At 10am on the final Saturday before the election, on a hot early summer day in New Addington, on the outskirts of Croydon, between 50 and 100 people of all ages and backgrounds were milling around on the grass, slapping on sunblock and Labour stickers and clutching red folders. They slowly assembled into groups of five to 10 and struck off into the streets. More arrived, made introductions, passed around more red folders and slowly assembled into new groups. Some were local members and supporters; others had received text messages sent out by the Corbyn-supporting organisation Momentum, asking if they would volunteer to go door-to-door to canvass voters, and pointing them to the address of a local hub full of clipboards, leaflets and stickers.

There have been some head-swimming statistics doing the rounds since Thursday’s general election. Despite beginning the campaign 24 points behind in the polls, Jeremy Corbyn increased Labour’s vote share more than any leader since 1945, adding more than 3.5 million votes to the 2015 total. Labour got more votes in England than they did in Tony Blair’s landslide in 1997. In a tumultuous night, Labour lost five seats to the Conservatives, but won 28 from them, as well as six from the SNP and two from the Lib Dems. Unexpected gains were made in places such as Canterbury and Kensington, and in many cases slender leads were not only defended, but turned into galumphing majorities – Tooting in south London, thought to be seriously under threat from the Tories, now has a 15,000 majority; other Labour-held seats now have majorities in excess of 30,000.

But it was the seats that changed hands that really mattered. With the Labour machine fearing the worst – its internal polling was predicting a heavy defeat – it directed its resources towards a largely defensive campaign. That left it to organisations such as Momentum to go after Tory and Lib Dem seats with slender majorities – directing thousands of mostly young volunteers via text message to marginals such as Battersea, Derby North, Croydon Central, Sheffield Hallam, Crewe and Nantwich and Brighton Kemptown.

One new tool that helped in this grassroots transformation of Labour’s ground game was mynearestmarginal.com – users would be given a list of their closest marginal seats, and detailed information of forthcoming canvassing and street stalls. (There was also a connected ride-share app, Momentum Carpool.) The website was briefly controversial, after some in the party claimed it was directing people away from key seats held by Labour MPs who had been “core group hostile” and critical of Corbyn. “Momentum appears to have given up on a series of Labour marginals across the north of England” ran a headline on ConservativeHome – but it was, Momentum said, simply a case of having not input all the data for all the constituencies before the launch, and, within a day or two, it was indeed sending activists to campaign for Corbyn critics such as Wes Streeting or Neil Coyle.

In the 20 months since it was founded, Momentum has been controversial, and troubled by both internal divisions, and criticism from other parts of the Labour party; not least because many on the Labour right were suspicious of the stated need for a “social movement” alongside the existing party. “Why not just join the Labour party?” was a common refrain. Walthamstow MP Stella Creasy said in March 2016 that the group was more interested in “meetings and moralising” than real campaigning, and a few months later, Ben Bradshaw called on Corbyn to “call off these Momentum thugs”, as the party’s internal battles reached fever pitch.

The opportunity to show off their organisational clout, and just to meet longstanding Labour party members face-to-face while out on the trail seems to have built some bridges. Sarah Jones, the new MP for Croydon Central, won her seat from the former Conservative housing minister (and May’s new chief of staff) Gavin Barwell with a 9.7% swing – and added more than 7,000 votes.

Jones is aligned with the Blairite pressure group Progress, and didn’t vote for Corbyn in either leadership election, but she is unequivocal in her enthusiasm for the new way of doing things. “I think everybody who didn’t vote for Jeremy has been delighted with what we’ve managed to achieve on Thursday,” she said. “I’m now sure we wouldn’t have done so well if someone else had been leading the party.” She describes thousands of new campaigners on the streets of Croydon in the past seven weeks – and at least 700 or 800 volunteers on election day to “get out the vote” (GOTV), including hundreds of first-time canvassers, coming from across London and the south-east. “There were so many young people; but not only young people, people diverse in their backgrounds, in their gender, everything – we’d buddy people up, a new canvasser with a more experienced party member – but then so many of the first-timers and Momentum people would come back, that was the amazing thing – and after two sessions they were the experienced ones, leading the new ones. It was a completely different kind of campaign.”

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For Jones, the activity of the new members, the strength of the manifesto and Corbyn’s popularity with previous non-voters, made the difference. “I went to Croydon College on Friday, to talk to them about what they wanted me to do – and to say, what was it that made you vote? What was going on? They said they really liked Corbyn because he had stuck to his guns for so long, for so many years, that he’d been out campaigning against apartheid years and years ago.” She asked where they had got their information. “Snapchat – they all said Snapchat!”

“There’s been this view from the right of the party that it’s all about winning elections, and of course that’s true – but I think what we’ve learned is if you pursue that in an incredibly targeted way, you stop reaching out to other organisations and having conversations at a wider level, and you sort of end up losing the purpose of what you’re doing.” This seems to recall former Peter Mandelson aide Derek Draper’s memorable line on New Labour’s use of focus groups: “A bunch of eight people drinking wine determined pretty much everything Labour did.”

By contrast, the enabling of a new kind of grassroots populism may have been a game-changer. “In Croydon,” continues Jones, “there were people putting out leaflets to young people, and putting things on social media, that were nothing to do with the Labour party. But they were supporting the Labour party because of Corbyn, and because of this huge new membership, and because their friends in Momentum [were] talking to them about politics. There was a reach beyond what the Labour party would normally be capable of. Lots of young people voted in this election, and we didn’t talk to all those people.”

One of those volunteers in New Addington, Ellen Buddle, 30, from east London, joined Labour this year, and was among the volunteers canvassing for the first time after Momentum put a call out on Twitter. “I was a bit nervous because approaching strangers doesn’t really come naturally to me. I noticed around halfway through that it started to feel like this lucky dip – you start to feel excited about each conversation, what that person will be like, what you’ll get to talk about – and you feel that way whether you’re told they’re voting Labour or Conservative. You just want to find out what they’re like. It was fantastic just to get a snapshot of what people care about.”

Max Munday, a 32-year-old volunteer from Momentum Sheffield, lives in the safe Labour seat of Sheffield Heeley, but describes a series of energetic campaigns in marginals across the north – some of them as DIY as they come. “In some cases we were mobilising where the Labour party machine weren’t putting in enough resources, frankly. You can’t really substitute for the party infrastructure in a local constituency, because the work has to be done over years, not a matter of weeks – but that’s what we ended up doing.” In Sheffield Hallam, Nick Clegg’s seat until Thursday night, the constituency Labour party (CLP) was “twinned” with nearby Penistone and Stocksbridge, where Angela Smith’s seat was under threat from the Tories – and its activists were told to head there. “There was clear guidance from the central Labour party that Penistone and Stocksbridge needed to be the focus of attention in Sheffield, but we looked at the fact that Clegg only had a majority of a few thousand, and we said: ‘You know what, maybe we can win here?’ So Momentum filled the gap. Everything, from the official campaign photos to making the memes for social media, to just arranging the printing of election materials. We didn’t have any glossy ‘Vote Jared O’Mara’ leaflets or posters, we just had a double-sided photocopied A4 bulletin – it didn’t look slick, to be honest. It was a really scratch campaign – one that without Momentum wouldn’t have existed – but it worked.

“On Monday night on four streets in Hallam, we had 40 campaigners, most of whom hadn’t ever done it before, in torrential rain; the papers were disintegrating in our hands, we were running around and speaking to people, and turning Liberal Democrat voters into Labour voters.”

Momentum have also pioneered some new approaches to canvassing, hosting 33 training sessions for volunteers across the country, in community centres and church halls, led by volunteers from the Bernie Sanders campaign. The approach sought to add to the traditional Labour method of “voter ID”-oriented canvassing: gradually honing lists of committed, likely or possible Labour voters ahead of polling day to enable the most efficient GOTV effort. This is vital, American volunteer Jeremy Parkin explained during the Sheffield training session last month, “but what hasn’t been done so much, I’ve been led to believe, is what we term ‘persuasion canvassing’ in the US: specifically targeting undecided voters”. They would encourage canvassers to make a personal connection, to humanise themselves, and use it to draw out key issues affecting the person on the doorstep, “and then you’re not just a button, you’re the person behind the button”.

For Munday, all parts of the Labour party have things to learn from the 2017 campaign, ahead of the next general election. Momentum members should be getting involved and communicating more with CLPs (indeed, from 1 July all Momentum members will have to be Labour members, too). Meanwhile, the party should be rethinking its strategy at a national and local level. “What we have to do now is bridge the experience of the older members and the enthusiasm of younger members. That exchange, where you get the experience passed on to the first-timers, combined with the enthusiasm and the energy to run around estates knocking on doors. We need to be ready to take hundreds of people into [new Tory gains] North East Derbyshire and Mansfield. I also think the established Labour party structures should learn from this, and how they allocate resources.”

Facebook Twitter Pinterest Labour supporters at Harlow Town Park, Essex. Photograph: Justin Tallis/AFP/Getty Images

Because of its open nature, the number of volunteers for Labour across the country, across the campaign, is impossible to pin down. But stories abound of marginal seats targeted by Momentum that were overwhelmed with volunteers – of groups of young canvassers arriving on the critical final two Saturdays before polling and being sent to other vital constituencies because they had run out of the clipboards that hold the electoral roll data and information on voting history. Essentially, volunteers were re-directed because every street already had a team of canvassers on it.

“I heard about people being overwhelmed in a few seats, after we sent out text messages directing volunteers,” says Beth Foster-Ogg, 20, Momentum’s membership organiser. “I got a call from Cambridge, from a local organiser saying: ‘You’ve sent me too many people! We’ve sent out all the boards and there’s still loads of people flooding in, we don’t know what to do.’ It happened in Leeds North West, too – they started the day, they had so many activists that they went: ‘Right, let’s scrap our whole strategy, we’re going to just print off the electoral register instead’ – and rather than focusing on likely Labour voters, which is what you would normally do, they knocked on all the doors on the electoral register – that’s unheard of.” The seat saw a 14% swing to Labour, overturning a Lib Dem majority of almost 3,000 and replacing it with a 4,000 Labour lead.

In Battersea in south London, Labour turned a Tory majority of 7,938 into a Labour majority of 2,416, on a 9.1% swing. The new MP, Marsha de Cordova, sounded overjoyed, after what was widely regarded as an especially energetic campaign. “I think for us in Battersea, when I first got selected over there, everyone had it down as an unwinnable seat,” she reflects. “But we had some amazing young talent, people who were willing to be out in serious bad weather at times, morning, noon and night. There was so much enthusiasm around our campaign, it galvanised people – we had a message of hope, and a really progressive manifesto that we could share on the doorstep and be proud of. Battersea is a seat with a lot of inequality, a lot of estates that have been abandoned – I haven’t got the final figures, but turnout on some of our estates, among the more impoverished communities, increased massively. What’s really important is we keep these young people who have been volunteering involved and engaged, and all the CLPs have to be really opening and welcome. But I also want us to look at how we can change the dynamic and do different things to keep them engaged, because local party meetings can be pretty dry.”

That means keeping people such as Ellen Buddle – and it sounds as if the party is on the right path. “I’ll definitely, definitely be canvassing again if there’s another election,” Buddle says. “But more than that, I’m excited to see what Momentum and Labour decide to do in the meantime. I want to get involved in community campaigns – and I can’t wait to jump in and help out in future.”

With speculation that there will be another general election before the end of the year, Momentum national organiser Adam Klug, 30, says that the organisation intend to move to a permanent general election footing, to put on more Sanders-inspired canvassing sessions, have a new membership drive (they have had 1,000 people join over the weekend) and target a slew of Conservative seats that now look like marginals. “I think we saw with Kensington, or Canterbury or Warwick, that there is no unwinnable seat for Labour now. So we’re going to target as many places as possible. Where we came from seven weeks ago to now, if you think we were polling at 24%, and ended up at over 40%, it’s incredible. But now we’ll just keep going, so we’ll be ready for that election, whenever it comes.”