Mr Sanders is no flash in the pan. Socialism found no audience in the US because most Americans felt they were middle class. High rates of social mobility gave most people the sense that their society was exceptional — and rightly so. As Richard Hofstadter, the US historian, said: “It has been our fate as a nation not to have ideologies but to be one.”

That is now in question. As recently as 2008, 63 per cent of Americans identified as upper middle or middle class. That has fallen to 51 per cent. Meanwhile, the share of Americans who self-identify as “working and lower class”, according to Gallup, has risen from 35 per cent to 48 per cent since 2008. Perhaps fittingly, the share of Americans who identify as upper class is 1 per cent. That number hasn’t changed. But the belief that they are rigging the system is now mainstream.

To be clear, I am not forecasting a red dawn in the US. It is hard to imagine even a small portion of Mr Sanders’s agenda being enacted. But the rise of the Democratic left is every bit as real as the Tea Party’s surge among Republicans. Until recently, political scientists talked of “asymmetric polarisation” — meaning Republicans were moving more to the right than Democrats were moving left. Now Democrats are catching up. Meanwhile, more and more Americans profess intolerance for other people’s political beliefs. Elections are generally won in the centre. But it is smaller than it used to be. By US traditions, next year’s election is likely to present an unusually stark clash of ideologies. Whatever else he does from here, Mr Sanders has already ensured that.