LONDON — In his apartment a stone’s throw from Tower Bridge on the banks of the River Thames, Jon Lansman is sipping strong black coffee and contemplating life after Jeremy Corbyn.

As founder and leader of Momentum, the insurgent campaign group that backs the opposition Labour leader, Lansman is as responsible as anyone for Corbyn’s against-the-odds rise.

In three years, Corbyn has gone from obscure backbencher to potential prime minister.

Corbyn’s ascent, backed by Momentum, makes Lansman one of the most influential figures on the left of British politics today, backed by an army of activists, personal ties to the leader and a seat on Labour’s governing body.

It’s a far cry from 2015, when Momentum was established with the primary aim of defending Corbyn from internal rebellion.

“Momentum will outlive Jeremy, no question. It’s going to outlive me. That’s my intention” — Jon Lansman

After a leadership challenge and a snap election, Corbyn is now safe and looking forward to the next national poll in 2022 — or possibly sooner.

That presents a new challenge for Momentum, and for Lansman: How to reinvent the organization so it stays relevant during a potential Corbyn premiership and long afterward.

“Momentum will outlive Jeremy, no question,” says Lansman. “It’s going to outlive me. That’s my intention.”

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Sitting in his flat, the shelves rammed with socialist books and left-wing memorabilia — a bust of Lenin, a statuette of Marx and a campaign poster signed by “Jezza” — Lansman is convinced the party is on the cusp of power.

Bad news for Britain is good news for Labour, Lansman believes — and he sees plenty of bad news: squeezed living standards, unaffordable housing, £9,000-a-year tuition fees.

“This is enormously fertile ground for us,” he says. “I think the future looks very rosy.”

Momentum’s influence has already revolutionized Labour, giving the party a battle-tested campaign force ready whenever the leadership is in danger.

The organization was out in force during the recent local elections in England, with mixed success. Its supporters, like Guardian columnist Owen Jones, say almost everywhere where Momentum was on the ground, Labour’s vote went up.

The Tories, meanwhile, have gleefully pointed out that Labour's electoral surge often did not translate into extra seats — and that the Conservatives were able to hang on to many seats outside the big cities that had been targeted by Labour.

Momentum has learnt on the job, first in the 2016 leadership challenge, then in the 2017 snap election.

It brought in experts from Bernie Sanders’ ultimately unsuccessful campaign to be the Democrats' presidential candidate in 2016 to hone its skills online. Combined with a mass of enthusiastic supporters, it brought a new dynamic to British politics.

The organization’s sheer scale spooked the Tory hierarchy to such a degree that some Conservative MPs have begun to call for a “Tory Momentum.”

Under Corbyn, Labour is now the biggest political party in Europe. Momentum itself has more than 40,000 members and rising — Lansman says 500 new members are joining every week.

Most importantly, in Lansman’s view, the Tories are swimming in a rapidly shrinking pool of older, white voters while young voters flock to Labour.

Before 2015, the biggest age cohort in Labour were those in their 60s. Now it is those in their 20s.

As new voters hit the voting age of 18 and older voters die off, the potential swing to Labour every year is the equivalent of 1.5 percent to 2 percent, he says — provided traditionally ballot-shy younger people can be persuaded to actually vote. “The Tories don’t have people on the ground. They have old people. They have tiny numbers of activists. You can’t mobilize people without having the people to start with.”

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Lansman, 60, has spent a lifetime embedded in the obscure outer reaches of left-wing politics.

When Corbyn entered parliament in 1983, Lansman stuck to the factional battle for the soul of the Labour Party.

By 1988, when the party’s leader Neil Kinnock crushed Tony Benn’s leadership challenge, the left appeared irretrievably defeated. From then on, Labour moved further and further right, first under John Smith and then Tony Blair.

Lansman’s journey from outsider to the head of one of the most powerful political movements in Britain was as rapid as it was unexpected.

By 2014, under Ed Miliband, Lansman was reduced to sleeping in a camper van in a car park outside a party policy meeting in Milton Keynes, trying to get a motion passed against austerity. He failed.

Lansman’s obsession with internal politics has created plenty of enemies.

One senior trade union official says: “Jon has a good heart and is a smart fella but his obsession is internal wranglings of the party, rather than bigger picture politics. Because he is so influential with the leadership, this has had a big effect — to its detriment.”

His supporters on the left say he has been instrumental in saving the Labour Party from the fate of other social democratic parties in Europe. Union organizer Sam Tarry, a director at Momentum, says: “Jon helped give a shot of adrenalin into the arm of the Labour Party. He has the ear of the leadership and is respected across the trade union movement and the left.”

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Lansman’s journey from outsider to the head of one of the most powerful political movements in Britain was as rapid as it was unexpected.

In 2015, Corbyn was squeezed onto the ballot paper to stand for leader after Miliband’s general election defeat to David Cameron.

For party insiders, it was a way to “widen the debate.” No one thought he could win — especially Corbyn. He turned to Lansman, his campaign chief, and half-joked: “You better make f--king sure I don’t win.”

Corbyn’s success brings its own challenges.

“The left in other countries look to us because Corbyn could be prime minister,” Lansman says. “That would be the first real left figure in government in the West in a long time.”

Momentum’s challenge is now to make that happen — and to make sure Corbynism is actually delivered in government.

“Yes, Momentum was primarily about keeping Corbyn in power,” one senior Labour Party aide says on condition of anonymity. “But it was — and is — more than that. It is also about creating a movement to bolster a Labour government."

“If you look at every single historical example, a party seeking to transfer wealth and power is faced with enormous institutional forces within the state from those who are losing out,” the aide adds. “Having a countervailing force in the country will help the government keep its radicalism.”

Lansman himself only hinted at this future role during the interview. “The extent to which we engage in public campaigning outside of elections is a significant part of our objectives,” he says.

Momentum is needed, so Corbyn’s most powerful supporters believe, not only to protect the leader — but to protect Corbyn’s socialist revolution when it comes.

First, though, Corbyn needs to win power.

“There will not be any voluntary departure before the election. I think he wants to be prime minister for a bit as well" — Lansman on Jeremy Corbyn

Lansman believes Momentum’s power in British politics will only grow over the next few years.

For starters, he wants Labour to authorize Momentum to spend far more of the party’s campaign funds at the next election, unlike 2017.

Under the electoral spending limits, third party groups like Momentum can spend just a fraction of the £20 million available to political parties.

The electoral rules would allow Labour to authorize Momentum to spend some of its campaign funds.

The potential is obvious. Momentum’s most watched 2017 campaign video — “Dad, do you hate me?” — was produced for just £20 but watched by 14 million people. “If they gave us £20,000, that would enable us to enormously increase the amount of money we were spending on Facebook advertising ... and have much more impact,” Lansman says.

He is convinced Corbyn will stand in 2022. “There will not be any voluntary departure before the election,” he says, adding: “I think he wants to be prime minister for a bit as well.”

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Under Corbyn’s leadership, Labour and Momentum are (almost) fully aligned.

The movement continued to back Corbyn even as he and senior members of his party faced charges of turning a blind eye to anti-Semitism, but it also released a statement warning its supporters that the accusations “should not and cannot be dismissed simply as right-wing smears.” Lansman, who is Jewish, repeated the message in the media.

The real oil in the water though is Brexit.

Momentum’s supporters are overwhelmingly opposed to the U.K.’s departure from the EU. Corbyn and some of his closest advisers are old-school Euroskeptics who view the EU as power unto its own — a check on socialism.

On Brexit, Corbyn has successfully fudged the issue to keep his coalition together, vowing to deliver on the result of the referendum while opposing the “Tory Brexit” on offer.

For now, Momentum has no official position on Brexit — or anything else. As a “pluralist” organization put in place to block a leadership challenge, it has no mechanism for taking policy positions.

One of its challenges, says Lansman, is working out exactly what it stands for — beyond support for Corbyn.

“One possibility, which there has been a bit of discussion about, is whether we should consider supporting policies if we have a large majority of support for them,” Lansman says.

In other words, tip-toeing toward the development of a policy platform — like a political party.

Lansman wants to go even further, to “develop our own, new ideology.”

But what of Corbyn himself? By the next general election in 2022, he will be 72, a month shy of his 73rd birthday.

“Over time, Jeremy will eventually go and if we survive we have to work out a new way of living without the same kind of commitment to the current leader,” Lansman says.

His focus is now on Labour’s internal “democracy review,” which will change the way the party makes policy, giving more power to activists over MPs.

“We want to make sure it’s democratic before he [Corbyn] goes,” Lansman says. “He’s of an age where he is incredibly fit and well today but who knows if he will be in six months' time. You cannot predict your health.”