You don’t convince people by insulting, bullying or threatening them. Most of us learn this in the school playground. But a few of us don’t. And a disproportionate number of those who don’t seem to go on to become politicians.

That, at least, is how I explain current affairs. Again and again, given an opportunity to convince moderates, political leaders instead choose to lash out, seeking the approval of their most hardline supporters.

It never works. When, for example, Madrid sent riot police to break up the Catalan independence poll, and later arrested several separatist leaders, Spanish nationalists imagined that they were bringing a renegade province to heel. Instead, inevitably, the Catalan separatist parties won last month’s regional elections.

The past 18 months in Britain provide a textbook demonstration. Following a 52-48 vote for Brexit in June 2016, those on the losing side could have reached out to the waverers. They might have tried to persuade the EU to offer a slightly looser arrangement, in the reasonable expectation that a show of flexibility would shift the requisite number of voters. Alternatively, they might have tried to build a consensus around a moderate and gentle form of Brexit, one which, while it accepted the legal fact of Britain’s departure, would seek to replicate much of the existing deal, in recognition of the narrowness of the mandate.

But they didn’t do these things. Instead, they made every error in the book. They sneered at the other side, calling Leave voters stupid, racist and selfish, and publicly hoping that — being, on average, older than the Remainers — they would die, and so alter the electoral balance. Again and again, leading Remain supporters publicly sided with Brussels against UK negotiators, even when Brussels was making deliberately outrageous demands as an opening bid. Worst of all, they positively slavered at the prospect of an economic downturn, repeating and retweeting every prediction of bad news while studiously ignoring the hard evidence of rising investment and falling unemployment. Wishing for a recession is never a good look.

Result? Instead of winning over moderate Leavers, they alienated moderate Remainers. According to the polls, around two thirds of British voters now want their government to get on with pulling out of the EU.

The same phenomenon explains why Donald Trump’s support has, if not exactly held up, certainly not plummeted in the way that most observers expected. No other president has faced so many political crises so soon after his election. There have been apparent conflicts of interest between the president’s business interests ad his public office. One after another, key officials have left his administration, often with extraordinary tales of how it works. At least a dozen of his tweets might have seen a predecessor removed from office. Every week brings new accusations of collusion with a foreign power.

But Mr. Trump is lucky in his enemies. Instead of couching their criticisms in a way that might appeal to Republicans, most of the President’s detractors have used the sort of language that they use against all Right-wingers: racist, homophobe, sexist, yada yada.

No doubt throwing these words around makes the liberals feel better, but it also makes the other side switch off. Racist? Yeah, even Mitt Romney was called that. Homophobe? How many other Republicans do you remember launching their campaigns with rainbow flags? Even sexist isn’t quite the right word. Try, instead, “lewd,” “unfaithful,” “seedy,” “ungentlemanly,” or “crude.” But Lefties generally don’t like to use these sorts of words. They prefer their comfort vocabulary, even though it fails to cut through with anyone else.

It’s a classic example of what happens when people talk only to their own tribe. When the esteem of our peers matters more than the opinion of strangers, we start looking for traitors rather than converts, striving to outdo one another in the intensity of our rage. “You think that Donald Trump is unpresidential, but he’s doing some good stuff on tax cuts? You’re as bad as he is! If you can’t see that he’s a fascist, you’re basically a fascist yourself!”

How do you suppose people react to being spoken to that way? How do you imagine they respond to the sneers from celebrities, the screeching on social media, the casual dismissal of every Trump voter as an under-educated oaf? That’s right: Like anyone else, they dig in. The President has no base in the Republican Party, and few friends in politics. But, with enemies like his, who needs friends?

Daniel Hannan, a Washington Examiner columnist, is a British member of the European Parliament.