There’s one thing I don’t understand about Jeremy Corbyn. How did this man, who rejects the post-92 low-tax consensus, who has almost nothing emollient to say to business leaders, who talks about socialist values and has a visible beard, get on the ballot in the first place? Surely the parliamentary Labour party has ways of dealing with people like him? Is something wrong with their immune system?

To that last question: yes. They’ve lost a lot of blood, and now nothing works. MPs supported his candidacy on the basis that someone with significant differences to the others might “open up” the debate, let in some blue-sky thinking. It didn’t occur to them that anybody might agree with his side of the debate, but many constituency Labour parties do – by last Thursday night, he had 40 local party nominations, to Liz Kendall’s five. Anonymous sources inside the party are clearly rattled, threatening a coup – which requires 47 supporters – if he wins. One told the Independent on Sunday: “We can’t just allow our party, a credible party of government, to be hijacked in this summer of madness. There would be no problem in getting names. We could do this before Christmas.”

This is a sound point: a Corbyn victory would amount to a complete rebellion by the grassroots of the Labour party, who have for years been treated as an embarrassment, the last people in the country to whom the parliamentary Labour party listens, after they’ve canvased the opinions of everybody else. Anthony King, in his book Who Governs Britain?, wrote of the huge scope of democratic power we currently enjoy, given that anybody can join a party and then vote for its leader. I rejected the thesis at the time: leader, schmeader. What does democratic power mean when party conferences (apart from the Greens’) have replaced any meaningful participatory creation of a manifesto with a set of panel discussions? But I was suffering the same lack of imagination as those MPs who nominated Corbyn in the first place. Leaders do matter: not for all the reasons we’re told they matter – for their statesmanlike traits or grace in eating sandwiches – but because, if they will not stay within the perimeters of what the party considers rational, reasonable, credible, not-loony, then they immediately,irrevocably – coup or no coup – change the party.

Ahead of the overthrow comes the battalion of grandees dispatched to sneer at Corbyn from the platform of reasonable, grownup current affairs programmes. Tristram Hunt and Chuka Umunna on Newsnight last week both essentially told Labour members to knuckle down and take their medicine: only a centrist could win the rest of the country round to their way of seeing things. Corbyn, was not only not a centrist; he also, in Umunna’s extraordinary phrase, wanted to increase benefits to “people who could work but choose not to”.

The question to Umunna and Hunt is: why isn’t it working? Perhaps that’s premature. It may yet work. This may be a summer of madness. The “shy Kendallites” might come out of the woodwork, or Corbyn may turn out to have simply punched out some space to make Andy Burnham seem like the adult. So let me amend that: why hasn’t it worked yet? Why has calling Corbyn a loony lefty, an unrealistic pinko, an electoral death ray to the entire Labour project, a narcissist, a dreamer, a dinosaur, failed to get him back in his box?

The charge of being unrealistic oxygenates rather than smothers the spark Corbyn has created

Part of the reason is a simple disconnect between Westminster’s idea of “realism” and everybody else’s. Many of Corbyn’s positions – that some industries should be renationalised, that we can afford the NHS so long as we’re prepared to raise money through general taxation – are actually shared by the majority of voters. Other Corbyn views – that Trident is a behemoth solution to last century’s problem and has very little to offer in a modern conflict – may not be shared by most people but are certainly shared by enough to warrant representation in the mainstream political offer. Calling Corbyn a nutjob doesn’t discredit him: he would prove his lunacy only by being miles away from everybody he talks to – which he has so far failed to do.

On a more profound level, though, they’re coming at him with the wrong truncheon. The charge of being unrealistic actually oxygenates rather than smothers the spark Corbyn has created, since it is Labour’s brand of realism – that the perils of the world are many and varied; that everybody’s main driver is “aspiration”; and that Labour, being the natural party of justice, can navigate these choppy waters so that everybody ends up with the money they want – that has lost its lustre. Labour doesn’t win with a message of caution, simply because the Conservatives do it so much better; Labour wins with a message of hope.

It is bizarre to observe the intricate debates about how best to recreate Blair’s success: was it his flexibility around the private sector, his love of social mobility, his toughness on crime and the causes of crime, his soundbites, the intensity of his relaxation around the super-rich? No. The most memorable, salient, powerful thing about Blair was that he embodied hope. His optimism was palpable and contagious. In 1997, things could only get better. That’s what made him unstoppable. And that, in the end, is what would neutralise Corbyn: not ever-shriller accusations of the danger he poses but a more forceful articulation of what hopeful Labour would look like, and what its hopes would be.