It is too soon to say what Labour will become after its leadership contest but it is clear what the party has already ceased to be. The case for listening to people who vote Conservative and thinking of ways to change their minds has been crushed. This was the ethos of “New” Labour, and it is dead.

In a Guardian hustings featuring the four contenders last week, I heard constant complaints about failure to “challenge myths” about the economy, benefits, immigration and other areas where Labour is deemed unfit to govern by the people who choose governments. The voters are wrong, and what is required is a louder exposition of their wrongness. It is like watching a relegated football team argue over the causes of defeat and concluding they spent too much time on their opponents’ half of the pitch.

Most MPs from the Anyone But Corbyn (ABC) camp recognise that they have lost the party and the argument. From the outset Liz Kendall’s supporters misjudged the mood and allowed their champion to be caricatured as a soulless Blairite nostalgist scolding members for their attachment to socialism. As the Corbyn bandwagon gathered momentum, the ABCs deployed rebuttals based on electoral logic: steering left was undesirable because it was impractical. In so doing they ceded idealism to the Corbynites.

They were complicit in the division of Labour into two spheres: principle, which belongs to the left, and cynical calculation, which is the stock in trade of the right. Even the old argument that principle without power is impotent contains a tacit recognition that Corbynism is pure in essence. The more blood-curdling the warning of ballot box catastrophe, the sharper the divide. Corbyn became the light of hope against Blairism’s dark heart of fear.

This moral high ground gave Corbyn strange immunity from criticism. When the anointed one is the incarnation of principle, his actions are beyond reproach. Scrutiny of his opinions and the company he has kept are “smears”. If he has invited to parliament men who justify terrorism or shared platforms with antisemites and homophobes it cannot be because his judgment is warped. It must be an enlightened strategy of engagement for the higher cause of peace. These fellow travellers include people who have called 9/11 “sweet revenge” and said Nazi gas chambers are a hoax to promote “Jew- worshipping” in Europe.

Corbyn denounces such views when they are brought to his attention. His conscience is clear, and that is good enough for the fans. Holocaust denial is brushed off as the dust that gathers on noble boots during the long march to a “principled” foreign policy.

When Blair’s descendants re-enter the debate they find their licence to moralise has been revoked

The younger ABCs, the likes of Chuka Umunna, Tristram Hunt and Emma Reynolds, understand better than the New Labour aristocracy that drawing arguments on a map of marginal seats has fuelled the Corbyn campaign. Pointing out that elections were won by Blair doesn’t rehabilitate Blairism – it diminishes the appeal of winning.

That trend was clear in the last leadership race. Ed Miliband gamed it when he promised to “turn the page” on New Labour. Thanks to the release of emails from Hillary Clinton’s private account, we know that even the US secretary of state understood it in 2010. She wrote to Sid Blumenthal, her unofficial adviser and aide, that the result was “clearly more about Tony than it was David or Ed”.

A problem for Labour moderates (or modernisers, or whatever you call them) is that Blair did not simply submerge old left notions of virtue in a pragmatic programme for power. He developed an evangelical zeal in his foreign and security policy that appeared to replace old morality with a new ethic that, in the eyes of many on the left, promoted war at the behest of George W Bush and jettisoned civil liberties on the way.

Now, when Blair’s descendants re-enter the debate about “Labour values”, acknowledging for example that the 1997-2010 government was too relaxed about inequality, they find their licence to moralise on domestic policy has been revoked by activists still enraged over foreign affairs.

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Meanwhile, Corbyn shows up on an Iranian state propaganda channel glibly conflating 9/11 and the death of Osama bin Laden as equivalent “tragedies”, and his supporters say he was only making a point about extrajudicial killing: nothing to criticise in a man of principle. What matters is what he meant in his heart, not what he said, or where, or with whom he said it.

A serious party cannot present this proposition to the country as a bid to take charge of Downing Street, and I’m sure many of Corbyn’s team know it. He is a transitional figure whose victory would be used as an opportunity to take control of the party machine. His failure to generate a nationwide socialist revival would be blamed on the disloyalty of MPs – the undead hand of Blairism strangling the infant movement. If a challenge came from the right, Corbyn would step aside, saying the whole thing was never about personalities and he is, after all, nearing retirement age. A younger, less problematic candidate would run for the left. The name often posited for that role is Lisa Nandy, the dynamic MP for Wigan.

The dilemma for MPs appalled by the turn events have taken is to acknowledge the potency of the campaign that has beaten them without surrendering the party to a platform of certain defeat. They must lay their flowers quietly on New Labour’s grave and develop a new set of arguments for a 21st-century party that marries political reality and moral authority: caring about the deficit because stable finances are sound in principle; reforming social security because a system that is resented by millions has failed in principle; addressing concerns over immigration because confidence in the way borders are managed is a condition of tolerance and cohesion, which are good in principle; weighing the case for intervention against dictators and terrorists on the grounds that sometimes action beats inaction on principle.

Jeremy Corbyn has no unique claim on virtue yet his record has become the touchstone of left integrity. If Labour sets its moral compass by the mythologised conscience of this accidental saviour, it will abandon rational politics – and that is harder to recover than practical methods for winning elections.