With hindsight, the earthquake that shook British politics three years ago this week feels more like a gentle tremor. Since then, commentators have been regularly overwhelmed by the temptation to recast the unlikely landslide election of Jeremy Corbyn as leader of the Labour Party as either the forewarning or even the direct cause of the more violent tectonic activity that has followed it. Without Corbyn there is no Brexit, with no Brexit there is no Trump, so the theory goes. But the counterfactual bears consideration. In the Corbyn-free branch of history, it falls to Andy Burnham to save the world. You may wish to draw your own conclusions.

For those who view Corbyn’s election as an unnatural disaster, the clean-up was meant to have been completed by now. The radioactive leaks contained, normality restored, the Labour Party area declared safe again. But that has not happened. Not only is his position as leader of the party both unassailable and entirely legitimate, but it would take no small amount of bravery to bet on anyone but Jeremy Corbyn emerging victorious whenever the UK gets around to having another general election.

The future of Brexit is anything but certain. Donald Trump may yet face impeachment. Jeremy Corbyn, meanwhile, is free and clear – and after some of the most bitter and vicious internal fighting in the party’s history, the consolidation of his control over it is reaching its end point.

He has earned the right. He has brought into the party hundreds of thousands of new supporters. In a general election that caught the party somewhat by surprise, he nevertheless managed to win 13 million votes, and gain seats when annihilation was predicted by all. He has also transformed the party into a vast social movement unlike any that has been seen in British politics before.

“We are in extraordinary times,” says Laura Parker, who is the national coordinator for Momentum, the now supremely powerful campaigning and activism arm of the Labour movement which grew from the “Corbyn4Leader” campaign group of 2015. “There can be no half measures. People don’t want Britain tweaking with. They want real change.”

It is Momentum that is compelling young people to get involved in politics in their tens if not hundreds of thousands.

“We are a bridge, really,” she says. “We sit between the Labour Party on the one side and a whole range of activists and groups, be they to do with health or housing or what have you. We are a bridge between the wider Labour movement and the party.”

What Momentum has correctly identified is that people who want to be involved in politics don’t necessarily want to be involved in the minutiae of local party organisation, the procedural stuff. And also, people don’t necessarily find that their local party is the place for them.

“Manchester Momentum puts on a disco every month,” Ms Parker says. “Bristol Momentum meetings have campaign stalls from every kind of campaign group right across the city. You do not get that from a local Labour Party meeting. We are a bridge to something beyond the party.

“It is about innovation. The Labour Party is a very large organisation, and changing it will take a very long time. You might want to get involved but be in a bit of a country where there have been no Labour MPs for a long time, or where it is all a bit stodgy, or you’re in a bit of the country where the local party has not embraced the policies of the new leadership.”

It is here where Momentum and the wider pro-Corbyn movement falls onto controversial ground. Two victorious leadership elections and an unlikely general election success later, the only area of the party still holding out against Corbyn is a large but shrinking portion of its MPs. Momentum, and others, stand accused of seeking to deselect them.

Ms Parker describes the infighting in the parliamentary Labour Party as “out of kilter”, and says that recent votes of no confidence in more centrist MPs in local parties are “a case of the parliamentary party catching up with the membership”.

Deselection is also not how they see it. “We are quite unique in Britain, in this idea that you get elected candidate for the Labour Party in a safe labour seat and its assumed its a job for life, for 40 of 50 years,” says Steve Howell, who worked as Labour’s deputy director of political strategy and communications during the 2017 election, and tells the story of it in his book, Game Changer.

“It’s part of the culture here. It’s not part of the culture anywhere else. In America, for example, candidates have to go through primaries for their seats all the time.”

But some of the tactics have been unsettling. The Derby MP Chris Williamson is currently touring the constituencies of outspoken anti-Corbyn MPs on what he has called his “democracy roadshow”. On Thursday night he posted pictures online of a local meeting in the Streatham constituency of Chuka Umunna, showing all hands raised in a “unanimous” vote in favour of “mandatory reselections”. You do not need to be the most diligent student of history to know what kinds of democracies feature 100 per cent of voters voting the same way.

One anti-Corbyn MP who stood down at the 2017 election could hardly be more scathing: “Before he became leader, I had never met him once. I never saw him. He would come in for foreign affairs questions in his luminous green jacket, ask about Palestine and that was it. He was just an eccentric.

“People are saying that there needs to be a new party in British politics. I saw earlier this week Alastair Campbell saying that there already is a new party in British politics, it’s just got an old name, the Labour Party. He’s right. It is a whole new party.

“The top team has changed, the headquarters has changed, the NEC has changed. It has all been comprehensively taken over.

“It was always a social democratic party that believed in a mixed economy. Now the fundamental values of the party has changed. It is a party committed to state socialism, to mass nationalisation. Internationally the leadership has a very different view on defence, counterterrorism, the special relationship, the nuclear deterrent.

“When he got elected leader, it was totally miserable. I had a feeling of impending doom. I thought, we are in a very bad place.

“I took a moral decision that i would speak out against him. I did lots of interviews, and I know it annoyed him.

“But, I thought, we’ve got to keep the thread of an argument alive about centre-left politics. About the need to win elections, about being in the right place on immigration, national security, defence. We can’t just let that argument die.”

Corbyn arrives in Commons to applause

At Labour Party conference in a couple of weeks, the party’s Jewish MPs are expected to hire bodyguards. A large number of MPs are suffering severe personal strain, from what one calls “the levels of orchestrated threats of physical violence and sustained social media abuse”.

One former MP says that Corbyn’s victory was the moment that life in his local party changed.

“Suddenly, a guy from [the trade union] Aslef turned up at my meetings to make long, angry, shouty speeches about parts of the world he’d never visited. He didn’t look like he’d ever even been on a train let alone driven one. These sorts of people took over.

“When you see the pictures of these people, at these meetings at the moment, they look unpleasant. There are profound mental health problems in those rooms, and they are changing the party. They are emboldened, spiteful. They are tolerant of antisemitism. These are not nice people. It is a cult.

“All this ‘kinder, gentler politics’ stuff. This ‘I’m comfortable with a little dissent’? He’s not. He’s profoundly intolerant, bad tempered, and not used to having to work very hard. When he does these petulant interviews it’s because he’s knackered. He used to wing by parliament once a month to raise the issue of Venezuela then fly out there as a guest.”

The ‘cult of Corbyn’ is a term you hear often, but is not entirely fair.

“Wherever he goes he can get a crowd, and this is very unusual in British politics,” says Matt Zarb-Cousin, a young Labour activist who worked in the Labour leader’s office for 18 months leading up to last year’s election.

“If Corbyn says he’s going to get a thousand people he’ll get more. There is something that appeals to people about him. They think he’s a genuine person. They think he’s trying to do the right thing. He cares about people. It is a charisma that we don’t normally associate with politicians. When people talk about politicians with charisma, usually they think of people who are sharp – like Blair or Macron or Cameron. People who look good in a suit.

“The charisma Corbyn has is different. People feel like they relate to him, like they know him.”

It is hard to argue with this point. The latest, and most serious, chapter in Labour’s antisemitism crisis has now been going on for four months, but it is doing the party no harm whatsoever in the opinion polls. It is a crisis the general public seem not to fully understand, with its complexities concerning clauses and definitions, and what wreaths were and weren’t laid where by whom.

When Danny Dyer appeared on national television to memorably call David Cameron a “t***”, not once but twice, another of his pronouncements in that short interview was subsequently all but forgotten. Though it may have helped that Jeremy Corbyn was sitting yards away from him, he was nevertheless described as a “decent” man.

The idea that Jeremy Corbyn is himself a racist, a legitimate question but one that has come to prominence via the front pages of the Daily Mail, is one that the public is not swallowing.

In two weeks the party will hold its annual conference, and there is much speculation about whether its anti-Corbyn MPs will seize that moment, or indeed any moment, to formally split from the party. In various MPs’ WhatsApp groups there is much disagreement about strategy and tactics.

“They have furious rows among themselves on these groups,” says one, a former member of Corbyn’s first shadow cabinet. “‘What is your plan?’ they’ll say. Or, ‘Well I’ve just won this vote on my local committee. The tide is turning’. Or someone will say, ‘What if Corbyn had left in 1997? We’ve got to stay and fight even if it takes 20 years’.

“I don’t know what the answer is on all of that. Personally, I think the moderates will just wither on the vine. But this idea that he is on the brink of winning an election? I don’t think so.

“The Tories will not go in to another election with Theresa May and a host of bad retail policies.

“And one thing they never managed to get traction on in 2017 was, ‘What will be the dire consequences of Jeremy Corbyn becoming prime minister, or if John McDonnell becomes chancellor?’ They could get no traction for because no one believed there was a remote chance it could happen. And Labour MPs were complicit in this. They knocked on doors and said, ‘Vote for me. I hate him even more than you do’.

“I joked about this with some of them at the time. I said to one of them, so your message on the doorstep is, ‘If you absolutely definitely don’t want to see a Labour government, then vote Labour’, and they said to me, ‘That’s exactly right’.

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That Corbyn and his supporters, having won two leadership election victories, want control of the party back from this section of the party is hardly surprising and hardly unreasonable.

Ms Parker, who was on the civil service fast stream and not a member of the Labour Party during the Blair years, says it is only through Corbyn that the party is reclaiming its traditional values, not abandoning them.

“I think the 20 years of neoliberal Labour – they were the aberration. That’s why it lost its way and lost hundreds of thousands of members.

“We are about a reaffirmation of Labour’s starting values. About community, about public ownership.

“Obviously it’s 2018, the challenges are different, but there is nothing that Jeremy Corbyn or John McDonnell stand for that is out of kilter with Labour values.”