Dear Jeremy,

These are exciting times for progressive politics. In the space of just a few weeks you’ve brought something into your party that’s been missing for far too long: hope. I’ve never felt so optimistic about a potential leader of the Labour Party. For the first time in my memory, the party of Keir Hardie and Clement Attlee looks likely to be led again by someone who dares to stand up for the radical changes demanded by the challenges we face.

I’ve shared many platforms with you, from opposing Britain’s disastrous and bloody war in Iraq to supporting investment in the economy in place of relentless and cruel austerity. Your inspiring campaign has put so many of our shared values into the centre of the debate in British politics.

The beauty of this moment, and what scares the political establishment most, is that the power of your campaign is coming from thousands of grassroots voices – not a diktat from above. It hardly seems a coincidence that the first truly democratic leadership election in your party’s recent history is producing such a powerful resurgence in optimism. People do indeed vote differently when they know their vote counts.

And it has been particularly exciting to see so many of the policies that have long been championed by the Greens resonating with such a substantial portion of the electorate. Like the Green surge of earlier this year, and like many of the social movements and new progressive parties that have sprung up across the world, an anti-establishment mood is manifesting itself into a real political force.

Jeremy Corbyn addresses over a thousand supporters at Middlesbrough Town Hall on August 18, 2015 (Getty Images)

However, to fully embrace this moment – and if Labour is to truly become part of a movement rather than remain just a machine – it’s crucial to recognise the multi-party nature of modern British politics. No one party has a monopoly on wisdom, or is capable of making the transformation alone: a diversity of progressive voices is essential for our democracy.

Greens, for example, bring vital and distinctive issues to the table – most crucially, and at the heart of our politics, is the fundamental belief that a new social contract will only ever be possible if it’s built upon the foundations of “one planet living”. Without a safe climate at the heart of our policymaking, progressive politics won’t ever take root. Indeed, there is no better argument for abandoning tribalism than the looming climate crisis we face. If we’re going to stabilise our environment and build a secure economy that serves our children and grandchildren, we have to work together.

Labour leadership: The Contenders Show all 4 1 /4 Labour leadership: The Contenders Labour leadership: The Contenders Jeremy Corbyn Jeremy Corbyn started off as the rank outsider in the race to replace Ed Miliband and admitted he was only standing to ensure the left of the party was given a voice in the contest. But the Islington North MP, who first entered Parliament in 1983, is now the firm favourite to be elected Labour leader on September 12 after a surge in left-wing supporters signing up for a vote. PA Labour leadership: The Contenders Liz Kendall Liz Kendall has been labelled the Blairite candidate throughout the contest, which partly explains why she has failed to attract the support needed in a party that has drifted even further from the centre-ground of British politics since the election. She has faced criticism over her relative lack of experience, having only served as an MP since 2010 and having no experience of ministerial or shadow cabinet roles. But that very lack of experience allowed her to initially make a pitch as the only candidate offering real change and a real break from the Blair/Brown/Miliband years, until Jeremy Corbyn entered the race and shifted the whole debate to the left. She is set to finish a disappointing fourth. PA Labour leadership: The Contenders Andy Burnham Andy Burnham started out as the front-runner in the leadership election, seen as the candidate of the left until Jeremy Corbyn entered the race. The former Cabinet minister has found himself squeezed between the growing populism of Corbyn’s radical agenda and the moderate, centre-left Yvette Cooper, not knowing which way to turn. It has attracted damaging labels such as ‘flip-flop Andy’, most notably over his response to the Government’s Welfare Bill. He remains hopeful he can win enough second preference votes to take him over the 50 per cent threshold ahead of Corbyn. PA Labour leadership: The Contenders Yvette Cooper.jpg Yvette Cooper has put her experience and achievements in government at the heart of her offer to the Labour party. She played a key part in setting up Sure Start in Tony Blair’s government and has pledged to continue her record on delivering for young families by promising a “revolution in the way families are supported” by introducing universal free childcare. She has also championed her role as a full-time working mother, taking pride in telling audiences that she does the school run for the kids before her day starts as a politician. But she has been criticised for being too wooden and lacking in passion and her attacks on Liz Kendall for “swallowing the Tory manifesto” at the start of the leadership contest have been criticised for helping Jeremy Corbyn brand all three mainstream candidates as ‘Tory-lite’. PA

For that reason, one of my few disappointments about your campaign is that it hasn’t focused more on reforming our ailing democracy. A truly progressive politics fit for the 21st century requires a voting system which trusts people to cast a ballot for the party they believe in. If you do win this contest I believe you should take this opportunity – and the huge amount of momentum behind you – to call a constitutional convention to allow people across the country to have a say in remodelling Britain for the future. A convention has the potential to energise even more people than your leadership campaign, or the Green surge, and to inspire the kind of feeling across the UK that swept Scotland in 2014.

In the short term, for the next general election – which will still be contested under First Past the Post – my personal view is that there is potential in considering local grassroots electoral pacts where progressive candidates are standing, so as to give us the best chance of beating the Tories in 2020. It’s only by winning that we have the chance to implement positive change.

By working together in the coming weeks and months we can continue to build upon the movement you’ve played such a huge role in creating. Not only can we provide real economic alternatives to austerity, defend the trade unions and make the argument for urgent climate action, but we can also start to imagine an entirely different future – of a new social settlement, an economy that provides decent pay and allows people to flourish outside of work too. Crucially, a new politics will provide a constitutional framework which hands power from Westminster back into the hands of voters.

The old politics is crumbling, not just in Britain but across our continent. We now have the chance to embrace a movement based not on greed or fear, but on resilient local communities, people working together and a stable economy that works for generations to come. I truly hope you win the contest on 12 September – and I look forward to continuing to work with you to bring about the progressive politics that has inspired us both for so many years.

In solidarity,