Why is America so alarmed by Brexit? Lest the reader be in doubt, remind yourself of this. Never before has a sitting US president visited a fellow democracy in a bid to sway an election. Nor, until now, have 13 former US secretaries of state and defence risked addressing a letter to a foreign electorate with the same motive. Ditto eight former Treasury secretaries and five former supreme commanders of Nato. Not only has the US establishment broken its non-interference rule over Brexit, it is stamping on its smithereens. If we did not know better, it might seem the UK was uniquely important to the future of the world.

Seductive though that thought may be — particularly for a Brit living in Washington — there is a domestic subtext that can be summarised in two words: Donald Trump. If the British are foolish enough to leave Europe, perhaps Americans are crazy enough to elect Mr Trump. Of course, no one would claim a causal link between what happens in Britain on June 23 and the US presidential election in November. Most American voters have never heard of Brexit. Nor would most feel strongly either way if they had.

Yet there are sufficient echoes to trouble America’s besieged elites. In much the same way US music companies test products in the British market, or TV production companies simply borrow what works, the Brexit referendum has become a trial balloon for the health of western democracy. Think of The Office, that dystopian Slough-set comedy that captivated British viewers. Not long after, the US Scranton-based version pulled off a similar hit. For decades, US and UK political trends have tracked each other. Margaret Thatcher swept to power in 1979, the year before Ronald Reagan was elected president. Bill Clinton’s New Democrats paved the way in 1992 for Tony Blair’s New Labour five years later.