Right from the beginning, the promise of Corbynism was that it would empower the grassroots. In his acceptance speech on becoming Labour leader in September 2015, the MP for Islington North spoke of enabling members to better influence party policy, of his desire to hear passionate debate, and to “make our party more inclusive, more democratic and the membership better listened to in the future”. Let the tail wag the dog.

On that basis, we shouldn’t be surprised by today’s announcement that Momentum, the youthful, 40,000-strong membership organisation established as an outrider for Corbyn’s leadership, will be campaigning for a “radical and transformational” new policy agenda. It is nonetheless a desperately needed and inspirational injection of energy. Over the summer, the group will seek to organise Labour’s mass membership to get motions passed in local constituency party meetings backing a green new deal (including achieving zero carbon emissions by 2030), the introduction of a four-day week, and the abolition of all migrant detention centres.

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The aim, they say, is to get these CLP motions taken forward to conference in September and made into party policy. That’s something pro-remain campaigners in the party did very effectively ahead of last year’s Labour conference, shaping party’s current policy on Brexit.

The idea is to create an election manifesto that builds on the widely praised 2017 equivalent – but takes it much further, and in doing so sees off Tom Watson’s more conservative Future Britain group, whom they see as “intent upon watering down and blocking Labour’s most transformational policies”.

Interestingly, three other small grassroots organisations are in the mix here, prompting Momentum’s turn to policy campaigning: Labour for a Green New Deal, Labour Against Racism and Fascism, and Labour 4 Day Week – each of them less than a year old. After a year of (largely enforced) malaise in Westminster, these groups provide a welcome reminder of what the point of a leftwing Labour party ought to be.

What is notable is how these three groups, and now Momentum, relate to a wider network of activist movements, thinktanks and campaigners. They are showing that they can be a conduit for existing political energy outside of the Labour party. This, again, is exactly what a leftwing party should look like: pluralistic, democratic and porous, rather than proprietorial.

The school climate strike led by Greta Thunberg and the phenomenal campaign by Extinction Rebellion inspired millions to take the climate emergency seriously; the thinktanks Autonomy and New Economics Foundation who campaign for shorter working weeks have been gaining influence; while everyone from the hunger strikers inside Yarl’s Wood to the groups Sisters Uncut campaigning on the outside, have helped make the plight of migrants in detention centres a mainstream issue.

Party politics shouldn’t exist in a vacuum, cut off from the political life, ideas and activity around it. And that doesn’t mean Labour should simply co-opt and dilute the demands of those extra-parliamentary activists, instead it should respond to them, work with them and be energised by them.

There is a bizarre fallacy popular in the rightwing press and Westminster among critics of the Labour party under Jeremy Corbyn: that it has become a “Stalinist”, “Leninist” and also – confoundingly – simultaneously “Trotskyist” organisation: that it is ferociously authoritarian in spirit and practice, and run with iron discipline from the centre. This line of attack displays not only a depressingly slight grasp of the basics taught in key stage 3 Russian history modules, but also a goldfish-like memory of even recent British politics.

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How easy it is to forget the macho culture at the heart of the New Labour project, the endless stories of bullying, abuse and “explosive clashes” in the heart of government, a party run by “the Burnley bruiser” Alastair Campbell, while former Blair adviser John McTernan wrote cheerfully about how great it was that Labour whips used to physically assault backbenchers, and the vital need to “crush” opponents. By contrast, the Labour party under Corbyn might be many things – tangled, messy, riven with factions and kvetching MPs – but an incarnation of Soviet democratic centralism it is not.

For months now it has felt as though the Labour party has been stranded in the purgatory of Brexit, distractedly rubbernecking at the clown-car pile-up that is Change UK, while radical grassroots energy flared up outside the party hierarchy, as it always had done prior to 2015. The PLP has been right to respond to Extinction Rebellion and the school climate strike so swiftly – declaring the world’s first climate emergency, and announcing a plan for a massive investment in solar energy just yesterday. But more needs to be done.

British society needs radical transformation to address the ever-worsening climate crisis, the way we work – for too little money, for too long, until it destroys our mental and physical health – and the way we treat migrants and refugees. It is right that Momentum is showing itself to be responsive to this. Above all, it is a reminder that Labour needs to be led from the bottom up.

• Dan Hancox is a freelance journalist and author