In the first of a series of letters to progressive parties, the chair of Compass encourages the Green Party to drive the political debate.

Dear Greens,

This is the first of a series ‘letters’ to the main progressive political parties in the UK. No one asked me to write and I guess only those that want to be constructively challenged will read them. But that is the spirit in which this and the letters to follow are meant, somewhere between generosity and hope, and realism and frustration that our political system isn’t working and none of our parties are yet good enough on their own – but might be together. So here goes.

Your party, in policy terms, is close to my heart. Proportional representation, a basic income, anti-austerity measures such as Green QE and of course real sustainability, not green wash, what is not to like? And a little over a year ago you were given national recognition through the membership surge you momentarily enjoyed. Thousands of new people joined and you got an unprecedented slice of the vote – with over 1 million backers.

It should have been the big break though. But of course it wasn’t. The electoral system is rigged against you and you won only one seat. Of course Caroline Lucas is one of the best progressive politicians in the Commons but isn’t that where the parliamentary road to sustainability ends? Under First Past the Post (FPTP) the one seat you have is likely to be only seat you will ever have. Where you are second you are so far behind you are unlikely to ever catch up and if you do – you are second to Labour. So even unlikely success does nothing to dent the Tory majority. The recent local election results confirm the mini-surge is over. Yes the excellent Sian Berry ran a good campaign in London, but in a Corbyn world you have lost support to Labour in key places like Norwich and Bristol. The moment in the sun on the Brighton council is over. Yes in Scotland under PR you won more MSP seats – but not as many as you thought.

What is your future, where is energy and purpose to come from and what is the place of the Green Party in the 21st century? With Natalie standing down, the election of a new leader, or even leaders, is the chance to have a real strategic debate. You must take it. Since the general election I’ve not heard much deep thinking and vigorous debate from you? It’s worrying. So here are some issues that might provoke some renewable heat and light.

Unlike the recommendation of the Guardian editorial that you should stick to green issues, your job is to show why sustainability has to be part of every aspect of policy and indeed core to any notion to the good society – a more seductive and fulfilling life precisely because it not about a global race and turbo consumption. I’ve always thought your name holds you back – you can’t be a single issue pressure group but a force for system change.

On the big picture, it seems to me that, given the harsh electoral hole you are in, your role is help change the weather of British politics, in terms of vision, policy and practice. Power is the ability to change things. It shouldn’t be confused with ‘being in office’. Yes the two things can coincide but the Greens have chased local and national office and lost the spark to change the terms of debate of British politics. Not just better policies but a different political culture. Look at UKIP, they are hardly represented in office but have had a massive effect on that national debate – swinging the country to the right on immigration and Europe. You need to be more a movement of ideas than just a not very good electoral machine with the voting odds always stacked against you. Run with big ideas like a basic income and a shorter working week more than you knock on doors to ask people to vote for you in elections you can’t win. Even if you could adapt a Lib Dem like pavement politics strategy – the planet doesn’t have time to wait for you. There are millions of people whose lives are anxious and insecure, who don’t want a treadmill like existence – who yearn for a different quality of life. Speak for them. You might find it pays off in votes – support coming from the fact that you offer a desirable future – not because you are playing the political game ‘seriously’ like the other parties.

Which takes us on to your influence strategy. The only hope, I repeat the only hope we all have in the short term is for a progressive alliance of Labour, SNP, Lib Dems, Plaid Cymru and the Greens to defeat the Tories and UKIP. Such an alliance needs to be thought through and built now. It’s going to mean difficult decisions and some compromises. But there is no option – unless you want the Tories back in. The cornerstone of a progressive alliance would be a commitment to introduce PR after the 2020 election. It’s not a panacea but it makes other big changes possible by letting multi-party politics and a future that is negotiated come to life. Labour is moving in this direction, the Shadow Chancellor John McDonnell is only the most senior advocate of a growing list of MPs from all sections of the party backing PR. The trade unions and the TUC are heading in this direction too.

The Green Party must wholeheartedly adopt this strategy and lead it. Caroline Lucas had been bravely leading the charge for a progressive alliance. But as a party you are not there yet. When I speak at Green Party meeting I always say how short sighted of Labour to stand competitively against Caroline in Brighton Pavilion. And you all clap loudly. Then I say how short sighted of the Greens to stand competitively in next door seat of Brighton Kemptown – and stop a progressive Labour candidate from winning and hand the seat to the Tories. And of course you don’t clap then. But this is the hard truth of an electoral systems rigged against you. You can only change it if there is a progressive alliance with a mandate for electoral reform. That is the prize of doing deals and making compromises now. We need to work through the details of all this – how an alliance can be built from the top down and the bottom up. But be built it must. And Corbyn is not Blair. Make him and his party do a deal.

And finally my Green friends, can you change your party? You are wonderfully democratic but you have a clunky and old feel to you. Experiment and modernise. Use new technology to make policy decisions, adopt the kind of Circles that Podemos use, try out more fluid ideas. Let members join other parties like the WEP do. This is a world of Facebook where people hold multiple identities all at once. Take a big look at the Alternativet party in Denmark and see how they link sustainability to innovation and enterprise – and make it all look cool. It’s called the future. Grasp it and use your party to prefigure the world we need to create.

Sorry if it sound like I’m lecturing. Just wait to see what I have to say to the other parties – especially mine. It’s just that it’s so frustrating watching you do the same thing expecting a different outcome. Ultimately the only thing we can change is ourselves. You have the chance of an amazing future and more opportunity to experiment than any other progressive party. We have to live within the limits of the planet but find a way of doing this that is more seductive than endless consumption. It means being braver than ever. Good luck. Compass is here to help. We are with you.