What happens if Jeremy Corbyn wins?

A lot depends on the scale and manner of the victory. If Corbyn wins a victory among “full” members and trade union affiliates – and not just thanks to the £3 registered supporters – then his leadership will be significantly more stable, at least in the short-term. There is a feeling among the parliamentary Labour party that the “registered supporters” scheme – where people can pay £3 to vote in the leadership election – gives too much power to a group of people “who’ll be off in a few months” in the words of one MP.

Regardless of the outcome, the registered supporter experiment is highly unlikely to be repeated, so if it those supporters who have handed Corbyn victory, while the membership and affiliated trade unionists have backed Andy Burnham or Yvette Cooper, I wouldn’t expect Corbyn to last very long. But if the Islington North MP triumphs among due-paying trade unionists and party activists, all but a few MPs will give him some breathing space, albeit, in the words of one frontbencher, in order to “give him enough rope to hang himself”.

Although many opposition frontbenchers will step down – including big beasts like Chuka Umunna and Yvette Cooper – there will be enough people unwilling to accept the reduction in the size of their offices and prestige, and enough volunteers from the left of the party to fill the gaps. Corbyn will be able to form a Shadow Cabinet and would remain in power, although radical change of the party’s structures and decision-making processes will likely be blocked by the parliamentary Labour party.

The party is also highly unlikely to split. As one senior figure on the Labour right commented to me: “We tried that before, and it didn’t work.” Don’t underestimate, either, how tribal most Labour right-wingers are; Liz Kendall’s comment that she could no more leave the Labour party than “her own family” speaks for most, if not all, politicians on the Labour right.

The small few who aren’t viscerally attached to the Labour party are also unlikely to leave because they hold Tim Farron, the newly-minted Liberal Democrat leader, in disdain. One staffer described the switch as a choice between “a party that’s not interested in beating the Tories with over 200 seats and one with under 10”.

So Labour is unlikely to formally split and there will be a Shadow Cabinet, after a fashion. But how long will Corbyn stay in post?

Whoever is Labour leader, the 2016 local elections will be a challenge. The London mayoral candidate faces a tough battle against Zac Goldsmith, while in Wales – the one remaining part of Britain with a Labour government – for the first time, Carwyn Jones will go into an election facing a politician just as popular as he is in the shape of Leanne Wood. A further bloodbath is expected in Scotland. Heavy defeats in all three contests would likely doom Corbyn, although victory in Wales and London would buy him another year.

Barring a political surprise to rival the surge that powered Corbyn to the leadership, the veteran leftwinger probably has a best before date of 2018. Anything other than victory in the European elections would mean the end of his leadership. There is, however, a small chance he could see out the full five years, even if midterm elections are a disaster.

A thumping win among the membership would probably be enough to shut down any hope of a coup against Corbyn. While Corbyn required 15 MPs to “lend” their support to get him on the ballot, as I wrote shortly after he first announced his bid, there is now a big enough pool of MPs from the party’s leftmost flank for Corbyn to get on the ballot unaided. A putsch against Corbyn raises the fear, as one MP gloomily observes, “of him just winning again”, leaving them looking both “unelectable and fucking stupid”.

Getting rid of a Labour party leader is, theoretically, quite difficult. It requires the signatures of 20 per cent of Labour MPs before party conference to trigger a leadership ballot, although, in practice, a leader would likely step down rather than carry on, bloodied, until the next Labour party conference.

The various attempts to unseat Ed Miliband failed for three reasons – the lack of a clear alternative candidate, the false comfort of the polls, and because the 20 per cent threshold was prohibitively high. The final attempt to remove Miliband in November of 2014 failed because Alan Johnson, the only plausible unity candidate, refused the crown, while Cooper and Burnham were unable to make a deal, each preferring their chances in a contested leadership election after the general election.

But one consequence of Labour’s rout is that there are fewer Labour MPs, meaning that the path to 20 per cent is now clearer. More than 46 Labour MPs hold seats that were Conservative-held after the 1983 wave and a few even remain in seats that Labour lost in 1992. Only a small handful of Labour MPs last time feared for their own seats under Miliband – many more will be nervous under Corbyn.

That said, they will be hampered by the lack of a plausible successor; both Burnham and Cooper will be discredited by their defeat, meaning that the identity of Labour’s next leader will be even murkier. Would Umunna let Dan Jarvis take the crown unencumbered? And if, as looks likely, Tom Watson becomes deputy leader, would his opponents in the PLP risk handing full control over the party to Watson? One insider described the transition as “swapping Trotsky for Stalin”.

The likelihood is, though, that if Corbynism proves as much of an electoral disaster as the pundits expect, that Corbyn would remain “in office but not in power”, until being replaced by a safe pair of hands, either from the 2010 intake, the deputy leader or a grandee from the old guard.

Unless, of course, his election sees Labour transformed in the polls and carry all before it, in which case all bets are off.