In both Britain and the United States, the resurgent left represents the best antidote to inequality, stagnating living standards and insecure employment. Yet it is increasingly clear that if the movements represented chiefly by Jeremy Corbyn and Bernie Sanders fail, it will be the authoritarian xenophobic right that will fill the vacuum.

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In the growing turmoil on both sides of the Atlantic, the old politics of Blairite and Clintonian centrism have no answers to a popular desire to break with a rotten social order. They have maintained a degree of influence not because they represent a massive constituency of public opinion, but because of long-established networks in their respective political parties, in the media and among economic elites. They are incapable of defeating the rightwing populism that menaces both nations: that mission, the outcome of which is uncertain, falls to the left.

Consider our respective political plights. A hard-right plutocrat is the most powerful man on Earth: a man who calls white supremacists “very fine people”, who demanded a total ban on Muslims entering the US, who separated crying children from their parents and locked them in cages, and has adopted far-right tropes about Europe “losing its culture” because of immigration. His elevation to the presidency was deemed a laughable impossibility: the Clinton camp reacted with “giddy disbelief” when he became the Republican frontrunner in early 2016.

Yes, Clinton suffered misogyny, and Russia undoubtedly preferred Trump and acted on it: yet the latter factor has become a Clintonian comfort blanket, a means to avoid a post mortem on the politics of the Democratic failure. As veteran Democratic Representative Barbara Lee told me just after Donald Trump’s “American carnage” inauguration speech, there was no shortage of political mobilisation against Trump in the US election: “What I didn’t see in there was the clarity of what our platform was.” Indeed, as a prominent US progressive campaigner, Ben Wikler, put it: “If you ask people what Hillary Clinton’s platform was, they couldn’t tell you … If you asked people what Donald Trump’s platform was, everyone could tell you.”

A Democratic campaign founded on economic populism could have defeated Trump, but Clinton did not believe in it, and was beholden to vested economic interests who would have suffered because of it.

A leftwing insurgency is undoubtedly making inroads against the Democratic party establishment. The triumph of a 28-year-old socialist, Alexandria Ocasia-Cortez – who defeated a party establishment kingpin, Joe Crowley, in a New York primary – was one striking example, and self-declared socialists have won from Virginia to Pennsylvania. New movements such as the Justice Democrats and Our Revolution are helping to catapult progressive candidates into elected office. But prominent campaigners tell me that the Democratic party establishment see the left as a mortal threat: that their desire is for a 2020 presidential candidate with perceived superior personal and moral traits to those of Trump, – someone charming and with supposed integrity, rather than someone who offers a genuine radical political alternative. If they succeed, no lessons will have been learned from 2016, and a Trump re-election beckons – meaning we are not even a quarter of the way through his presidency.

Here in Britain, the manner in which the official leave campaign conducted itself in the EU referendum legitimised the xenophobic right. The Tory hard right is in the ascendancy, and a fascist street movement – led by convicted fraudster Tommy Robinson – represents a growing threat. What could be described as a Far-Right International is developing: Trump’s Mussolini-praising former strategist Steve Bannon tours Europe meeting politicians including Tory hard Brexiteer Jacob Rees-Mogg, while wealthy “alt right” extremists in the US are set to plough financial resources into the resurgent British far right.

Unlike the US, the left has assumed the leadership of the Labour party. In 2015, Labour was heading for political oblivion, suffering one of its worst postwar defeats and splintering in three different directions: to the civic nationalists of the SNP, the xenophobic right of Ukip, and the more radical Green party. Corbynism could not secure victory for Labour within two years of this rout, but saved the party from going the way of the German Social Democrats and the French Socialists.

And yet. Chatter abounds of either a national government – floated openly by Tory remainers, privately by a small grouping of Labour MPs – or a new “centrist” party. If I was a hard-right strategist, I would be aching for either option. The former would mean a stitch-up of politicians to form a government no one voted for, united around reversing a referendum result: vindication of the right’s narrative of betrayal.

For the politics of a new “centrist” party, look no further than Anna Soubry: a Tory politician who has not only mostly voted with her party on Brexit, but has voted for the bedroom tax, cuts to disability and low-paid benefits, slashing taxes on the rich and big business, hiking VAT, hammering trade unions, and privatising the NHS and Royal Mail. That would be the politics of a new centrist party: combining social liberalism with policies that favour the rich at the expense of workers and the vulnerable. At its most successful it would simply divide the anti-Tory vote, handing power to a Conservative party de facto led – as Soubry herself says – by Jacob Rees-Mogg.

There has been a longstanding myth that political upheaval and crisis belonged to mainland Europe, that radical impulses were somehow alien to British or American political culture. Both nations are now in turmoil because the neoliberal project promised freedom and instead delivered insecurity. Disappearing jobs, creaking public services, a lack of affordable housing: all have led to a widespread desire for a rupture with the old order. The radical right is feeding from this disillusion. Only a left that offers a genuine alternative – to hold powerful vested interests to account, rather than scapegoating migrants and Muslims – can hope to defeat this political poison.

• Owen Jones is a Guardian columnist