In Derby North earlier this month, Labour organisers were describing a welcome situation the like of which they hadn’t seen in years. That was days before the nightmare of the Manchester terror attack, which shocked the country and put the election campaign on pause.

Back then, before the horror struck, Labour campaigners were enthusiastic over the response to My Nearest Marginal, an electioneering website launched by Momentum, the grassroots group of Jeremy Corbyn supporters. The site invites activists to punch in their postcodes and directs them to their nearest swing seat – constituencies such as Derby North, in the East Midlands, where the Labour candidate, Chris Williamson, lost to the Conservatives by 41 votes in 2015. It’s the country’s second most marginal seat, after Gower.

Immediately after the website launched, in early May, more than 100 Momentum activists turned up in carpools – another organising feature of the site – to Derby North, from nearby Leicester, Sheffield, or Burton-on-Trent, to knock on doors for Labour. The Saturday I visited, a steady stream of supporters, including six teams from Tottenham, London, and returnees from the previous week, arrived to leaflet in the near-steady stream of rain – demolishing the stacked boxes of leaflets at the campaign HQ.

I’m not a Momentum member, and it was striking to see the age range – from millenials to the middle aged and upwards – across a group often cast as idealistic youngsters.

Now back in flow in the resumed campaign, a general election mobilisation like this was meant to be one of the advantages of a group such as Momentum, and one of the benefits of Labour’s swelled membership following Jeremy Corbyn’s election as party leader. With Labour membership hitting half a million, there was much talk of the campaigning boost this would bring, although it was just as often dismissed: what use were a group of cultish, Trotskyist-infused Corbynista clicktivists, hopelessly out of touch with normal people, when it came to the crunch?

Whatever you make of Corbyn’s leadership, it has brought supporters out during a critical election period

Well, it’s crunch time now. And the answer, it turns out, is: quite a lot. My Nearest Marginal received 50,000 unique visits in its first week – more than Momentum’s membership, of 22,000. It has since clocked some 300,000 page views. Those who have campaigned through the website say its appeal isn’t just the coordination element, but the framework it offers first-timers (many of those turning up have not canvassed before).

You can meet and travel with others in your area and get trained in canvassing – including a rundown of issues pertinent to a particular constituency – before taking to the streets, paired with experienced campaigners. One 56-year-old arriving to Derby in a three-car pool from Leicester said this was reassuring before going door-to-door, where she found people keen to discuss issues such as education and health care.

According to the constituency’s Labour candidate, Chris Williamson, the site brings not just numbers but energy: the boost of new blood, passion for the policies and enthusiasm (buoyed by recent positive polling) for getting a Corbyn leadership in government. Williamson supports the party’s direction of travel, and says its manifesto is the best in decades. I wondered whether Derby North was a skewed example, since it’s easy for Momentum members to support a Corbyn-friendly candidate. But speaking to constituency campaigners where the local candidates are, let’s say, less enthusiastic about the leadership, it seems this mostly is not a factor – what mobilises is seat marginality. One veteran campaigner in the south east told me that the shared purpose of the election campaign was healing Labour divides locally.

But as well as driving campaign efforts around the country, Momentum may be changing the dynamics of electioneering. US campaigners for Bernie Sanders are helping Momentum and providing training in canvassing; some 2,000 people in 10 cities around the country have attended workshops.

These differ to the voter ID method that’s often used – if you’ve been doorstepped, you’ve perhaps experienced this somewhat deadening approach that simply seeks to extract past ballot behaviour and current voting intention.

The Bernie campaigner style is about building connection, finding out what drives someone’s voting intention: it’s usually a key concern, and often people vote for emotional reasons, according to one of the trainers I talk to. This apparently works well with undecided or disengaged voters and brings the focus on to policy, away from personality – a useful device if Corbyn is cited as a concern (from my experience in Derby North, this comes up, although, like Brexit, not as often as anticipated).

Momentum is also running election-pegged community events around the country, and has launched a phone-banking app now also adopted by the Labour campaign. The group has also launched Electiondaypledge – where you sign up to helping Labour get its voters to actually go out and vote on election day.

The party’s current boost in the polls could be down to a combination of factors and it’s not clear how you’d quantify the impact of Momentum’s organising – not least because people don’t turn up waving Momentum flags. But it seems a palpable shift is taking place. Whatever you make of Corbyn’s leadership, it has brought supporters out during a critical election period and spawned a new kind of engagement on the ground, between the party and the people who can put it into power.