By Robin Ashington

Progressives have much to celebrate, but we have fallen far short of what we might have achieved at the general election had we worked together more effectively. Despite huge advances in cross-party cooperation and tactical voting, we didn't go nearly far enough. If we had done, Jeremy Corbyn would be in No.10 right now.

It's worth taking the time to review what the progressive alliance movement has achieved so far, what went wrong this time around, and what we must do differently to ready ourselves for the next election.

The 2017 campaign heralded historic breakthroughs for cross-party co-operation on the left, spearheaded by 40 local Green parties making brave decisions – independently of each other – to stand aside in the service of the greater good. In seats such as Oxford West and Abingdon or Westmorland and Lonsdale, it is beyond doubt that this was the primary factor in denying the Tories a victory. In others, the Greens' courage gave a progressive candidate the head of steam they needed to propel them towards a large majority.

This was also the election in which tactical voting went mainstream, with hundreds of thousands using online tools such as the Progressive Alliance Vote Smart website to check which party was best-placed to beat the Tories in their own constituency.

I've spent much of the campaign volunteering in Compass's Progressive Alliance HQ in London, as well as helping to facilitate cross-party dialogue in my own county of Sussex. It has been a joy to witness seasoned party activists drop their long-held allegiances with gleeful abandon and squadrons of fresh-faced volunteers embrace an entirely new model of electoral activism. When I asked "which party would you like to help?", the most common refrain was "whichever needs the most help".

The age of post-tribal politics has dawned, and it is not going away. This is not about floating voters: there is an increasingly confident breed of both activists and voters that is prepared to make common cause beyond party identity, that recognises that no-one has a monopoly on hope. We won in 22 of Compass's 31 primary target seats.

In my neck of the woods, Sussex Progressives created a platform for cross-party collaboration on an unprecedented scale. We enjoyed success in four of our target seats in Sussex, taking Brighton Kemptown and Eastbourne from the Tories and returning huge increases in the existing majorities for the Greens in Brighton Pavilion and for Labour in Hove.

While we achieved much to be proud of here, Sussex also offers a case study in our nascent movement's failure to scale-up quickly enough to meet the challenge of the political crisis facing our country. A little more collaboration between the three parties in Sussex alone would have been enough to deprive Theresa May of her ability to form a government. If Labour in Lewes had recognised that only the Lib Dems could beat the Conservatives there; if just a fraction of Lib Dems in Hastings and Rye had voted Labour; and if Greens, Lib Dems and supporters of independent candidates in Worthing East & Shoreham had backed the Labour candidate, it would surely be game over for Theresa May by now.

Some might say that all we needed in Sussex was a little more tactical voting, but in my view the fault lies not with the voters but with the politicians. We cannot expect every voter to be clued up enough to succeed in the increasingly complex game that first-past-the-post forces us to play. A simple quid pro quo between Lib Dems in Hastings and Labour in Lewes could have been sufficient to win an extra seat for each party. Neither had any chance of winning on their home turf. They had nothing to lose but their pride. Hastings was our local tragedy: we were so close, yet so far, from taking the scalp of a home secretary whose policies strew shattered lives behind them from Calais to Carlisle. But while the Labour party in Kemptown were taking all the help they could get, the Hastings constituency party was bogged down in debates about whether to accept the help of volunteers from outside the party fold. And Hastings Lib Dems weren't willing to sacrifice a single one of their nearly two thousand wasted votes unless Labour could offer them some tangible benefit in return. Too many of us remain ensconced in our tribal identities, oblivious to the harm they cause.

Across the country, there are 62 constituencies won by the Conservatives last Thursday in which the combined progressive vote would have trumped the Tories. There are the obvious ones with tiny majorities in which the Labour vote cost the Lib Dems a seat only they could win (Richmond, St. Ives), the dozens of seats in which a tiny Lib Dem vote cost Labour a victory (Pudsey, Thurrock, Stoke-On-Trent South), but also several in which the Green vote share alone would have been sufficient to hand Labour victory had they stood aside there too (Southampton Itchen, Calder Valley, Norwich North).

We cannot go on like this. Greens and Lib Dems are wilfully denying ourselves the pluralist future we so need. Had the the left-of-centre vote not remained so terribly split, the 2017 election could have returned a whopping progressive majority of 124 seats, whose first Act of Parliament might have been to reform the archaic voting system that demands such game-playing of us in the first place. Remember that all of this is a one-off – once we have instituted proportional representation, we will never again need to build such electoral alliances. Our parties' very different political and philosophical traditions will each blossom in the fertile ground of meaningful constitutional reform, but the relationships we build as we till the soil will stand us in good stead for the grown-up politics of collaboration and coalition that must follow.

None of this election's failures detract from what we have begun to build: the remarkable alliances we we took into this election were cobbled together in just a few weeks, in response to a snap election that few expected. Given more time to convince the naysayers, to plan, to dream and to bargain if we must, we can and will achieve the promise of the progressive alliance. Each day that the Tory-DUP regressive alliance retains its fragile grip on power is a day gifted to us to build our movement. We will not have long. We must remain on an election footing, must be ready to learn from our mistakes and to build the movement that will usher in not just a change of government, but the democratic reforms that will change our political landscape for good.

Robin Ashington has worked in various political groups, mostly on environmental issues. 

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