Try one of the following sentences. "President Trump has made some excellent appointments, but it is unacceptable to mock the family of a deceased American soldier." "I'm glad that the president is cutting taxes, even if he never released his own tax returns as promised." "It's right to scrap Obamacare, but it's wrong to threaten tariffs against Mexico."

The point is not that these statements are right or wrong — plainly people can disagree about that — it's that you never hear them. American politics is an all-or-nothing affair, in which you root for one team and despise the other.

For what it's worth, the sentiments expressed above would have been pretty mainstream among conservatives during the primaries. But, once the nominations were decided, tribalism took over. Republicans who would have voted for almost any other candidate before Trump began to block out stories that were unflattering to him, and focus on those that put him in a good light. Democrats, naturally, began to do the same with Hillary Clinton.

People were bound closer to "their" candidate by criticism from the other team. No patriotic American can watch anti-Trump demonstrators burning the flag without feeling their sympathy swing towards the new president.

I use the team analogy for a reason. Think of a contested refereeing decision. Rival fans won't just see it in different ways; they will become convinced that theirs was the only possible way to see it, and that those who insist on interpreting it differently are either fools or liars.

George Orwell noticed a similar phenomenon in the 1940s. Those who became most exercised about the Nazi atrocities in Europe, he wrote, were generally unaware of Stalin's genocide in Ukraine. It wasn't that they had weighed the two crimes and judged one to be worse than the other; it's that they had chosen not to see Stalin's record at all.

This is an inescapable human trait, part of how we are made. If you became angry when Barack Obama sidelined Congress to alter immigration policy, say, the chances are you were relaxed when Trump did the same thing, and vice versa.

As the behavioral psychologist Jonathan Haidt puts it: "Morality binds and blinds. It binds us into ideological teams that fight each other as though the fate of the world depended on our side winning each battle. It blinds us to the fact that each team is composed of good people who have something important to say."

The odd thing is that tribalism is self-defeating. In politics, as in life generally, you rarely win people over by insulting them.

The Democrats have lost both legislative chambers, most governorships and the White House. If they're going to win power again, they need to convince quite a few people who didn't vote for them. Yet they rarely work up the energy to woo the other side; it's so much more fun to insult it.

If someone has qualified reservations about the current administration, the likely reaction from anti-Trumpers, especially online, will be scorn rather than encouragement. They demand 100 percent conformity with their values. They don't want Trump attacked because he is a Russophile or a boor. They want him attacked because he is a racist and a homophobe. Never mind that these things will strike most observers as untrue — how many Republican candidates can you remember launching their campaigns waving rainbow flags? The point isn't to convince others; it's to flaunt their own superiority. Racism and homophobia are, in their eyes, the worst possible sins, ergo anyone they don't like is a racist and a homophobe. It is more comforting to stick to the old incantations than to complain about, say, executive overreach.

Now here's the tough bit. The chances are that, if you're a conservative, you were nodding along at that last paragraph. But try flipping it around. We have our sacred values, too, and they can be just as off-putting to the undecideds. Refusing to allow that a Republican president has vices is every bit as silly as refusing to allow that he has virtues. Deflecting criticism by saying, "Yeah, well Clinton was worse" is, to most neutrals, both irrelevant and irritating. Assuming that your opponents are unpatriotic, or that every critic of the regime is represented by the flag-burning idiots, won't convert anyone.

It has become a commonplace that voters are fed up with nuance, that they want clarity and conviction. But that is not the same as wanting pig-headedness. There is an electoral prize for whichever side grasps this first.

Dan Hannan is a British Conservative MEP.