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One of Jeremy Corbyn’s most powerful weapons in the 2019 general election campaign is located in a cramped, frugal office in a scruffy part of north London. Here at the headquarters of the left-wing activist group Momentum, a team of committed socialist filmmakers has produced a succession of viral videos that have transmitted Labour’s campaign messages to a staggeringly big audience — around 16 million individual voters.

In the five weeks since the election was announced, Momentum’s video operation — consisting of 15 full-time producers and editors, supported by around 700 volunteers around the country who scour the internet and broadcast media looking for material that can be turned into memes — has achieved more than 50 million views on Facebook, more than most of the professional media organisations covering the election.

On this afternoon, with just days to go before the December 12 vote, Momentum’s headquarters is humming with activity. Around 50 staff are crammed elbow-to-elbow in the open-plan space. Scrawled on whiteboards and signs around the office is a reminder of the group’s political objective, in case anyone was in doubt: “SOCIALISM.”

Emil Charlaff, the head of the video team, stands over a colleague’s monitor reviewing one of their upcoming productions, a scripted comedy sketch attacking the Conservatives’ austerity measures. A wiry 33-year-old with a shaved head and trimmed beard, Charlaff wears a grey T-shirt, black jeans, and trainers. Watching the footage, he’s concerned that it’s too slow, so he tells the editor to cut it.

“It’ll take another couple of days to get this perfect,” the editor says.

“We’re cutting it fine,” Charlaff says.

“We’ve got Tory Robin Hood to get out first.”

On a whiteboard above Charlaff, someone has scribbled Momentum’s key social media statistics during the campaign so far: video views on Facebook and Twitter and the time users spent watching their clips. In the first week of the campaign, they got 4.2 million video views on Facebook. By the fifth week, that had risen to 16.7 million. “It was a good week,” Charlaff says dryly. He says he’s got more viral content still in the pipeline for the final days.

In this election, commentators have praised Boris Johnson’s Conservatives — not unfairly — for improving their social media presence with a much more imaginative and engaging approach than they’ve had in the past. But much less attention has been paid to the way that pro-Corbyn groups on the left, with Momentum at the forefront, have been breaking new ground in the battle for attention online.

“They’ve got a smart operation that has delivered brutally effective content on the NHS and inequality that has reached far more people than the TV debates have,” said Mike Harris, chief executive of 89up, a digital agency that has been closely tracking social media activity during the campaign.

One example of Momentum’s reach came on a morning in late November. It was the day after the Tories launched their campaign manifesto and one of their main initiatives was backfiring: The party had promised there would be “50,000 more” nurses working in the NHS in a decade, but under scrutiny, it turned out that only 31,000 of those would be new recruits. Nicky Morgan, the Tory culture secretary, was sent out on ITV’s breakfast show, Good Morning Britain, to do damage control.

The interview, aired at just after 7am, was an excruciating mess, the presenters growing increasingly exasperated as Morgan stuck stubbornly to the party line. But for Momentum, it was a golden opportunity. Watching the show that morning was one of the volunteer army that the group has been using to track the political shows looking for just this sort of thing: footage that can be cut into catchy, emotive videos that promote Corbyn and embarrass their rivals.

The volunteer dropped the clip into a Slack channel the spotters used to surface their potentially viral discoveries, and back at Momentum headquarters one of Charlaff’s editors picked it up. They cut the GMB interview into a six-minute clip emphasising the anchors’ irritation and Morgan’s obstinacy, adding subtitles and a provocative headline: “Biggest Car Crash Interview Ever!”

Momentum posted the video on its Facebook page, and soon it was going viral.

At 8 o’clock that night, it had been seen more than a million times.

By midnight, it had hit 2 million. And it kept climbing.

Eventually, Momentum’s spin on the Morgan interview was watched 8.3 million times on Facebook, making it one of the three most-viewed videos of the 2019 election, according to BuzzFeed News’ analysis.

“That kind of stuff does go far,” Charlaff said. “People like seeing politicians caught out or tripped up or confronted. That one went particularly far because [Morgan] stuck with it, and it was just so blindingly obvious to anyone watching that it was indefensible.”

The Morgan clip is one of thousands of posts published by pages loyal to Corbyn that have gone viral on Facebook since the election was announced at the end of October. According to an analysis by BuzzFeed News of sharing data from CrowdTangle, an analytics platform owned by Facebook, a loose network of hard-left pages generated around 27 million interactions (likes, comments, and shares) and 175 million video views during the campaign — vastly more than a cluster of right-wing pages backing Johnson received.

In 2017, at the last election, the emergence of a crop of aggressively loyal digital advocates was a major reason for Labour’s unexpectedly good performance at the ballot box. This “alt-left” media lost its dominance over the online political conversation in the next two years while a new generation of hyperpartisan pages dedicated to Brexit soared in popularity. But when the new election was announced at the end of October, the Corbynites ramped up their online activity again.

“They’re outbidding us,” Labour told its supporters in a fundraising email on Tuesday, after reports that the Tories were planning to flood social platforms with targeted advertisements in the final days of the campaign. But this obscured the bigger picture. Advertising has been more widely discussed by pundits during the campaign, but “organic” reach — where users share content with their friends because it moves them, without the creator having to pay for it — is just as important for persuading voters. And on that front Labour has clearly dominated.

For all the attention that Johnson’s videos received, they were eclipsed by the top clips on Labour’s key themes. According to BuzzFeed News’ analysis, the most-viewed posts of the campaign related to the NHS and were skewed against the Tories.

A vox pop by asking British people to predict the cost of health services in the US was seen by 38 million people on Facebook and Twitter. An argument by the American actor Rob Delaney against privatising the NHS, which ran first on Corbyn’s Facebook page, was watched 14 million times on both platforms. And then there was Momentum’s clip of Morgan defending the Tories’ contested promises about recruiting nurses.

In contrast, Johnson’s take-off of Love, Actually was watched 5 million times on Twitter and Facebook. His meandering tea-break chat at Conservative headquarters early in the campaign got around 6 million views on both platforms.

Corbyn’s personal Facebook page has been the biggest driver of political content in this campaign — getting more than 60 million video views, according to the CrowdTangle data — and the Labour party’s page has also been popular. Then there’s the network of satellite groups that aren’t run by the leader’s office but are fiercely loyal to the cause, led by Momentum, which have spread his ideas to a wide audience.

The groundswell of social media activity hasn’t been enough to shift the polls in Corbyn’s favour. Most observers expect Labour to lose on Thursday. However, it suggests that the opposition leader’s ideas and anti-Tory attacks have resonated widely with the public, which could have significant consequences for the future of both the Labour party and politics in Westminster generally.

More broadly, it also illustrates the seismic changes taking place in the UK’s political media. The information landscape has become fragmented and volatile, with voters’ perceptions increasingly shaped by hyperpartisan groups that present an unashamedly biased (and sometimes misleading) view of political issues. At little cost, those groups are amassing massive, highly engaged audiences that in some cases exceed those of mainstream media outlets, giving them enormous influence over the online political conversation.

The rapid rise of this partisan ecosystem has enabled wider participation in the democratic process and aired viewpoints overlooked by the traditional media — but it has also amplified the discord and division. Regulators and policymakers have been caught flat-footed. There are few rules and little oversight. Misinformation has flourished and new channels have opened for “dark money” to secretly influence elections and policy-making.

In this election, an array of fact-checkers has struggled to stay on top of misleading and sometimes entirely bogus assertions, like the claim this week that a photograph of a 4-year-old boy lying on the floor of a busy hospital ward had been staged. Other disinformation included a fake tweet allegedly sent by Corbyn after the London Bridge attack accusing the police of “murdering” the terrorist.

In this new environment, some of the most influential actors are campaign groups that exist outside the traditional party structures, which advocate positions that were once confined to the fringes of politics, and whose operations are often little scrutinised or understood. On policy issues like Brexit and the NHS, it is increasingly these campaigns that are framing the arguments online.

“Satellite campaigns are useful because they provide different kinds of networks for parties to spread their messages through,” said Kate Dommett, a senior lecturer in politics at the University of Sheffield. “We know that people are often unwilling to be directly engaged with political parties, but are more open to broad social issue–based campaigns… [And] research has shown that people are more receptive to messages shared by their friends.”

Momentum’s video operation has complemented Corbyn’s campaign by spreading its reach to millions of new viewers who don’t follow the Labour leader’s social media output, according to Charlaff, and by acting as a sort of digital innovation lab, testing out formats and ideas, pushing messages that are too edgy for Labour’s official channels, and providing some members of staff to the leader’s social team.

A professional filmmaker and photographer, Charlaff was introduced to the Labour leadership shortly before the last election in 2017, when he was doing social media training for charities and left-wing organisations. He spent that campaign with Momentum, one of five people on their video team at the time, and stayed on.

In the two years after that election, Charlaff and his colleagues laid the ground for a new campaign, studying what works on the various platforms, building out their network of volunteers, and examining what left-wing campaigners in other countries were doing on social media, particularly Bernie Sanders in the US and Jacinda Ardern in New Zealand. (One of Momentum’s first big videos in this campaign, a 30-second explanation of Labour’s Brexit policy, was inspired by a viral clip of Ardern giving a rapid-fire account of her government’s policy achievements. A few weeks later, Corbyn used a similar format to promote Labour’s manifesto.)

When it became clear this year that an election was imminent, Charlaff pulled together a core team of 15 producers at Momentum headquarters. They’re making content for all platforms, but Facebook remains by far their biggest source of eyeballs. “I don’t think that’s really changed as much as people thought it would have,” Charlaff said.

Their output has been a mix of relatively long scripted sketches featuring professional actors, which satirise Tory policies, pieces to camera by “real” people telling personal stories, and clips taken from TV shows, usually of Tory politicians saying inflammatory things. (One of the striking features of this election has been the way that partisan campaigns have edited footage from news programmes such as Question Time and Politics Live into posts that amplify their agendas, turning even obscure appearances by junior ministers at off-peak hours into viral flare-ups that could damage the parties.)

Do the shares translate to impact offline? “That’s always a concern for us,” Charlaff said. “Views on Facebook are great, but how do they translate into real-world results?” He insists it does have a big impact.

Momentum’s social media presence helped it raise more than £100,000 in the first day of the campaign. And it has helped put boots on the ground in seats that Labour needs to win. Inspired by the “distributed model” Sanders is using in the race for the Democratic presidential nomination in the US, Momentum has established dozens of volunteer silos in this election, with more than 1,000 volunteers taking time off work to pitch in.

Charlaff also points to the group’s voter registration drive on Facebook. He claims that around 125,000 people, most of them under 35 and in marginal constituencies, signed up to vote in this election after clicking on a link accompanying one of Momentum’s videos.

Charlaff is standing over one of his editors looking at the viewing figures for one of their most recent videos. It’s a piece to camera by a man Charlaff describes as “Paul the Bricklayer”, arguing for a second referendum on the UK’s membership of the European Union. “This is the last Brexit video you will need to watch,” the video says.

In a few days, the video would rack up 1.3 million views on Facebook. That’s far from Momentum’s biggest of the campaign, but it’s a good example of what, in Charlaff’s view, works in political campaigning on social media right now — a professionally produced video calculated to appear authentic, presenting a highly contentious, partisan viewpoint in a simple, relatable, seemingly trustworthy way that will activate viewers’ emotions.

“We’re not trying to push propaganda down people’s throats,” Charlaff said. “Obviously we have partisan messaging, but it’s more about trying to tell a story that people can engage with.”