For many years, relative stability was a marked feature of US politics - as compared, say, with Britain. And this surely worked to America's advantage. Perhaps US politics is starting to look more like postwar British politics. If so, it is safe to say that the country will not like the results.

But one wonders whether even more may be at stake than the capacity to form sound and steady policy. So inflamed are the US political classes that a deeper breakdown begins to be imaginable.

Historically, the US has both accommodated and benefited from a remarkable degree of cultural pluralism - with sufficient civic tolerance, mutual (if sometimes grudging) respect and unashamed patriotism to bind the whole together. Now, more than ever, the instinct of politicians and their energised supporters is to divide. Mr Obama seemed to promise a corrective, but that hope is fading. Old and new media, obsessed with gladiatorial politics, offer no remedy. They either take sides or act as fight promoters; in any event they worsen the polarisation and leave the centre unserved. The internet's echo chambers stir brainless anger and push the poles still further apart.

In the coming years, the US has enormous challenges to face - not least, like Britain before it, the trauma of relative economic decline. Right now, its polity looks unfit to cope. "A house divided against itself", said Abraham Lincoln, "cannot stand."

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