From the other side of the Atlantic, the British left watches the Democrat 2020 primary field take shape with a sense of déjà vu. The impossible has been done. On health care, climate change, and inequality, the protest fantasies of yesterday are, we are assured, the party dogmas of now. Centrists no longer get to tell the Left what’s popular and what isn’t. “Capitalist realism” — if not broken — is at least looking a bit scuffed and shopworn.

When Bernie Sanders launched his first presidential campaign in 2015, no one thought it would achieve such change in the language of the Democratic Party and in American politics. But now, in the calm of decision — and with a second Trump administration at stake — progressives are wavering about whether it’s time for “Sandersism without Sanders.” Time, that is, for a candidate who has absorbed Bernie’s progressive agenda, but one younger, with less of the battle damage of years of activism, less free with the “S” word, less white and male, and less needlessly antagonizing to the Clintonites in the party.

Fredric Jameson and Slavoj Žižek refer to the figure of the “vanishing mediator,” who like Mary Poppins or Alhambra in Ali Smith’s novel The Accidental, completely transforms a situation in the action of entering and then withdrawing from it. It took this cool old guy to give the Left its confidence back, but isn’t it time he accepted this function of temporary catalyst and made way for another candidate?

The lesson from the UK is: we’ve been there. Don’t do it. Bernie needs to run.

In the weeks following Britain’s 2016 EU referendum, Jeremy Corbyn’s leadership of the Labour Party was hollowed out by a succession of resignations from his Shadow Cabinet and a vote of No Confidence from members of parliament. Most of the Parliamentary Labour Party (PLP) had been appalled at the old socialist backbencher foisted upon them as leader by party members in the previous year. Now, the confusion of the defeat of the Remain campaign gave them a pretext to act.

Pleading his continued support among the membership, Corbyn scandalized the PLP and commentariat by refusing to resign, forcing the party establishment to move awkwardly behind one, and then another underprepared challenger in a new leadership contest. Pandering to the newly radicalized membership, the challengers paid tribute to Corbyn’s policies, and claimed that they intended to continue his anti-austerity project (to the point of inventing an honorary role of “Party President” for him) — offering in effect, “Corbynism without Corbyn.”

Like American progressives today, in other words, the British left was faced with the dilemma of whether to embrace the policy gains made under Corbyn’s surprise ascendancy, but to accept that his idiosyncrasies as a candidate meant it was time to move on. As it happens, the decision in the UK case was simplified by the fact that the alternative candidates were far more underwhelming than those Sanders supporters are currently faced with (even if it would be flatly foolish to rush into thinking that most of Sanders’s rivals are any more sincere and any less cosmetic than Corbyn’s were in their newfound progressivism). There was no question of Labour members taking the bait that time, yet — ironically — plenty of us were left uneasy about the fact that we had originally supported Corbyn with precisely such a “vanishing mediator” role for him in mind.

It wasn’t very likely that a general election would be won by this scruffy far-left MP whose record of anti-imperialism had led him to share platforms with Sinn Féin and Hamas (before, you know, everyone did). Yet the disaster of the 2015 election led Labour members to conclude that the shock of a lunge to the “unacceptable left” was the only way to reset the party’s coordinates and face down a party establishment eager to pull right.

There was a big question mark hanging over this strategy, however. A deliberate starvation of left-wing Labour candidates during the “Third Way” New Labour years had only very recently begun to be reversed. With no obvious successors with sufficient footing in the party, we went into Corbynism pretty unclear about how we were supposed to rebuild after the explosion it was going to provide.

Such anxieties were misplaced. Soon after Theresa May decided to capitalize on Labour’s piss-poor polling and called a snap election in April 2017, the writer Owen Hatherley attended a meeting of activists aligned to Momentum, the grassroots campaigning organization set up to support Corbyn’s leadership from outside the Labour Party. Hatherley described his surprise at

the contrast between my expectation of disaster and the enthusiasm and optimism of nearly everyone else in the room. A Tory leader with a gigantic lead in the polls had set out to crush the left for a generation — an obvious and, it seemed, easily achievable aim — yet Momentum seemed unfazed. We can do this, they insisted. Don’t trust the polls.

The anecdote — recognizable to anyone campaigning at the time — contains the core of how Corbynism took the radical left from being an irrelevant rump in the most neoliberal country in Europe, to humiliating a popular right-wing leader, depriving her of her majority, and, at the time of writing, being poised to take power, probably soon. This was achieved not despite the idiosyncrasies of the choice of leader, but precisely through the interplay between him and his supporters.

Bernie-skeptic arguments from the Left often point out that we are not supposed to be beholden to individual leaders, but to be building social movements. This is true, if a bit glib. One could conclude from the statement as given that it’s therefore irrelevant who the candidate is.

But Sanders, like Corbyn, has a unique reputation among extra-parliamentary political movements. Under Corbyn, Momentum meetings and “The World Transformed’ festival events are populated by the best of the country’s activists, many of whom quite reasonably wouldn’t have been seen dead near the Labour Party before his tenure. Sanders, meanwhile, has helped inspire and publicly supported an extraordinary wave of radical resistance in Trump’s America, from the Democratic Socialists of America, to the teachers strike wave, and the campaign for a living wage for Amazon workers. As president and prime minister, both men would show what a pact between executive and grassroots could really achieve. But it takes years of trust-building for a leader to be in position to make that pact — any other elected official could not be substituted in for either of them.

The “social movements, not leaders” position also risks playing into a pious and simplifying opposition between “vertical” leadership and “horizontal” group action, with a young left raised on Occupy and anarchism accustomed to privileging the latter. Bernie 2016 was indeed “our revolution,” just as Hatherley’s Momentum activists were right to put the stress on the “we” in “we can do this.” But in both cases these horizontal efforts were given their energy and faith in themselves over “the polls” (even to the point of absurdity) by what initially seemed like shortcomings in the leaders involved.

While Corbyn’s age, his association with an until-now failed political project, and his messy foreign policy associations had seemed like liabilities before the 2017 election, the election period revealed them instead as crucial to the Left’s success. As William Davies observed shortly after it, “both Corbyn and Sanders have an impressive archive, appearing in photographs as young men being manhandled by police as they protested against racial segregation. It isn’t just their words that persuade people they offer a break from the status quo, their biographies do too.”

In a time of low trust in politicians, to have been demonstrably committed to the same thing for decades suddenly becomes its own kind of electoral currency. While Hillary Clinton lost to Trump in key swing states, and as social-democratic parties across Europe continue to hemorrhage support to the far right, Corbyn has been protected from the particular venom of mass resentment directed at these establishment parties and figures, precisely by his historical estrangement from the official party’s machinery.

From the perspective of having weathered three and a half years of internal party sabotage against Corbyn by disgruntled right-wing MPs, the ease with which many Bernie-skeptic commentators have bought into progressive Democrats’ overnight conversions to left policies is troubling. Progressive as she is by recent standards, Elizabeth Warren, the candidate closest to Sanders, is a card-carrying defender of the wisdom of markets who was a Republican for most of her life. One only has to look at the hauteur with which one of the most exciting politicians in the world (“Bernie + Cardi” as she called herself), Alexandria Ocasio-Cortez is treated by the Democrat establishment to see the extent of internal pressure any progressive candidate would be placed under to moderate. The Left needs Medicare for All and environmental reform to be the tip of the iceberg of a far more transformative program, not the opening offer in a negotiation. The ambition of new left ideas coming out of Corbynism since the 2017 election show how much more there is to be gained.

Perhaps the most troubling part of the Bernie-skeptic critics’ case is that Trump is going to lose whoever the Democrats put up. From the midst of the ongoing rise of right-wing European populism, Trump’s re-election seems frankly more plausible than his election was in the first place.

It took the 2017 election for the UK left to grasp it, but the idiosyncrasies and intransigence of a candidate like Corbyn or Bernie are not something leftists and progressives can afford to throw away. As they begin pondering who to run for president in 2020, Americans shouldn’t make that mistake.