One of the three local councillors for the Highgate ward in Camden, north London, brims with radicalism. She is an astute campaigner, who is hard-wired with the values of social and environmental justice. And yet Labour is hoping to oust her in the local elections in May.

This is Siân Berry and she is a Green. Ipso facto, for Labour, she is the enemy and must be replaced with one of its own. Missing out on the seat by only 75 votes last time, the Labour juggernaut is well-placed to get its way this time. Berry is the only Green on Camden council and one of only four across the whole of the capital. (She is also on the management committee of Compass – the progressive cross-party organisation I chair.) But as a long-time Labour member I see only the value that people like Berry bring to radical politics, and the danger of extinguishing their voices.

In neighbouring Islington, Caroline Russell plays a similar role as the only Green councillor in that borough. But her seat, too, will be relentlessly targeted come May, leaving Labour with an absolute monopoly of control and the lack of accountability that goes with it. This is not just bad for democracy, but bad for Labour. Just as it would be bad for Labour if the party were to defeat the Green party’s sole MP, Caroline Lucas in Brighton Pavilion, at the next general election.

In over 60 Tory seats the progressive vote was bigger than the regressive vote

Not everyone in Labour is committed to monopoly control. But the predominant sentiment of Labour, from left to right, is the electoral extermination of anyone who isn’t a card-carrying member of the party. Owen Jones’ article last week, arguing that the Greens should join Labour, was a good example.

Challenging this monopoly socialism has never been more necessary. Jeremy Corbyn would now be prime minister if Labour had given even an inch to the idea of a progressive alliance last June. In over 60 Tory seats the progressive vote was bigger than the regressive vote – but division on the left meant that purity was preferred to power. While Labour cleaves to the myth of one more heave, the reality is that such an alliance is likely to be needed again.

Labour dreams of recreating the “1945 moment” – but in so doing misreads history. Yes, it was Labour that won the seats and pulled the then functioning levers of the state to remake a postwar society, but it was the Conservative Quintin Hogg who coined the term social security, just as it was William Beveridge and John Maynard Keynes, both Liberals, who invented the new system of social security and the new economic order to pay for it. Labour itself was a rich, diverse movement. Today all that is remembered is Clement Attlee and a cabinet that seemed to do socialism to people and not with them. It is worth recalling the words of Attlee: “The foundation of democratic liberty is a willingness to believe that other people may perhaps be wiser than oneself.”

There is no doubt that given the chance Labour would take every seat in every council, assembly and parliament across the land – and that, as a consequence, all other voices must be annihilated. Tony Blair once claimed that New Labour was “the political wing of the British people” as if there could be no other choice – a tent so big it was the home for literally everyone.

‘I see only the value that people like Siân Berry bring to radical politics, and the danger of extinguishing their voices.’ Photograph: Nils Jorgensen/Rex

During the last election many Greens reached out and backed Labour and looked for solidarity in return. As Green candidates stood aside and Green, Liberal Democrat and Labour supporters voted tactically, they saw something bigger than their single-party interest. Some in Labour reciprocated, but others were punished for doing so. In South West Surrey, three Labour members who backed a National Health Action party candidate were expelled from the party.

The question is: can Labour give up on the illusion of monopoly power? It’s not a right-left issue. From John McDonnell to Chuka Umunna, there is a rising tide of electoral reformers in Labour’s ranks. It must be extended further – and fast.

Much depends on Momentum. The organisation seems to hold two cultures at the same time: old-school command and control, and an openness to pluralism. A generational divide represents this schism, with younger members not having the same tribal loyalty to Labour as older comrades who endured the Blairite years and want “their” party back. But when Jon Lansman, the chair of Momentum, says: “I want an inclusive, pluralist Labour party” there could be hope.

It will be essential if the Corbyn project is to succeed. Taking on the City, house builders and privatised industries, to name just three obvious and powerful sectors, in the context of Brexit will be hard enough. To believe it can be done without broad alliances is simply reckless.

Labour needs to shift from the politics of the Big Tent, with everyone under its suffocating roof, to the politics of the campsite, in which progressive parties, movements and organisations keep their identities but coordinate and help others. The party could be the biggest tent in a new ecosystem with the power to transform our country.

The cold war is long over. It is time for cold politics to be over, too. It is time for warmth, generosity and, dare I say it, love. For Labour, like all of us, it’s only when you let go and trust other people that meaningful and lasting change becomes possible. Maybe Labour could start by trusting the likes of Siân Berry?

• Neal Lawson is chair of the pressure group Compass and spokesperson for the Progressive Alliance, and author of All Consuming