It appeared the ultimate summer ‘silly season’ story: that Labour would choose an unrepentant, self-consciously unspun bearded leftie as its leader. But, as ballot papers for the leadership election are dispatched, the story is threatening to close with a nightmare final chapter for the party. This week the pollsters YouGov had Corbyn 20 points ahead of Andy Burnham, his closest rival, and in a position to win the contest in its first round. Labour thus faces the prospect of a defeat in 2020 that could make Margaret Thatcher’s 1983 landslide look small-scale.

But while Corbyn’s rise may not have been predicted, it was eminently predictable. Labour has consistent form when it comes to such self-harming behaviour: after it lost power in 1951, 1970 and 1979, the party engaged in vicious internal warfare and then moved sharply to the left.

In each of these fights, the left has trotted out its hackneyed narrative about the ‘great betrayal’ supposedly committed during Labour’s time in office. In the 1970s and 1980s, Tony Benn led the denunciations of the governments of which he had been a member. After 2010, the condemnation of New Labour’s record was given added legitimacy thanks to its source: the former Treasury special adviser, Secretary of State for Energy and Climate Change and by now leader of the Labour party, Ed Miliband.

Miliband not only provided the intellectual groundwork for the Corbyn insurrection, he also, albeit unwittingly, provided the organisational opening. Desperate to placate the increasingly truculent unions that had helped elect their boss, Miliband’s team, says one observer, ‘gave a free rein and turned a blind eye’ as the unions tried to squeeze their favoured candidates into parliamentary seats.

This had two results. First, the unions managed to ensure left-wing loyalists were picked in a swath of constituencies so safe that not even Miliband could lose them. MPs elected for the first time in May figure disproportionately among those who nominated Corbyn.

Second, in the wake of revelations in 2013 about an alleged union stitch-up in the Scottish seat of Falkirk, Miliband sought to change the party’s rules for electing its leader. His aim was to give the appearance of reducing the unions’ influence but to do so in a way that their bosses would go along with.

This is not a system designed to encourage the kind of mass participation seen in US primary elections. Instead, the compromise Miliband forged — abolishing the old electoral college in which the unions held a third of the votes, but allowing party ‘supporters’ to register for £3 and union members to do so for free — flung Labour’s doors wide open to Corbyn’s growing army of hard-left activists and starry-eyed youthful idealists. The parliamentary party, which under the electoral college system controlled a third of the votes and used them to keep such barbarians outside Labour’s gates, is now reduced to a bystander.

The left’s strength has been augmented in the leadership contest by the support Corbyn is generating from far-left campaign groups such as the People’s Assembly Against Austerity and the Stop the War co-alition, as well as the efforts of Len McCluskey’s Unite union to encourage its members to support him. Many terrified Labour MPs fear the impact could be as politically catastrophic as the Militant entryism of the 1980s. Of the 190,000 new members and supporters who have signed up to the party since May, it’s estimated that two thirds have done so to back Corbyn.

Facing this advancing army are the beleaguered forces of the Labour right. Its condition suggests that, in many ways, the party’s position is more parlous than it was under Michael Foot. Then the right was bolstered by ‘big beasts’ such as Denis Healey, Roy Hattersley, Peter Shore and John Smith, who had served in the Callaghan government and opted not to flee to the SDP but to fight the Bennite enemy within. But with the exception of Alan Johnson, their contemporary equivalents — David Blunkett, Jack Straw, Alan Milburn and John Reid — have departed the battlefield. Depleted in parliament, the right has been decimated in the unions. In the 1980s it was union leaders like Frank Chapple, Eric Hammond and Bill Jordan who helped wrest the party from the clutches of the hard left; nothing of the sort could happen today.

As Jeremy Corbyn will no doubt know, Marx said that history repeats itself, first as tragedy, then as farce. For Labour, that prediction may come horribly true next month.