There is only one debate that matters in the US. Can Donald Trump be beaten this November and by whom? But the themes behind the debate are not confined to American Democrats. The same issues, albeit in different national contexts, confront the centre and the left across the west – whether taking on the Brexit right in Britain or responding to the rise of German fascists.

What is the best strategy to counter the politics of grievance, identity and strident nationalism that are mushrooming everywhere, but personified by Trump? Is it to bet on politicians who can appeal to the centre – whatever that is – or is it to meet fire with fire and develop a brand of left populism?

There are divisions aplenty among the Democrat politicians vying for the presidential nomination, but on one thing they are agreed – the overriding need to beat Trump. The question dominated the Iowan caucus and Friday’s television debate amongst the candidates before Tuesday’s primary vote in New Hampshire. Is the route to the White House via the “democratic socialism” of Bernie Sanders or Elizabeth Warren’s softer variant? Or is it best to plump for moderation?

And if it is the latter, is the right choice the articulate, charismatic 38-year-old, gay mayor from South Bend, the Afghan veteran Pete Buttigieg? Is it the elderly, hyper-experienced former vice-president Joe Biden? Or the billionaire, ex-Republican Michael Bloomberg, now engaged in the most expensive advertising blitz in modern political history in the run-up to the super-Tuesday primary vote in early March when eleven states vote simultaneously?

The experience of Labour is casting a baleful shadow over Sanders’s chances, notwithstanding his undoubted ability to sway even Republican-leaning audiences with his savage but persuasive indictment of today’s American capitalism.

Then there’s the Democrats’ own success in the midterm elections in November 2018, when a swath of candidates campaigned hard on their effectiveness to deliver on bread-and-butter issues. Sanders, who so nearly beat Hillary Clinton four years ago, remains the darling of Democrat activists, echoing the once near-messiah status of Jeremy Corbyn; even a fortnight ago, he looked certain to be the outright winner in both Iowa and New Hampshire. But not now.

It was Buttigieg who drew large and enthusiastic crowds and who emerged the victor in Iowa, if by the narrowest of margins. In Friday’s debate, he had been catapulted into the candidate with momentum, who the others united in trying to take down; running a small city is no qualification to run the US charged Biden. And under his watch, whatever his rhetoric, black arrests and detentions in South Bend had risen, said Warren. He punched back – the straight-backed young man easy in his own skin and who knows just how far to push progressive politics. Suddenly, the left candidates who have been making the running looked uneasy. Warren spoke little and Sanders found himself attacking an unexpected rival rather than making his case.

However, Sanders has made his mark and in a sense his job is done. Buttigieg’s “moderation” is far more left of centre than Clinton’s was. He is clear that socialism in the US doesn’t play beyond parts of New England and makes his pitch within the mainstream, but it is more clearly to the left. His positions on gun control, strengthening trade unions, extending Obamacare, public education, climate change – all reflect the move left in Democratic politics driven by the growing dismay about the direction of capitalism and society, on which Sanders has so successfully capitalised.

Life expectancy in the US is falling, particularly among men between 25 and 64, as deaths from drugs, suicide and obesity climb. Blue-collar wages remain stagnant, while billionaires proliferate. Too many companies are sliced and diced for the short-term gain of transient owners. Whether it’s Boeing or Facebook, the feeling is that those that run such companies are too careless about their wider societal responsibilities.

Donald Trump is believed to fear the candidacy of the former Republican and ex-New York mayor Michael Bloomberg. Photograph: David Goldman/AP

But the Democrats’ dilemma is that blue-collar and southern voters too easily believe Trump’s lines about it all being the fault of foreigners and the Washington swamp. The unspoken fear is that Trump could still lose the popular vote and win the presidency via votes in the electoral college, as he did in 2016. To block him, especially in key states such as Pennsylvania, Michigan and Wisconsin, there must be an appeal to independent and moderate Republicans, as Buttigieg – and Bloomberg – keep repeating.

The winning coalition has to include Sanders on the left and liberal Republicans, repelled by Trump, on the right, and it has to be anchored in a critique much more leftist than American centrists have made until now. They are beginning to build an emerging majority of Democrats who think so too.

If so, the nomination will fall to a “moderate” who can build this coalition, although even a few years back today’s moderate position would be characterised as left wing. Biden is losing momentum: the struggle will be between Bloomberg and Buttigieg. Trump is wary of both, but probably more of Bloomberg, his money and his credibility. Warren could have an important role – she would be a “guarantor” the administration would be progressive. So what will be would be offered is not a milk and water centre platform, but a combination of the centre and left. Of the two options, Bloomberg-Warren is marginally more likely to succeed, but there will be many twists, turns, setbacks and scandals before November. Predictions made now are certain to be confounded.

But the main strategic outlines are becoming clearer. There is no point – in America as in Britain – of being in politics to declaim while the right in power changes our world for the worse. The political aim must be to build the coalition that wins. In Britain, Keir Starmer’s increasing support among activists reflects the same judgment; he is palpably much more left than any “moderate” predecessor, but with the key advantage that he is plainly electable, with putative deputy Angela Rayner his “guarantor”.

The challenge of beating a Trump or a Boris Johnson remains awesome, involving answers to issues of identity as much as economics. We know the populist socialist left can’t do it. The last card is a centre-left coalition, electably led, but with firm roots in the left. Wish the Democrats luck – the impact of another Trump term, on not just the US but the world, are too destructive to contemplate.

• Will Hutton is an Observer columnist