Leftwing politicians are in electoral retreat across most of the western world. The one exception is the United States. At 15 per cent in the Democratic polls, Bernie Sanders, the senator from Vermont, is riding higher than any US socialist since Eugene Debs ran for the White House a century ago.

The fact that Mr Sanders has very little chance of unseating Hillary Clinton is beside the point. His popularity is dragging her leftward. If he flames out, other left-wingers, such as Martin O'Malley, the former governor of Maryland who entered the race at the weekend, are ready to pick up the baton. Elizabeth Warren, the populist Massachusetts senator, will continue to prod Mrs Clinton from outside the field. The more Mrs Clinton adopts their language, the harder it will be for her to reclaim the centre ground next year. Yet she is only following the crowd. A surprisingly large chunk of Democrats are happy to break the US taboo against socialism.

To most students of US politics, the phrase American socialism is an oxymoron - like clean coal or the Bolivian navy. A century ago, Werner Sombart, a German scholar, asked "Why is there no socialism in America?" It was a question that confounded Marxists. As the most advanced capitalistic society, the US was most ripe for a proletarian revolution, according to their teleology.

At 15 per cent in the Democratic polls, Bernie Sanders, the senator from Vermont, is riding higher than any US socialist since Eugene Debs ran for the White House a century ago. Getty Images

Yet the US refused to live up to its role. Europe's finest intellectuals would have done better to have listened to the Irish immigrant in 1893 who on landing at Boston docks proclaimed: "If there's a government here, I'm agin it." They might also have read the first three words of the US constitution: "We the people". For all the crimes committed against Native and black Americans, the US republic came into being without an aristocracy or feudal serfdom. It was born a middle class country with equality of opportunity as its creed. That made it a radically different place to the old world it had left behind.

Such differences are no longer obvious. No one, including Mr Sanders, is talking about nationalising chunks of the US economy. Yet his policies are radical by American standards. He wants a single-payer healthcare system, along the lines of Canada, or the UK. He would abolish tuition fees for instate higher education. He would drive big money out of US politics, redistribute income, mandate paid holidays and increase social security benefits. He would also break up the "too big to fail" Wall Street banks. "Are we prepared to take on the enormous political and economic power of the billionaire class," asks Mr Sanders, "or do we continue to slide into . . . oligarchy?"