On the night before he was defeated by Neil Kinnock in the 1988 Labour leadership election, Tony Benn told a gathering of supporters: “I do not want anyone to think that tomorrow is the end. It is the beginning. It is twice as good as we thought it might be.” The following day, the left-wing challenger won a mere 11.4 per cent of the vote. “It was appalling,” he recorded in his diary.

When most Labour MPs refer to the “wilderness years”, they have in mind the party’s spell in opposition from 1979 to 1997. But the left is describing an internal exile that lasted until 2015. Before Jeremy Corbyn’s victory, he and his allies had never come close to winning a leadership election. They are now on course to do so for a second time in September.

The predictability of this should not negate its significance. When Corbyn was elected by the members last year, MPs hoped that it would prove no more than a summer fling. Yet the romance has endured. All of the evidence suggests that Corbyn retains a comfortable lead among both members and registered supporters. At the time of writing, he had won 134 constituency nominations to Owen Smith’s 25. Even before the high court ruled that 126,592 members who joined since January were unlawfully excluded from the contest (a decision challenged by the Labour Party as the New Statesman went to press), victory for Corbyn was one of the safest bets in politics.

Smith’s supporters hope that he will narrow his opponent’s lead but none anticipates winning. Corbyn, the most left-wing leader in Labour’s history, will almost certainly be confirmed in power.

Vast political energy has been spent in trying to dislodge Corbyn, but to little effect: 81 per cent of Labour MPs voted to declare that they had no confidence in him; 65 frontbenchers resigned. Neither act diminished support for the leader among the only group that counts: members and supporters. Indeed, the failed “coup” has harmed Smith more than his opponent. Most members agree when Smith laments that Labour has “never looked more disunited”. But they blame the rebels, not Corbyn.

Some opponents had long believed that there was little purpose in challenging him before 2017 – if then. Those who had invested heavily in Corbyn needed to be given time to “see him fail”, they said. It was the shock of Brexit and the fear of an early general election that accelerated the process.

When it became clear that Corbyn would not depart voluntarily, rebel MPs spoke of turning the leadership rules to their advantage. They would seek to recruit hundreds of thousands of new supporters through the £3 scheme, harnessing “the 48 per cent” aggrieved by Brexit. An unpublished poll by Greenberg Quinlan Rosner suggested that 10 per cent of the public would participate. Yet the decision by Corbyn’s opponents on the National Executive Committee to impose a £25 fee foreclosed this possibility. The hope was that shrinking the selectorate would benefit the rebels. But most of the 140,000 £25 supporters are believed to favour Corbyn. As in 2015, the left’s recruiting capacity was underestimated. Corbyn’s allies believe that there are still more who could be mobilised to defend his leadership. “We’re not at the bottom of the well,” one told me.

A second leadership victory would be an unprecedented triumph for the left but its mastery of Labour is not yet complete. Had Corbyn departed, an alternative candidate, such as the shadow chancellor, John McDonnell, or the shadow defence secretary, Clive Lewis, would have struggled to achieve the requisite 38 MP/MEP nominations (15 per cent of the total) needed to get on the ballot. The left’s devotion to Corbyn is both personal and tactical.

His allies identify two ways to consolidate their position. “You change the rules or you change the MPs,” a shadow cabinet minister told me. The deselection of some of Corbyn’s opponents is regarded as “inevitable”. At a recent rally in Brighton, the leader stated that he would not “interfere” if members sought to oust the Hove and Portslade MP, Peter Kyle. By simultaneously replacing retirees with left-wing loyalists, a more Corbynite parliamentary party will be built.

In parallel with this, the Bennite Campaign for Labour Party Democracy is seeking to reduce the nomination threshold to 5 per cent. The earliest possible date for this rule change is 2017 – another reason for Corbyn’s tenacity. By the time he departs, the left’s hope is that its supremacy will be assured. Labour will have been transformed from a party in which MPs hold the whip hand to one in which the activists do.

It is this that leads some to regard a split as inevitable and desirable. Yet it is not a course that MPs intend to pursue if Corbyn wins again. To adapt Adam Smith, there is a great deal of ruin in a party. MPs’ tribal loyalty to Labour militates against a breakaway. Many expect Theresa May to trigger an early general election next year, with Labour’s defeat resulting in Corbyn’s resignation or removal. But he has refused to guarantee that he would go. As the left is fond of noting, Kinnock remained in post despite losing in 1987.

Before his defeat the following year, Benn threatened annual leadership challenges. Many of Corbyn’s opponents now echo this warning. For the left, this role reversal is the reward for its past refusal to split. “You know what, comrade, we’re just in it, aren’t we?” Benn once told Corbyn when discussing why they had remained in the party.

Back then, the left, no less than the right, assumed that its marginalisation was permanent. When Corbyn won in 2015, it was as an insurgent. He is now on course to triumph as an incumbent. Never has the left been closer to transforming Labour for good. Never has it been harder for their opponents to stop them.