More than 150,000 people have joined the Labour Party since May’s defeat, a figure which exceeds the total membership of any other political party in the UK. Over 60,000 have joined since Jeremy Corbyn became leader, more than either the Liberal Democrats or Ukip can boast among their ranks. The composition of the party is changing too. The average age of the party membership fell by 11 years over the summer – from 53 to 42 – and more women than men joined. Something similar happened with the SNP after the independence referendum, when its membership, in a nation of only five million, surged beyond the 100,000 mark. There, too, new members were younger and most of them were women.

At his party conference speech last Tuesday, Corbyn touched on all of this, pointing out that the deluge of new members to Labour – on a scale unseen for five decades – represents a break with trends across Europe for parties of the centre-left. The hypothesis that the continent's historic parties of social democracy, now bereft of a coherent ideology or social base, were facing inevitable decline, now has an almighty outlier in the British Labour Party.

Still, for many commentators, membership figures matter less than the opinion polls that show the Conservatives well ahead. What that misses, however, is that in politics, resources – money and members – always matter. By 2020, Labour could have more of both than any of their rivals, so much so that overcoming a hostile mainstream press would not be unthinkable.

Labour now has more than 350,000 members, and Corbyn says he wants a million before the next election. The Conservatives, meanwhile, have not filed any data on the matter for almost two years, but Conservative Home calculated in 2013 that the party probably has fewer than 100,000 members. That activist base is getting older as well as smaller, with the average member somewhere between 59 and 68. When it comes to campaigns on the ground, age matters. It can make the difference between no leaflets through the door and six. A large, young base can also play a central role in getting more voters registered over the next few years, with Corbyn – as he tacitly admits – needing a huge increase in turnout to stand any chance of becoming prime minister in five years’ time (Obama’s victory in 2008 was based on an increase in voter turnout of 7 per cent compared to eight years earlier).

The question of whether Labour could leverage the rhetoric and dynamics of a social movement to come to power, as Obama did, is no longer academic. Britain is seeing the revival of social democracy – and socialism – as a mass movement. The Tories, for all their money, mainstream media influence and Australian spin doctors, might find it too much.