We have grown used to identity politics. We know how intense the passions stirred by race, gender or sexuality can be, how they can seem to trump the old allegiances of class or economic self-interest. But now we might need to add a new form of identity to the familiar categories, one that feels just as much about belonging. It is political allegiance itself.

The thought is prompted by the rise and rise of would-be Labour leader Jeremy Corbyn, but not only by him. Bernie Sanders, the lone self-described socialist in the US Senate, is enjoying a Corbynesque surge in enthusiasm in Iowa and New Hampshire, the states that vote first on whether he or Hillary Clinton should be the Democrats’ presidential nominee in 2016. Like Corbyn, Sanders calls for an increased minimum wage, free university tuition and an economy no longer tilted towards the super-rich and the corporations. Like Corbyn, he is dismissed as an old white guy whose views were out of date a generation ago. And like Corbyn, he is enthusing the very young people usually written off as being disengaged from politics.

The similarity goes further. Opponents of both Corbyn and Sanders insist that even if the egalitarian Utopia of which these men dream might be desirable, it is out of reach, if only because the voters will never vote for it.

Conventional understanding of politics assumes that that kind of rational argument is devastating: if you amass the historical data and the foreign examples, point to defeat after defeat for Corbynist programmes or Sanders-like candidates, surely their supporters will glumly lower their placards and come to their senses.

That’s what’s been happening in the Labour party this week, as Tony Blair and others tried to sit the kids down and say: “Look, you’ve had your fun. But take it from us, even if Corbyn is right – which he isn’t – he is never, ever going to get elected. This crusade is doomed. Come back home.” And it hasn’t worked, chiefly because it misunderstands the attraction, even devotion, of Corbynites to their cause. Listening to some of his young backers this week, it became ever clearer that they are not supporting him because, having made a calm assessment of the four candidates, they believe he has the best chance of returning Labour to power. On the contrary, that has nothing to do with it. This week’s YouGov poll of Labour supporters – and we should remember our post 7 May vow never to listen to another poll again – found that, among those backing the Islington MP, only 10% thought it was important that the party leader “understands what it takes to win an election” (the figure was 63% among supporters of Liz Kendall).

What’s needed instead, one enthusiast for Corbyn told me, is “someone who can articulate what you feel”. The key is “to have someone who represents what you believe in. Why does it matter whether other people believe it or not?”

I suspect that gets to the nub of it. For a lot of those taking part, choosing a party leader is not about assembling a governing majority, winning power or even making a change in society. It is about identity. It is about being true to yourself. In this sense, joining the Corbyn tribe becomes something non-negotiable, or at least impregnable to routine political arguments about electability, popular appeal and the like. Those kinds of calculations are held to be cynical, because they require you to compromise something fundamental about who you are.

They are doing it not because they think he can win, but for the same reason people retweet images of same-sex weddings

The unkind reading of this is to suggest that support for Corbynism, especially among the young, is a form of narcissism. In the current New Statesman, Helen Lewis notes the tendency of people on social media towards “‘virtue signalling’ – showing off to your friends about how right-on you are”. She sees the current stampede of constituency Labour parties to nominate Corbyn as an extension of this same habit. They are doing it not because they believe the 66-year-old can win in 2020, but for the same reason people retweet images of same-sex wedding ceremonies. As Lewis puts it: “They are doing it to signal that they are on the side of right and good.”

I suspect there are other reasons at work, especially among the young. Part of it is simply age and memory. Those over 45 can remember the four Labour defeats from 1979 to 1992, the 1980s lurch towards Bennism and the civil war over Militant. They contemplate the prospect of a Corbyn victory and say, “We’ve seen this movie before: we know how it ends.” But for someone in their 20s or 30s, all that is so much ancient history. They look at today’s landscape, at the SNP or Syriza or Podemos, and think: why not?

What’s more, when they hear power held up as the decisive criterion, they’re not convinced. Based on recent experience, their view of the centre-left in office is mixed at best. So when Blair’s smiling face appears on TV they don’t associate his three election victories with the chance to implement a minimum wage or a windfall tax on the utilities, but rather with the invasion of Iraq. (In the US, Sanders feeds off the disappointments in Obama, whether the failure to close Guantánamo or to halt the galloping enrichment of the 1%.)

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Blame for that is shared. Some belongs to New Labour itself, for internalising pragmatism to such an extent that it entered its soul, shutting itself off from the stirring language of moral fury at injustice. The result is that New Labour’s second generation often looks like a smooth, besuited set of careerists, elite members of a spadocracy against which Corbyn appears the paragon of rumpled authenticity.

But blame also attaches to the party’s outgoing leader. Ed Miliband’s positioning as the man who came to bury New Labour subliminally taught younger supporters to believe that little of value was achieved in those three hard-won terms. They absorbed the lesson that maybe power was not essential if all it led to was privatisations, Iraq and an engorged City.

All this has consequences for those who would like to halt Corbyn’s march to the leadership. It means they have to find a different way to talk to those drawn to the rebel backbencher. Sounding like the grownups lecturing the kids won’t do it. Hurling insults won’t help either. Nor will talk of electability, if what’s at play here is a matter of identity. They’d be talking at cross purposes.

Instead, Labour’s pragmatists will somehow have to match the excitement that’s been unleashed. The prospect of Labour’s first female leader could be a starting point. Having the chance to oust the Tories before today’s 20-year-olds turn 40 might be another. But ultimately those unwilling to face a lifetime of opposition will have to persuade their fellow party members that an identity built on the purity of impotence is not much of an identity at all.