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 rightwards. 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 60, Lansman is now more influential than at any point in his long career. He founded and chairs the activist group Momentum, which helped secure Corbyn's 2016 re-election and drove Labour's advance at the 2017 general election. In the party's National Executive Committee (NEC) elections, Lansman was one of three left-wing candidates elected by a landslide on 15 January. "Really honoured to now represent almost 600,000 members on the national executive of @UKLabour," he tweeted as the results were announced. "At last the 21st century version of the Socialist party I joined 44 years ago."

Lansman, who is immediately recognisable from his psychedelic dress and omnipresent backpack (and, now, a long, Rasputin-esque beard), is being rewarded for his persistence. A Corbyn ally described Lansman 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 told me when I interviewed him in 2016. "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."

The NEC result gives the left a clear majority on Labour's ruling body for the first time since Corbyn's election. Lansman, who has long championed activist rights, has declared his support for rule changes such as a reduction in the nomination threshold for leadership candidates. At present, a Corbynite successor would require the backing of 15 per cent of Labour MPs/MEPs to make the ballot. This is now set to be reduced to 10 per cent or less. As Tony Benn, Lansman's mentor, always hoped, power is being redistributed from MPs to grassroots members.

Jonathan Lansman was born in 1957 and was raised in an Orthodox Jewish family in Southgate, north London. He attended the independent Highgate School and worked on an Israeli kibbutz when he was 16. "It was actually a very politicising experience," Lansman later recalled. When I did my bar mitzvah I saw myself as a Zionist and I think after I went there I felt it less. I was more interested in the kibbutz and what I liked about it was the pioneering spirit, the sense of community and radicalism of it."

Lansman 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."

He studied economics at Clare College, Cambridge, and ran for student union president on the same slate as BBC presenter Andrew Marr, then known as "Red Andy", who produced campaign cartoons. While at university, Lansman befriended the young Corbyn, then Labour's election agent in Hornsey. After graduating in 1979, Lansman established himself as one of the left's leading activists while still in his 20s. He helped lead the Campaign for Labour Party Democracy (which fought for members' rights) and was secretary of the Rank and File Mobilising Committee, the primary vehicle for Benn's 1981 deputy leadership bid.

During the campaign, Lansman was falsely accused by Benn's opponent Denis Healey of organising the heckling of speeches he gave in Cardiff and Birmingham. When Lansman retorted that he had been in Spain during the Cardiff meeting and travelling to Aberystwyth during the Birmingham gathering, Healey, the former chancellor, was forced to apologise. The producers of the Weekend World, the TV programme on which Healey made his accusations, accepted Lansman had been slandered and made an out of court settlement.

For decades, Benn's narrow 1981 defeat was the left's high-water mark. During Neil Kinnock's leadership, the left lost control of the NEC and policies such as unilateral nuclear disarmament and European Community withdrawal were abandoned. With the election of Tony Blair as leader in 1994, and the revision of Clause IV (which committed the party to "the common ownership of the means of production, distribution and exchange"), Lansman and his allies were further marginalised.

After the death of his wife, Beth, from breast cancer at the age of 39, Lansman withdrew from politics for several years . But he re-engaged in 2007, when McDonnell and Michael Meacher ran for the leadership from the left. Though neither candidate made the ballot, Lansman later became a parliamentary aide to Meacher and established the website Left Futures. Ed Miliband's 2010 election was welcomed as an unambiguous break with New Labour. "I did see Ed as someone who would open up the debate rather further and that did prove to be the case," Lansman told me last year.

Unlike Blair and Brown, Miliband did not define himself against the left and aligned with it on the Iraq War, privatisation and taxation. He spoke warmly of Benn and other left-wingers, some of them friends of his late father, the Marxist academic Ralph Miliband. At times, the Blairites appeared determined to drive the left out of the party, transforming Labour into a version of the US Democrats. Under Miliband, the left was part of the family once more.

From the moment Miliband's 2015 resignation triggered a leadership election, Lansman saw the potential for a left candidate to thrive. "The other candidates utterly failed to recognise what had happened," he said. "The assumption that they were fighting for Blairite votes! [Chuckles] Tony Blair never had troops on the ground - ever - not committed to his project. They were always a small majority in my view - always. Labour Party members never believed in privatising the NHS, they never believed in academy schools, they never believed in the Iraq war. They were never with Blair on policy, that’s why democracy was prevented. As soon as the members had the power to elect the leader, it was going to be a battle for the left and centre votes, not for the right-wing vote."

Labour MPs have long feared the left will use its new hegemony to deselect Corbyn critics. During the 1980s, Lansman championed mandatory reselection - under which incumbent MPs automatically faced an open contest - which was abolished by Neil Kinnock in 1990. But though he remains a personal supporter of the policy, Lansman pledged on 12 January "not to campaign to deselect anybody". But as he has noted, MPs can already be deselected under existing rules (albeit with greater difficulty).

"I see that as a matter for local members," Lansman told me last year. "I think if there are any [deselections] they will be very small in number. To be honest, I think that any decent, hardworking Labour MP who campaigns alongside his or her members and treats them as an MP should has anything to fear from any selection process at all. People are very unwilling to get rid of someone they know who they work alongside. But it is their right, there have been procedures for mandatory reselection of council candidates for years and it’s not been a big deal. Yes, people sometimes get deselected, sometimes they get selected somewhere else, new people get selected, it’s not a problem."

"Bob Wareing [a left-winger] was deselected by Stephen Twigg [the Blairite MP], under a trigger ballot procedure there may be one or two, maybe a handful."

But though Corbyn still has few ideological supporters in the PLP, the left's power has never been greater. The leadership, MPs agree, will be Corbyn's for as long as he wants and he is well-placed to anoint a personal successor. I asked Lansman how Tony Benn would have responded to Corby's triumph.

"He would have been absolutely delighted and he would have been surprised," Lansman said. "That wasn’t what he was expecting when he died, which was not that long ago [2014]. 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."

"After decades in the wilderness," Lansman likes to say, "the promised land is not far off." 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."