Nothing about Jeremy Corbyn’s surge suggests a blip. Few at Labour HQ, busy checking credentials of new signers, think people are flocking to sign up to vote for anyone but him – some 60,000 new voters as of Monday. JC is a phenomenon.

Neil Kinnock warns of “Trotskyite forces with malign intentions” and entryists from the “Telegraph right”. But evidence from overflowing Corbyn meetings pumping with passion is that these are not resurrected Militant but essentially Labour-type people who just agree with Corbyn. The atmosphere is heady – as it was when Michael Foot filled rallies to the rafters. He refused to believe he was losing badly when finding whooping support everywhere he went. All of us on the left risk the myopia of inhabiting a universe of the like-minded.

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Labour’s dilemma is that most Corbyn policies are supported by most Labour members. The dispute is not, as Corbynites claim, about core beliefs previously betrayed – but about tactics on how to win so as to put them into practice. Blown-away Blairites now in the private defence, health and finance industries may no longer share recognisably Labour values.

But most still inside the fold, especially those rooting for Yvette Cooper and Andy Burnham, even Liz Kendall supporters, may agree with much that Corbyn says. Leave aside his alarming prevarications on the EU referendum and his “friends” in Hamas and Hezbollah, many of his policies stir deep longings in Labour hearts.

He has opened a floodgate of dreams, making people feel good about themselves. He is right about welfare, austerity, tax avoidance, renationalising rail and mail, Trident, housing and myriad other touchstones. He’s authentic to the tip of his beard, reassuringly no grand orator in the grandstanding George Galloway or Michael Foot tradition. At hustings he shines by offering virtue, while the rest wrestle with the wretched realities of British politics.

Once in power, with the levers of persuasion, you can take people further than you dare tread in opposition

The Labour question is always the same – how far can you go and still bring enough voters with you? As Labour’s divide deepens, those of us not supporting Corbyn have been assailed as “neoliberal”, “siding with the elite”, “betraying the poor”, “hypocrite” and worse. Like many Labour people, free to dream I’d go further than Corbyn: I’d go for a windfall wealth tax to pay off the deficit, make the Queen be Elizabeth the Last, abolish faith schools, private schools and inheritance, tax millionaires at 70%: add your wishlist here. I don’t know how far you can go – but you have to win power to get anywhere at all. Once in power, with the levers of persuasion, you can take people further than you dare tread in opposition.

What happens if Corbyn wins, marking a seismic shift in the party? Those declaring they wouldn’t serve in his shadow cabinet make a futile gesture. Only 20 of Labour’s 232 MPs nominated him (the other 15 threw him a lifeline). He says MPs will vote for his shadow cabinet – so will it be entirely confrontational? Hotheads threaten another leadership election pronto, as it takes just 47 MPs to demand one, but that would be suicidal: the same question would get the same answer with knobs on. Besides, Labour MPs due to lose some 50 seats in David Cameron’s boundary changes will face reselection from the party that elected Corbyn, so they will veer leftwards. What will happen when Labour councils have to set budgets imposing 40% cuts? Will some local parties, encouraged by Corbyn, demand illegal budgets – as in Liverpool and Clay Cross? What would Corbyn say? After 500 rebellions against the whip, his own whipping hand would be weak.

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Next May Labour risks losing up to 1,000 councillors, a loss that hollows out parties. Next June may bring the EU referendum: what should be a falling apart of Tories may become Labour strife. Disunity kills, say the pollsters.

Give Corbyn a go, say some: if Labour sinks, after a couple of years elect someone else. But the disarray, bitterness and credibility forfeited would be irrecoverable for 2020. Any split is a nonsense: our monstrous electoral system bulldozes new parties, viz the SDP in 1983, Ukip and the Greens this year. If Corbyn does win, MPs need to behave well, no plotting or undermining. Let him try to spread his message beyond the narrow confines of the existing pool of left-leaners. See if he can arouse Syriza, Podemos, SNP passions among other voters who show little leftward inclination.

The TUC’s own polling sheds a cold light on why Labour lost: it was “too profligate, too soft on welfare, too incompetent”. Labour wasn’t trusted with the economy, would spend too much, be too generous with benefits and make too many concessions to the SNP. Why vote Tory? To keep the economy growing, press on with deficit reduction: Cameron was the preferred prime minister. Labour scored a miserable 30 on competence – and this is TUC not Daily Mail polling. Those who manned Labour clipboards door-to-door talking to real voters know why Labour failed: my guess is that few of those troupers are Corbyn believers.

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Jack Dromey, MP for Erdington, says Labour had plenty to say about the plight of the poor, but little to offer his 4,000 better-paid Jaguar workers: the party needs to win them too. He recalls the People’s March for Jobs in 1981, incidentally Yvette Cooper’s first political demonstration, when hundreds of thousands cheered them on. But both Dromey and Cooper learned through 18 bitter years that false hope is easier to arouse than fulfilling that hope by winning. The social democratic vision of gradual change, achieved slowly and incrementally, is less thrilling, but more successful.

Can Corbyn overcome all with sheer conviction? I wish it were so. But Labour people, motivated by the plight of the needy in a grossly unjust society, shouldn’t gamble the future of the weak on such a slender chance. This Tory government is now bent on demolishing the state: yesterday the FT listed Osborne’s plans for a £32bn sell-off of assets. By 2020 what’s left of the state will need rescuing. Every Tory government impoverishes the poor and enriches the wealthy. Every Labour government makes landmark social progress. Winning next time matters desperately. A Cooper leadership offers an infinitely better hope of success than a Jeremy Corbyn/Tom Watson ticket.