On the well-kept shores of Martha’s Vineyard, Bill and Hillary Clinton and the Obamas arrived this month, still drawing their respective following of acolytes for book signings, cocktail parties and informal think-ins about what Democrats need to do to turn back the tide of Trumpism. The local joke is that when they leave the tiny airport “the left turns right” to their down-island destination of identical clapperboard summer retreats. Beyond this cloistered world of J Crew and clam bakes, the destination of travel looks very different. As America prepares for electoral combat season in the midterm elections this November, the centre-left is being lured to turn harder to the left.

The hot movement for opponents of Donald Trump’s aggressive populism is the Democratic Socialists of America, fiercely competing for political oxygen with Democratic party candidates.

'Some commonalities with fractious British politics and the takeover of centre-ground Labour are glaring'

For a democracy once said by a Soviet visitor to offer a choice that was “the difference between Coca-Cola and Pepsi Cola”, this is a whole new brew. The rise of the Democratic Socialists is the afterlife of the 2016 Bernie Sanders campaign, but with roots in a movement reaching back a century and that last peaked in a post-Depression quest for a New Deal. Candidates endorsed by the movement (it is, like Momentum, structured to avoid calling itself a conventional party) have recently won nomination battles in New York and Pennsylvania and a slew of others in urban centres are either associating themselves formally with it or identifying with its rhetoric.

Some factors behind its rise are specific to the ordeal of living under a political poltergeist noisily overturning the arrangement of bipartisan politics and challenging America’s place in the global order. Determination to get rid of a reactionary president alternates with an underlying anxiety that Democrats as usual may not have the breadth or star appeal to combat the force of his message.

Systems and cycles differ but some commonalities with fractious British politics and the takeover of centre-ground Labour are glaring. As Corbynites fight to turn an insurgent grab for party membership into a credible recipe for government, many shared themes recur – a declared distrust of capitalism and a vague but passionately voiced belief that “something else” is on the way. It also represents a further hollowing out of the centre and divides senior Democrats on whether to hug the new breed close as they bring fresh blood and energy into the fight or seek to maintain their own claim to a radical centre.

The Martha’s Vineyard gatherings of ancien regime Democrats look suspiciously at the disruption. They acknowledge that Trump has a good story to tell on America’s buoyant economy and fear voters will reward that simple message. There’s also an element of mourning for past glories. The outpouring of grief for Aretha Franklin and Obama’s tribute to the soul queen’s place in America’s “quest for redemption and hard-won respect” meet the needs of emotional politics. But they add to a sense that leading Democrats are better at reviewing recent US history than previewing the next chapter.

Driving that is the widening gap in profound views. A new Gallup poll concludes that support for capitalism as a system has fallen to below 50% among Democrats for the first time in the postwar era. In a Newsweek analysis, the commentator Peter Roff reckoned that the driving force was that “Democrats are losing faith in capitalism, not growing more attached to socialism”. The instant winners, however, are candidates who can channel these dissatisfactions most noisily and engage younger voters unconvinced by the messages of “hope” and “change” on their parents’ Obama posters.

The most prominent among them is Alexandria Ocasio-Cortez, who won an important New York Democratic party primary election, beating an established incumbent in the Bronx-Queens. This posits the question that centrist predecessors least like to dwell on: if we were so great, why are we losing out to the old hard left and Trump? And it poses an urgent tactical question for the midterms and beyond as to how far the party machinery should aid candidates running on platforms that differ from the leadership’s. A leading figure on Hillary Clinton’s campaign says it boils down to “a devil’s pact with a lot of small print. Does the party machine help candidates who don’t share our politics over the line in tight races to get rid of a greater threat? Or do we end up driving the working-class voters we lost even further away?”

'It’s a mush but it’s appealing mush, if you don’t ask how anything would happen that solved real problems' Former White House aide

Ocasio-Cortez’s brio and life story are impressive: she was until recently working in a cocktail bar to fund her activism. She ran her campaign on a platform that could have been designed by the shadow chancellor, John McDonnell, with its anti-free-market rhetoric and the left end of the Franklin D Roosevelt New Deal. She espouses free college tuition, universal healthcare and rejects the gentrification of outer boroughs as serving the interests of “big property” – a jibe at Trumpworld. There is the obligatory promise to help women and non-white Americans and a good deal of fudging about which bits of intersectional politics she thinks most important. The economic message echoes elements of the president’s protectionism and a sense that providing good-quality jobs can be centrally mandated. The word “uncosted” doesn’t begin to cover the implications of a vast spending hike. “It’s a mush,” said a top former White House aide, “but it’s appealing mush, if you don’t ask how anything would happen that solved the real problems of American workers and their families.”

Nonetheless, she did something that has given tough old Democrat strategists pause for thought: galvanised voters who had lost the habit of loyalty. Tom Perez, the Democrats’ chair, has embraced the insurgents as “the future of our party” and suggested that it is moving towards fielding candidates of different ideological hues in different races. The risk is not hard to see – a cacophony of voices with no single uniting message might get more Democratic candidates over the line in the midterm races, without beginning to answer the question of what kind of figure and platform the party is seeking for the next presidential race.

The sense of a retreating American Dream now available only to the well-to-do has echoes in Theresa May’s sporadic attempts to address the pressured lifestyles and budgets of “just about managing” workers in Britain or vaguely contoured attempts among centre-left thinkers to define an “inclusive capitalism”. The 2008 financial crash pushed this into sharp relief for a rising generation. When I speak to the admissions dean of a prestigious state university, he says “around a third” of students who enter on hardship schemes do so because their parents lost their homes in the wake of the sub-prime housing market collapse.

So yes, this is a justified frustration that gives birth to the dream of a new politics, at once egalitarian and energetic. It has, however, few answers that would not risk making matters worse. Like the siren songs of Corbynism, it seeks to sound innovative and forward looking, while resting on the shaky old pillars of a Marxism that failed in practice, not least because it could not encompass innovation. Ocasio-Cortez talks of “late-stage capitalism”, but hardly anyone outside a narrow band of ideologists really believes that this is where we are. The Harvard economics professor Ken Rogoff points out that many of the dissatisfactions it seeks to address would be more effectively achieved by a rigorous assault on monopolies than a take-down of the free market.

Students of Labour’s present rhetoric will also spot the tendency to seek to dismantle “neoliberalism” one minute and then refer to Scandinavian tax rates of public services – the result of stable, wealthy capitalist societies – as a model the next.

For all the deepened divisions of America, many voters occupy the big prairies of belief in between the populist surge of Trumpworld and those who think that the answer to modern economic woes is the promise to make more stuff free. If the extremes of politics feel like the exciting place to be, however, centrists have to define their own radical agenda with much greater verve. Outsourcing that will leave centrist Democrats with little to run on but their pride and legacy – and that never carries the crowd.

Anne McElvoy is senior editor at the Economist