On 27 September 1981, minutes after Tony Benn’s defeat in the Labour deputy leadership election, his 24-year-old campaign co-ordinator, Jon Lansman, told ITN: “To be less than 1 per cent below Denis Healey is a terrific result . . . The campaign for the policies and the campaign for party democracy will go on – and there’s nothing that’s going to be stopping it.”

The campaign went on but Lansman and the left entered what he calls “the wilderness”. In the 1988 leadership election, Benn suffered a landslide defeat to Neil Kinnock (89-11 per cent) and the party moved right. For decades, Lansman was a figure on Labour’s fringes, doughtily championing the policies of 1981. Few believed that the left would regain its past prominence. And then Jeremy Corbyn was elected Labour leader.

At 58, Lansman is more influential than he was at any point in his career. He is the founder and chair of Momentum, the group established by Corbyn’s allies. He is also one of the party’s most feared figures. Because of his lifelong advocacy of mandatory reselection, critical MPs believe Momentum will be used as a vehicle for their removal.

The Cambridge economics graduate rarely gives interviews and this has added to his mystique. But on 16 May, he agreed to meet me in Westminster’s Portcullis House. Lansman, a former aide to the late MP Michael Meacher, was dressed in a beige jacket, pink shirt, jeans and desert boots. On the day we met, a senior left-winger described him to me as “the person who got Jeremy on the ballot paper”. While Corbyn and John McDonnell were said to be all but resigned to failure, Lansman “wouldn’t give up”.

He was one of those outside the Parliamentary Labour Party (PLP) office on 15 June 2015 when the future leader achieved the requisite 35 MP nominations. “I was very persistent. It was a last-minute operation,” Lansman recalled. “I brought us all together on the final day and it made the difference.” By unfortunate coincidence, he was also in demand as a registered kidney donor. “I had to go and have a load of blood taken out of my arm mid-morning.”

On the day of Corbyn’s election, Lansman’s joy was tinged with melancholy. “I’m very happy. But sad too that it took 30 years,” he tweeted. I asked him how surprised he was by Corbyn’s decisive victory. “I absolutely did expect that if Jeremy got on the ballot paper, we would have a surge . . . I thought there had been enough of an opening-up of debate under Ed [Miliband] that if Jeremy was on the ballot paper we would do quite well. I didn’t expect to win. I thought that ‘quite well’ would be 35 to 40 per cent of first preference votes.

“On policy issues, I don’t think the membership ever stopped being on the left,” he told me. “They were never in favour of the Iraq war, they were never in favour of privatising the NHS, they were never in favour of academies and foundation hospitals. They voted for Tony Blair after many years of Tory government because they wanted to win. They saw him as a winner.”

I asked Lansman how he believed Corbyn was faring. “We knew we’d have a challenging press and we’d have a challenging time with some members of the PLP. It’s gone reasonably well.” On Corbyn’s foes, he said: “I don’t think it’s a matter of letting Jeremy down, I think it’s a matter of letting the membership down . . . [It doesn’t] like to see members of parliament trashing the party’s electoral prospects.”

Lansman has publicly stated that Momentum will not campaign for the deselection of MPs but he remains personally supportive of members having the right to do so. “I am in favour of accountable elected representatives. I think that’s what party members expect. I think it’s what the public expects. Zac Goldsmith is a bit of a bête noire at the moment but when he moved his recall bill he got a lot of support from the left of the Labour Party and he got support from the public . . . I don’t think that the general public thinks that MPs in safe seats should have a job for life. I actually think there should be some mechanisms for accountability and, yes, I still believe in those. But I do understand that when Jeremy is faced with a PLP, the vast majority of whom did not vote for him, that is a problem and it’s important that he’s able to reassure them.”

Though Labour has not reintroduced mandatory reselection, which was abolished under Kinnock in 1990, the government’s forthcoming boundary changes will force many MPs to stand again. “I would expect party members to make choices that reflect their views as closely as possible,” Lansman said. “There’s been a change in the complexion of the party membership as a result of the surge and that will have an effect.”

When a Labour seat falls vacant, the media almost always tout Lansman as a possible candidate but he told me that he had “no interest in standing for parliament”. Never? “Never is perhaps putting it too strongly. But I have no intention of standing.”

Lansman said that Miliband made an “enormous mistake” by “doing nothing to build up his own support network” (a view the former leader is said to share). It was not a mistake that Corbyn’s allies repeated. Less than a month after his election, Momentum was founded on 8 October 2015. Lansman was keen to dispel the view of the group as a tool for far-left entryists. “That isn’t what Momentum meetings are like. The vast majority of people are entirely new to politics. In some areas, yes, you have some returners but most of the returners aren’t Trots. This is not an entryist operation in any way.”

I asked him to respond to the description of Momentum by the deputy leader, Tom Watson, as “a rabble”. “I think that Tom was misled. He spoke immediately after Stella Creasy reported two things that were actually untrue. First of all, they hadn’t marched on her house; secondly, although they’d gone to the office, there were no staff inside. And they lit candles outside, it was no big deal . . . I don’t hold it against Tom.” (After publication, Creasy said that she did not claim protesters marched on her house, prompting Lansman to reply: "If I fell for media misinformation, I apologise".)

Midway through our conversation, Seumas Milne, Corbyn’s strategy and communications director, passed by and cast a sceptical eye at the proceedings. After finishing his phone call, he returned and sat in for ten minutes. Lansman and I discussed Labour’s recent anti-Semitism feud. An atheist Jew, he was sharply critical of Ken Livingstone and has called for the left to abandon its pejorative use of “Zionism”. “Most Jews in Britain don’t see it as an ideology, they see it as indicating support for the existence of Israel as a Jewish state. Most British Jews . . . genuinely support two states, unlike the current government of Israel.” He added: “It’s wrong to talk about Zionism as a single ideology or a homogeneous group of people.”

Lansman was raised in an Orthodox Jewish family in Southgate, north London, and worked on an Israeli kibbutz when he was 16. He wrote in 2014 that it was the “pragmatic anti-Toryism” of a schoolboy “forced to work on A-level essays by candlelight during Heath’s three-day week” that drew him into Labour activism. He added: “Disillusionment with Wilson’s leadership, the IMF crisis and the realisation that Labour members had so little influence on our own government soon pushed me to the left.”

Even before Corbyn became leader, some in the party were plotting to remove him. I asked Lansman whether he believed there would be a challenge. “At the moment people clearly feel that a challenge is too risky and I don’t expect the arithmetic to change that much. Jeremy got a massive mandate and he will get a massive vote again . . . We’ve now got 120 local groups around the country. We have the basis for a very well-run operation. Part of my role has been to ensure that Momentum is equipped to campaign to defend the legitimacy of Jeremy’s leadership.” The day after we met, the Times published a YouGov poll showing that support for Corbyn among full Labour members had risen to 64 per cent (up from 49.5 per cent at the time of his election).

Lansman told me that he expected Corbyn to fight the next general election and that suggestions that he plans to hand over the leadership to McDonnell, the shadow chancellor, were “without foundation”. Of the party’s electoral prospects, he said: “I don’t think there’s any leader who would find it easier to win than Jeremy next time.”

Labour recently became the first opposition to lose council seats since 1985 but Lansman defended the results as “very good”: “They compared very well, in fact were in some ways better, than the high point of Miliband.” He added, “That isn’t bad, considering the media hostility we’ve had and, I’m afraid, the appearance of division which a small number of MPs have provided.”

Does he believe the UK’s conservative institutions would tolerate rule by a left-wing Labour Party? “I’ve still got my Harry Perkins badge,” he quipped, referring to the fictional prime minister of A Very British Coup. “Is that a realistic scenario? You certainly can’t dismiss it. I remember, immediately after the 1981 deputy leadership election, having dinner with Madame Allende and Tony [Benn]. The threat from those quarters felt very real . . . If Tony had won, we would have faced some pretty tough opposition from sections of the British establishment.”

Lansman was close to Benn until his death in March 2014. How would the left’s most revered leader have responded to Corbyn’s success? “He would have been absolutely delighted and he would have been surprised. That wasn’t what he was expecting when he died, which was not that long ago. I think he saw some improvements with Miliband but he was also disappointed. He was loyal to his family, he was very pleased with Hilary [Benn’s] role, so that mattered to him. I think there were complex personal things in the mix.”

At the close of our conversation, I asked Lansman if he was confident that the left would avoid again being consigned to the wilderness. He replied with a degree of self-criticism that may surprise his detractors.

“I think we made mistakes in the Eighties. With hindsight, I don’t think we helped ourselves. We allowed ourselves to be marginalised; we were too unforgiving of people who some saw as betraying our principles and didn’t give them a way back. My view has always been that if you don’t take the mainstream of the Labour Party with you, you’re going to lose. There is no point in taking hard-left positions which are not going to appeal to the centre ground.”