Even at first sight it is exhilarating. The overthrow of one of the most mainstream and senior Democrats in Congress by a 28-year-old Democratic socialist with a radical programme and one tenth of his funding is, you might think, interesting enough.

But since Alexandria Ocasio-Cortez beat Joe Crowley in the Democratic primary in New York’s 14th district (meaning she will almost certainly enter Congress in November), I’ve been interviewing some of the people who lit the fuse that caused this detonation. What has emerged is just how marginal and improbable their movement was when it began, and how quickly it is now gaining momentum. A revolution has begun in America, and it is time we understood what it means.

While the effort to find and run insurgent candidates arose from the Bernie Sanders campaign in 2016, the handful of young people who launched this movement struck out entirely on their own. They had no resources and no political standing. Neither Sanders nor any others in the old guard were prepared to support them or endorse the candidates they found.

Play Video 3:47 'This is the beginning': Alexandria Ocasio-Cortez's victory speech – video

In a way, this tiny group, Brand New Congress, which evolved into the Justice Democrats, marginalised itself. It wanted nothing to do with a traditional left it saw as being obsessed with positioning. It wanted to escape the shadow of people who seemed stuck in the 1980s, who didn’t take environmental issues seriously or understand the need to challenge structural racism and gender inequality, or to reach millennials trapped in terrible housing and miserable non-jobs. They were mocked, ignored and dismissed as well-intentioned but hopeless idealists. One of them told me how he was literally patted on the head by an older Democrat.

At first, it was chaotic. Most of the volunteers they recruited had little or no experience. Some turned out to be wonderful, others less so. Their original aim was to find 400 candidates to challenge both Democratic and Republican incumbents. They sought bartenders, factory workers, small-business people, community organisers, teachers, nurses – ideally people who had never held public office before. While Democratic candidates are usually chosen on the grounds of how much money they can raise, the Justice Democrats looked for people who could not be seduced by big funders. They reasoned that if the people they met had served their communities instead of themselves, they were unlikely to sell out once they were elected.

They found plenty of brilliant potential recruits, but without mainstream support they didn’t have the credibility required to convince hundreds of people to give up their lives for an improbable cause. They managed to persuade a few dozen, however, and among them was Ocasio-Cortez. They phoned her, invited her to dinner, and asked her to attend a meeting in Kentucky with other potential candidates, in the hope that they would inspire each other to run. She took her time and toured the 14th district before she agreed.

Does Alexandria Ocasio-Cortez signal an earthquake in Democratic politics? Read more

She was, as we have seen, a fantastic candidate: determined, indefatigable, brilliant at explaining complex issues simply and directly. Alexandra Rojas, the campaigns director of Justice Democrats, tells me: “She has a way of making issues that others see as radical seem simple, straightforward and pragmatic.” Everyone I spoke to remarked on her grace and stability, and how she calmly absorbed the dramas that surrounded her bid. Extraordinary local campaigners combined traditional fieldwork with the big organising tactics developed during the Sanders campaign: using proliferating networks of volunteers to fill the jobs usually reserved for staffers.

Remarkable as she is, there are others like her. Cori Bush in Missouri, Jess King in Pennsylvania and Kerri Evelyn Harris in Delaware are just a few of those now fighting for Democratic nominations or seats while renouncing big money, relying instead on the enthusiasm of the communities they hope to serve.

The Justice Democrats are not expecting all these candidates to win, but hope for a few spectacular victories at the congressional elections in 2018 and 2020, not only replacing corporate, money-tainted Democrats, but flipping a couple of Republican districts as well (look out, for example, for the campaigns by Brent Welder and James Thompson in Kansas). As soon as such people take their seats in Congress, Saikat Chakrabarti, one of the core organisers, tells me the aim is to “legislate the hell out of everything, like the Republicans do … proposing the boldest, biggest ideas on day one”. By 2022, using the momentum gained from a few strategic victories, they hope to run a full slate of new or re-energised candidates. The aim is to create a genuinely populist Democratic party, which neutralises Trump’s brutal demagoguery and speaks to people across the political spectrum who have been alienated by the corruption and drift of mainstream politics.

Alexandria Ocasio-Cortez: who is the new progressive star of the Democrats? Read more

Thanks in part to the supreme court’s disastrous Citizens United ruling, which removed the caps on political spending by lobbyists, US politics is dominated by billionaires and corporations, buying the candidates and policies they want. They can’t be outspent, but they can be outmanoeuvred, by recruiting incorruptible people who can speak past the money. Eventually, the Justice Democrats hope, there will be enough strong and inspiring people in Congress to overthrow this ruling and purge the institutional corruption from US politics.

So far, the Democratic party has reacted in two distinct ways. Some senior figures, such as Nancy Pelosi and Tammy Duckworth, dismiss the significance of what Ocasio-Cortez has achieved. Others, such as Kirsten Gillibrand, have suddenly switched positions in response to her victory, echoing her call, for example, for the abolition of the Immigration and Customs Enforcement agency, which has been separating children from their parents on the Mexican border. Both are forms of self-preservation, but if more revolutionary candidates win their races, the second variety is likely to prevail.

By understanding how the great reversal in New York happened, we can begin to understand what this movement of outsiders might achieve. It could yet change the world.

• George Monbiot is a Guardian columnist