Pundits have long predicted that if Boris Johnson became prime minister he would “go full Trump”, and this week we finally saw what that means. With Dominic Cummings suggesting that MPs getting death threats should just “get Brexit done”, and Johnson sparking outrage with his comments about Jo Cox, the atmosphere in parliament is becoming increasingly poisonous.

But how should Johnson’s opponents respond? Their almost exclusive focus on deploring the use of words like “surrender” and “betrayal” may only play into his hands. Johnson is appealing to a British public fed up with Brexit who want politicians to get back to sorting out the real problems affecting them: a creaking NHS, stagnant living standards, crumbling infrastructure. Just like Trump – as progressives should have learned after Hillary Clinton’s disastrous campaign – he cannot be beaten by political grandees defending a broken status quo, but only by a compelling alternative vision for change.

If you looked only at the headlines, you could be forgiven for thinking that Labour conference had little to offer in this respect, instead being dominated by bitter factional disputes over Brexit and Tom Watson. But away from the glare of the media spotlight, Labour activists have been quietly building an agenda for transformative change from the bottom up. Labour conference passed transformational policy on a whole raft of issues – originated not in the leader’s office but in organised grassroots campaigns.

Labour for a Green New Deal pushed through a world-leading climate change policy including a target to reach net-zero carbon emissions by 2030. John McDonnell announced that the party would reduce the working week to 32 hours within a decade, following policy put forward by unions and the Four Day Week campaign.

Brexit and climate protests at Labour party conference: In pictures Show all 12 1 /12 Brexit and climate protests at Labour party conference: In pictures Brexit and climate protests at Labour party conference: In pictures Labour leader Jeremy Corbyn walks past anti-Brexit campaigner on the second day of the party conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures A protester holds a sign mocking Labour leader Jeremy Corbyn's Brexit stance at the party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Activists from anti-climate change movement Extinction Rebellion march on the Labour conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures Delegates take part in an informal vote on Labour's Brexitr stance at the party conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures Shadow foreign secretary Emily Thornberry leads a march in support of a second referendum on Brexit Getty Brexit and climate protests at Labour party conference: In pictures Anti-Brexit protesters take part in a People's Vote rally during the Labour party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Extinction RebelliActivists from anti-climate change movement Extinction Rebellion march on the Labour conference in Brightonon EPA Brexit and climate protests at Labour party conference: In pictures Extinction Rebellion activists march on the Labour conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures A delegate arrives carrying a briefing from the Labour Representation Committee to the Labour party conference in Brighton Getty Brexit and climate protests at Labour party conference: In pictures Prominent anti-Brexit campaigner Steve Bray protests on Brighton beach during the Labour party conference Reuters Brexit and climate protests at Labour party conference: In pictures Anti-Brexit protesters take part in a People's Vote rally during the Labour party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Activists raise a banner calling for the Labour party to restore the original Clause 4 of its constitution, which would pledge the party to nationalising all industry Getty

Thanks to tireless work by the patient-led campaign Just Treatment, the party committed to use legal powers to suspend drug company monopolies if they fail to offer the NHS a fair price. And the Labour Campaign for Free Movement successfully overturned the party’s commitment to ending free movement after Brexit, with conference also voting to close all immigration detention centres.

It is still unclear whether all these measures will be adopted by the leadership (Diane Abbott almost immediately appeared to backtrack on free movement). All radical parties of recent times, from Spain’s Podemos to Greece’s Syriza, have struggled with the tension between a commitment to mass democracy and a centralised party hierarchy – and Labour is no exception. But in Brighton, the mass membership asserted itself in no uncertain terms – and it has a lot to say.

Away from the conference floor, things were getting even more interesting. Just up the road from the conference centre, activists pitched up in Brighton’s Old Steine Gardens for The World Transformed – a four-year-old festival that has rapidly usurped the official conference fringe as the cutting edge of movement discussion on policy and strategy.

Amid marquees, bunting and a huge Hollywood-style sign reading “SOCIALISM”, activists participated in a series of “Policy Labs” on issues from crime and youth violence to education and health. Their deliberations formed the basis of a “Manifesto for the Movement”, written collaboratively in real time and presented hot off the press to John McDonnell at the end of the festival.

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This innovative and exciting experiment in participatory policy making helps to explain why the mood at The World Transformed was strangely upbeat given the UK’s rolling political crisis. There is a genuine sense that activists are doing something new and important – that politics is being reinvented for a new generation and given fresh life and energy.

Activists are under no illusions that things are falling apart, whether it’s the black hole of Brexit, a stuttering economy or catastrophic climate change. This is precisely why they feel a historic responsibility to help build something better. They know that if the radical left cannot provide answers that match the scale of the systemic crises we face, the far right will do so instead.

That’s also why this year’s festival saw a new determination to engage seriously with the challenges of implementing a radical agenda. From a strategy game on the first 100 days of a Corbyn government, to reading groups based on my own book with Joe Guinan, People Get Ready!, activists grappled with the headwinds that a radical government might encounter – from resistance by finance capital to an uncooperative civil service to a resurgent far right.

The contrast with a few years ago is nothing short of astonishing. Corbyn’s unexpected rise to the Labour leadership has forced the left to grow up fast. It has moved rapidly from being stuck in oppositional mode – railing against austerity and the banks but short on positive alternatives – to creating real solutions and taking responsibility for how they can be put into practice. If there is a solution to the constitutional crisis we find ourselves in, it will surely come not from the corridors of Westminster but from people-powered spaces like The World Transformed.

Jeremy Corbyn was propelled to the Labour leadership from below. Labour’s new members, decried as far-left entryists, were mostly just young people desperately seeking hope for a better future. The Corbyn project promised to be driven by the participatory energy of this mass movement. Until recently, that promise remained unfulfilled. The movement ecosystem was simply not mature enough to shape Labour policy in more than a patchy and inconsistent way. But no longer. Underneath the firestorm engulfing formal politics, this is a movement coming of age.