Imagine it is 2024 and Boris Johnson is running for another term as prime minister. He is up against the fourth Labour leader to try to win an election since Tony Blair resigned 17 years before. A combination of boundary changes and the loss of Labour heartlands in Scotland give the incumbent an edge. But why is everyone certain that the Conservatives are going to lose?

The answer is a united opposition, not just of Labour politicians but of people across the centre and left of British politics. United by agreement on a package of democratic reforms, including an elected second chamber – with proportional representation (PR) as the cornerstone of a transformational agenda. This wouldn’t be simple expediency but a reaction to the divisive, bickering politics of the Brexit years that had been fed by dark money and the manipulation of social media. It would give the Electoral Commission sharper teeth and enable electoral law to catch up with the digital age.

It’s a dream, but a good one.

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According to a new YouGov poll, three quarters of Labour members want the party to support PR, but it is yet to be mentioned in the run-up to Labour’s leadership election. This seems odd given that Johnson ended up with a massive parliamentary majority despite barely getting more votes than Theresa May in 2017. At the moment, Labour contenders are quickly unpicking the compromises of their Brexit policy and shifting blame one way or another, while ignoring the fact that most of the electorate voted for parties in favour of a second referendum. If we had PR, then that is what our new government would be organising right now. First past the post (FPTP) distorts the outcome of elections but those on the left have held on to the expectation that their turn will come.

So why is Labour the only socialist party in the developed world that supports FPTP? Some are worried it would mean no more majority Labour governments, and therefore an end to the advancement of equality and the redistribution of wealth. Except we are 32nd in the OECD’s rankings of income equality, way behind the rest of western Europe, where all countries have PR. Of the top 39 countries on the OECD list, only Canada and the US share our love affair with FPTP. The US has awful extremes of rich and poor, while Canada does manage to come ahead of us in 19th place. Yet Canada has never had a majority socialist government. Meanwhile, progressive governments in Denmark, Norway, Germany, Iceland, Finland and Sweden have almost never been the result of a single socialist party’s majority, but are made up of leftwing coalitions – and they do OK.

As we’ve seen, the argument that FPTP delivers strong and stable government is nonsense. I know from my own experience on the London assembly, elected via a system of PR, that a more consensual and positive politics is possible. We fought each other in elections and then worked together between them.

PR doesn’t guarantee that things will get better, but it enables us to generate a consensus about the direction of travel, whether that is ending austerity, stopping the NHS being privatised or ending domestic violence. As Friends of the Earth has pointed out, the electorate favoured parties who take the climate emergency seriously – yet we have ended up with a government that doesn’t. This is a failure of FPTP.

I hope that the Labour leadership will not only start discussing PR, but also debating how best to work with other parties to beat the Conservatives next time. Although I find compromise with other parties very hard, I didn’t enjoy the constant battle of wills on social media about tactical voting between people who were all keen to see the Tories gone from government. The Greens, Liberal Democrats and Plaid did show that it is possible to work cooperatively, but while Labour was invited to talk, it wouldn’t engage.

The other side did it better: the Brexit party stood down in Tory marginals and then split the Labour vote in the north to open up the red wall and allow a Conservative victory. The lesson is clear, if progressives want to win a FPTP election then we have to get our act together.

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The only two things I can predict with certainty about 2024 is that the climate emergency will have got a lot worse and that the Conservative government will have done relatively little about it. To rekindle the politics of hope, progressive politicians need to talk about what we can agree on. When the Green party’s Caroline Lucas and Labour’s Clive Lewis brought a bill to parliament to launch the green new deal, I felt real optimism that we could create something that is both popular and also radical enough to begin to solve the climate crisis. Let’s do the same with renewing our democracy.

• Jenny Jones is the former chair of the Green party and a former deputy mayor of London