The volume and velocity of Donald Trump’s moral offences mean that it can be hard to keep track of them. A mountain of outrage builds up, and countless misdemeanours are submerged in the process. So here’s a relic from last August: questioned on the clashes between “alt-right” neo-Nazis and anti-Nazi protesters in Charlottesville, Virginia, Trump remarked that there were some “very fine people on both sides”. The equivalence was sickening, though wasn’t helped by media references to the protesters as the “alt-left”.

Trump pushed the logic of “both sides” much further than most politicians or pundits would be willing to do. But the political upheavals of the past few years have driven liberal observers to pose some related questions. Are the polarities of left and right really so different from each other? Does Corbynism not share something with Trumpism?

Part of this can be characterised as “horseshoe theory”, the idea that the political spectrum is shaped like a horseshoe, with right and left starting to converge on each other as they become more radical. But other things are muddying the waters as well. A new strand of economic nationalism has emerged, advanced by the likes of Steve Bannon in the US, Marine Le Pen in France and the Five Star Movement in Italy, that channels resentment towards immigration and international capital simultaneously. In Europe, this is producing new xenophobic defences of the welfare state, as sometimes deployed by Ukip. National independence movements can be equally difficult to place on the political spectrum.

But there is another reason why “left” and “right” appear similar right now: populism. We live in what the Belgian political theorist Chantal Mouffe characterises as a “populist moment”, in which politics has become impassioned, confrontational, angry and unpredictable, dispensing with all the rules and expectations that have governed liberal democracies since the 1970s. If the distinction between left and right has become foggier, this is partly because a similar set of forces are being unleashed on both sides, including devotion to leaders, suspicion of the media, street-level mobilisation and an emotional sense of injustice.

‘Nationalists do not have a monopoly on populism’ demonstrators march through Paris on Labour Day last year past a campaign poster for the socialist politician Jean-Luc Mélenchon. Photograph: UPI/Barcroft Images

Mouffe is conscious that the term populism has more pejorative connotations in Europe than in the United States, and seeks to rehabilitate it. It was in America that populism first emerged, with the foundation of the People’s Party in 1891, which mobilised farmers and small businesses against the elites of big business, professional politics and government. The key characteristic of all populism, Mouffe writes, is the identification of a “people” who are distinguished from some kind of adversary, a distinction that serves to unite and mobilise them. Nationalists can point to any number of adversaries, from foreign powers to immigrants to “enemies within” (the liberal media, socialist intellectuals, Jews), all of whom can be charged with harming “the people”. But nationalists do not have a monopoly on populism, as Podemos in Spain, Jean-Luc Mélenchon in France, Syriza in Greece and Corbynism in Britain demonstrate.

The distinction between “people” and adversary is the fundamental starting point of all politics, Mouffe argues, adapting the ideas of political theorist and Nazi jurist Carl Schmitt. Technocrats are oblivious to this, and the conflictual (or in Mouffe’s term agonistic) nature of politics gets concealed for long periods of hegemony, during which politics becomes “a mere issue of managing the established order, a domain reserved for experts”. Margaret Thatcher succeeded in building just such a hegemony, painting her policies as the only way of acting in the nation’s interests, such that a common-sense view of the economy was already in place by the time New Labour came to power.

With good strategic leadership, a radically democratic and egalitarian movement can be a match for nativism

The 2008 financial crisis signalled the end of that Thatcherite hegemony, and the start of our present populist moment. At such times, we enter a period of radical indeterminacy, in which everything is up for grabs until some kind of new “people” is assembled and a new hegemony established. Oddly for a political theorist, Mouffe recognises that theory is of little use in such situations, given that so much is shaped by the contingencies of each situation. Everything comes down to strategies, tactics and the ability to seize the initiative before the adversary. The battle to achieve a new common sense encompasses party politics, civil society and the media, influencing how ordinary people feel as well as think. Thatcher had the Rupert Murdoch press on her side. Today, one might point to the battles taking place on social media.

The question remains, how exactly do we distinguish right from left in this bewildering new landscape? Or perhaps we no longer need to. Now that the man who saw “very fine people on both sides” in Charlottesville is building concentration camps, and Italy’s deputy prime minister, Matteo Salvini, has called for a “mass cleansing” of migrants, merely opposing fascism might be grounds enough for a popular mobilisation. What distinguishes left populism, says Mouffe, is that “the people” is constructed democratically rather than on the basis of nation or race. With good strategic leadership, a radically democratic and egalitarian movement can be a match for nativism.

There is certainly piecemeal evidence that this is true. What made Syriza thrilling in its early days was the resistance it offered to the far-right Golden Dawn as much as to the Troika. Corbyn’s Labour has avoided the electoral collapse that has afflicted virtually every other established centre-left party in Europe. Maybe Bernie Sanders would have held those crucial mid-Western states that Hilary Clinton could not in 2016. But Mouffe offers no guidance as to how left populism can fight and succeed, nor any reassurance that it will. No doubt that’s in keeping with her view of democracy: nothing in politics is real, until it has been constructed through struggle.

Yet there is something disconcerting here that she doesn’t address. If politics is about the naming of enemies, doesn’t the right start with a huge advantage over the left? Or when the left starts to play this game, isn’t there a risk that certain aspects of fascism (such as antisemitism) start to creep into its programme? When Mouffe strives to articulate what distinguishes left populism, it sometimes tips into the banalities of any moderate politician of the past thirty years. “The objective of a left populist strategy is the creation of a popular majority to come to power and establish a progressive hegemony” could almost have been written by Tony Blair. Nor is she prepared to rule out the appeal to “nation” as a tool for collective mobilisation. To be fair, she is arguing partly with the hard left who (unlike her) want nothing to do with parliamentary politics, and believes underlying historical forces will eventually see them triumph. However, “Yes, but how?” is the recurring question this short book provokes, not out of scepticism but from an urgent need for answers.

The US journalist John Judis writes in The Populist Explosion that leftwing populism is “diadic”, whereas rightwing populism is “triadic”. The former opposes “the people” to an “elite”, whereas the latter always adds a third party, typically immigrants, whom the “elite” are accused of favouring. This is far more categorical than Mouffe is willing to be, given her insistence that politics is riven with uncertainty, emotion and conflict, but it does clarify what exactly is at stake. If the political task right now is to construct a “people” from which a new common sense can be built, the question of how that can be done so as to include strangers and newcomers may be the most important one of the next few years.

• William Davies’s Nervous States: How Feeling Took Over the World will be published by Cape in September. For a Left Populism is published by Verso. To order a copy for £10.99 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.

• This article was amended on 16 July 2018. Italy’s deputy prime minister is Matteo Salvini, not Mario Salvini as an earlier version named him.

