Nearly two years may have passed, but the parliamentary Labour party barely need reminding how Jeremy Corbyn won a place on the 2015 leadership ballot. Moderate MPs, almost all of them unsympathetic to his politics, were persuaded to lend him their nominations to “broaden the debate". Corbyn squeaked over the threshold of 35 nominations and on to the shortlist with moments to spare. The trajectory his leadership has taken since means few, if any, will be so generous come the next leadership contest.

The consequences are naturally problematic for the left. Barring some unforeseen compromise or an influx of Corbyn loyalists, they face being locked out of a future leadership race by the arithmetic of the rules. At present, these require candidates to be nominated by 15 per cent of Labour’s MPs and MEPs.

The battle, however, is not yet lost – and like all great factional battles within Labour, it will be fought and won on the conference floor. With an eye on Corbyn’s inevitable departure, the Labour left is pushing for an amendment to the leadership rules. This would lower the nomination threshold from 15 per cent to 5 per cent. The reduced quota of 12 MPs – down from 37 – would all but guarantee a left-wing contender (or, indeed, contenders) a place on the ballot.

Dubbed the McDonnell amendment in honour of the shadow chancellor John McDonnell – whose two abortive bids for the leadership in 2007 and 2010 were scuppered by his inability to reach the 15 per cent threshold – the rule change will be floated at Labour conference in Brighton in September.

Critics of the proposed change (and there are many of them) say it undermines the fairly fundamental principle of parliamentary democracy. By contrast, its proponents speak with varying degrees of sincerity on the need to better represent Labour’s new mass membership. Indeed, Momentum’s Jon Lansman – a veteran of the Bennites’ long war with moderates in the eighties – was last week revealed to have told new recruits that securing the amendment was “absolutely crucial” to the future of the project.

But will it pass? Though Labour moderates have been spooked by the prospect of the amendment ushering in infinite Corbynism, many remain bullish as to their chances of seeing it off. The rule change will definitely be debated on the conference floor, but many moderates, having secured a series of important victories at a regional level last year, are confident that the make-up of conference delegates – plus the likely support of unions including Usdaw, the GMB and Unison – will allow them to block the left on this occasion as well.

Ultimately, whether or not the left will get their way depends on their ability to organise effectively at this grassroots level.