baiting, xenophobic, religious bigot”, “ISIL man of the year”, “utterly amoral”, a “narcissist at a level I don’t think this country’s ever seen”, and a “terrible human being” who has made “disgusting and indefensible” comments about women, to quote but a few of the things said about him by senior Republicans.

I have not gone to the trouble of collecting a comparable series of insults about Johnson. But in the latest London Review of Books, Ferdinand Mount calls him “a seedy, treacherous chancer”, and there is plenty more where that came from.

Trump and Johnson speak well of each other, but are in important respects quite different. Johnson is better educated, more charitable, more favourably disposed towards immigrants, more loyal to the institutions to which he belongs or has belonged, and more anxious to unite people, and to restore friendly relations when he has annoyed them.

But both men have benefited, at various points, from being underestimated by their critics, who perhaps supposed that no one could survive such fierce attacks.

And supporters of Trump and Johnson sometimes get the impression they too are being written off as evil and repulsive people. Hillary Clinton was explicit about this. She said at one of her fundraisers that you could put half Trump’s supporters in “the basket of deplorables”, for they are, in her view, “racist, sexist, homophobic, xenophobic, Islamophobic – you name it”.

This is not very good politics. The easy hit of self-righteousness, the casting into outer darkness of one’s opponents and their followers, enables one to avoid the more difficult task of scrutinising what those opponents are saying, and working out which bits of it constitute a legitimate response to the understandable concerns of, say, car workers who worry their jobs are going to Mexico.

Johnson benefits from the same lack of proper scrutiny. In recent weeks he has made announcements on such matters as health spending, police numbers and prisons which might equally well have come from a moderate Labour leader.

The Opposition has been reduced to silence, or to fringe subjects like grouse shooting. It informs us from time to time that Johnson is a liar, but this means it cannot respond to what he actually says. By indulging in character assassination, it has deprived itself of an opponent with whom it could have an argument.

On Brexit, it insists Johnson is leading the country to perdition, but its warnings are often put in such apocalyptic terms that voters wonder whether things are going to be quite as bad as all that; wonder indeed whether it is the Remainers who have lost touch with reality.

The case against exaggerating your opponent’s faults was well put by Tony Blair in his memoir, A Journey. Here is his defence of the gentle art of disparaging understatement:

I defined Major as weak; Hague as better at jokes than judgment; Howard as an opportunist; Cameron as a flip-flop, not knowing where he wanted to go. (The Tories did my work for me in undermining Iain Duncan Smith.) Expressed like that, these attacks seem flat, rather mundane almost, and not exactly inspiring – but that’s their appeal. Any one of those charges, if it comes to be believed, is actually fatal. Yes, it’s not like calling your opponent a liar, or a fraud, or a villain or a hypocrite, but the middle-ground floating voter kind of shrugs their shoulders at those claims. They don’t chime. They’re too over the top, too heavy, and they represent an insult, not an argument. Whereas the lesser charge, because it’s more accurate and precisely because it’s more low-key, can stick.

Trump will probably defeat himself in the end. So perhaps will Johnson. Their opponents seem unable to find the right words.