The wait is over, the nominations are in, and with half of the Internet hitting refresh on Twitter until the very last minute, Jeremy Corbyn is a candidate for Labour leader – confounding the predictions of large swathes of the commentariat and much of the Labour left itself (including me). Once the social devastation wrought by a Tory majority becomes apparent, the outcome of the 2015 general election will go down as one of the greatest ‘what ifs’ of recent years, and, within the reality that we now face, Corbyn’s nail-biting success in reaching the ballot may prove to be a turning point for the fate of the British left as a whole.

None of this success should on its own be a vindication of the strategy of the left’s participation in Labour. For now, all that has happened is that a serious, principled left wing candidate has been able to run, not because of the strength of the Labour left, but because of a combination of public campaigning – much of it egged on by the left outside the party – and the charity of some of Labour’s more moderate MPs. Many on the right and centre of Labour chose to put Corbyn on the ballot for a mixture of principled and tactical reasons – hoping to manipulate the dynamics of the election, or positioning themselves in the London mayoral or deputy leadership contests. As many of the more astute hard line Blairites are observing, this decision will haunt the Labour Right.

The presence of Corbyn on the ballot paper will immediately allow the left to articulate itself within the contest and on a national stage, though its arguments will be ignored as outsider views by many in the mainstream media. What may come as more of a surprise is that Corbyn could well produce a serious shock in terms of vote share. Had Dianne Abbott run under the party’s new electoral rules, she would have come third in 2010. Corbyn – by the looks of it, a more galvanising and uniting figure on the left, and personally a very popular figure in the party – should do much better, and that is without taking into account the most decisive factor: the popular legacy of the struggles of the past five years.

Far from representing a fringe constituency, Corbyn is a representative of the only political current, with the exception of the Scottish nationalists, to have galvanised popular opposition to the Coalition and Conservative governments: the radical left. In high politics, within the wider labour movement, and within social movements, the imagination and strategies that have brought people in and won disputes in a new era of reactionary government have come by and large not from centrists or trade union leaderships, but from grassroots activists and radical political minorities. On many core issues, the radical left is now the only political force articulating what a majority of the public wants: economic democratisation, tax justice, and public ownership.

The problem for Corbyn’s campaign is that, just as anti-neoliberal political sentiment has risen, particularly among the young, the tidal process that brought activists from social movements into the Labour left is weaker than ever. The new generation of leftwing sentiment has found its limited political expression outside of Labour, mostly in the Greens; to its detriment, much of this distance from Labour politics comes not so much from any particular long term strategy, but an atomised political culture which prises moral purity and in which party membership works much like brand affinity. The quirk of this process is that the same people who constituted the ‘Green surge’ of 2014-15 may now be persuaded to sign up and vote for Corbyn.

For much of the newer and older left, which has based so much of its political strategy and identity on being outside of the party, the idea of investing time and energy into a Labour leadership election may seem like a step backwards. But in order to progress, that is precisely what the wider left must do, not just because Corbyn might get a lot of votes, but because this Labour leadership election is about more than it seems.

It could be that, with an unprecedented surge in leftwing members and supporters, Corbyn really could win; and it could equally be possible that, in spite of all the hype, this is actually the beginning of the swan-song of the Labour left – another moment at which many of its number finally decide, from a position of strength, to start or join something new. In between, a series of other positive outcomes are possible – exposing the contradictions within Labour, winning arguments over ideas.

In any of these scenarios, it is essential that Corbyn’s campaign is large, well-supported, and directly linked to grassroots campaigns and social movements – and in order to deliver that, it will need forces far broader than the Labour left can currently muster on its own. If the trickle of leftwingers now signing up for the leadership election turns into a flood, the Labour Right will complain of a coup. Precisely the opposite is the case: if Labour is really the party of working people, then the wider left, Labour’s extended family, has far more of a claim to a stake than the machine politicians and hacks that populate so many of its offices. For almost every tendency and shade of the left, all roads now point towards backing Corbyn’s leadership campaign.