Not since Andrew Johnson followed Abraham Lincoln have successive US presidents contrasted more vividly than Barack Obama and Donald Trump. Their handover in 2017 suggested a late-game football substitution: the languid maestro giving way to the tenacious brute, the brisk handshake a portrait in mutual distaste.

Such eye-catching polarities of style can distract from underlying likenesses. In this case, there is one of world significance. As president, Mr Obama challenged the idea that, while other countries have interests, the US has a calling. He favoured realpolitik and wondered aloud whether a nation can have an exceptional place in the moral order when others claim the same. (Citing Britain and Greece, he forgot France, Switzerland, Japan, India, Russia and China.) A domestic backlash later, he caved, stressing his belief in US exceptionalism with “every fibre of my being”. Cliché from the otherwise eloquent is a reliable clue to insincerity.

That Mr Trump commits the same heresy, minus the repentance, is captured by such headlines as “Donald Trump and the death of American exceptionalism” (The New Yorker), “Trump’s America: Smaller, meaner and increasingly unexceptional” (The Week) and — to cut through the pussy-footing — “RIP American exceptionalism, 1776-2018” (Foreign Policy). Again, the “exception” he is meant to have betrayed is the moral specialness of the US, whether as a passive example to the world or as an active, blood-spilling rescuer of it. His focus on national gain is written up as Old World atavism shamefully revived in a higher-minded land.