It ain’t over until the ballot box sings. So it is just possible that bookmakers are exhibiting premature capitulation by paying out in advance to punters who have bet that Jeremy Corbyn will be the next leader of the Labour party. Some Labour MPs contend that there remains a chance that the spectre that fills the vast majority of them with horror will not come to pass when the coronation ceremony takes place in Westminster on Saturday. One frontbencher speculates: “Perhaps we will wake up on Saturday afternoon and find it has all been a bad dream.”

He didn’t sound like he really believed that. Most Labour MPs I speak to now sound resigned to what they regard as a catastrophic outcome for their party. Members of Liz Kendall’s campaign team are talking openly about what they will do when the hard left seizes the commanding heights of Labour while close supporters of Yvette Cooper and Andy Burnham speak as if it is all over bar the counting.

“I would love to think that there is a last-minute pro-Yvette surge,” says one significant Cooper supporter. “But I fear it is already lost. I have a horrible feeling that Corbyn is going to happen.”

Even if he doesn’t win, something very profound has happened to the Labour party. As Tom Baldwin, a former senior aide to Ed Miliband, has written: “Labour is not the same party it was 10 years, or even 10 weeks, ago.”

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It wasn’t supposed to be like this. The MP for Islington North set off as the 200/1 outsider with the backing of just 20 of his parliamentary colleagues. The only political prizes he had previously won were Beard of the Year and Worst Dressed MP. He scraped on to the ballot paper only thanks to charity nominations from Labour MPs who lent him a signature in the name of “broadening the debate”, a fabulous example of why it pays to be careful what you wish for.

A common response among Labour MPs to what has since unfolded is anger. Anger, in the words of one mainstream MP, that “we have allowed our party to be hijacked by people joining by text for three quid”. Anger that any veteran Labour MP who has the temerity to question whether Jeremy Corbyn is a viable candidate for prime minister is now vilified as a Tory, a fascist or worse by flash mobs of people who only declared their support for the party five minutes ago and might as readily find a different hobby five minutes later. Anger about what it will mean for the millions of less affluent Britons who need an electable alternative to the Tories. And anger among Labour MPs with themselves for allowing this to happen.

The soul-searching is already underway and the darkest night of it is being experienced by the Blairites, as most of them no longer call themselves. Back in May, they casually assumed that the defeat of Ed Miliband would be the cue for the party to collapse gratefully back into their arms. Today, they contemplate Labour falling into the grasp of personalities and ideologies that they thought they had out-argued decades ago. I was talking recently to someone who was one of Tony Blair’s most senior and loyal aides during his time at Number 10.

He sighed: “There are many people to blame, but a lot of it is Tony’s fault.” The charge against the former prime minister from some of his fellow Blairites is that his brilliance at winning elections was not matched by sufficient effort to ensure that Labour did not lose the habit once he had retired. He did not do enough to promote potential successors who might have revitalised and sustained the New Labour approach and was feeble about protecting them from Gordon Brown’s King Herod strategy of strangling any rival for the succession. As a result, the party began to drift from the electorate under Mr Brown, slithered further away in the care of Mr Miliband and is about to put light years between itself and swing voters with the most left-wing leader in the party’s history.

There is something in this. But fashionable as it is to blame Mr Blair for everything from the Iraq war to your malfunctioning washing machine, it is too simplistic to heap all the blame on his head. Labour moderates as a wider group will need to think about how they lost control of their party.

Some already are. Tristram Hunt has correctly observed that they became trapped in a technocratic, desiccated, uninspirational account of social democracy that is hopeless at making emotional connections with people. It is not all that often that Peter Mandelson admits to a mistake so it is worth listening when he does. “Labour modernisers have also been at fault,” he wrote recently. “In failing to acknowledge past mistakes and define what New Labour should mean for new times, we have allowed critics within the party to create a caricature of modernisation as a sectarian creed alien to the party’s values and history.” One of the triumphs of the Corbynistas has been to define the contest as a struggle between principle and idealism, as supposedly embodied by the bearded messiah, against unprincipled electoralism, as allegedly personified by his three mainstream competitors.

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Labour MPs, as a collective, are still getting their stunned heads round what is happening to their party. Because the hard left had been fought – and apparently crushed – so long ago, it simply did not occur to them that it might revive, least of all in the person of a 66-year-old colleague who has been an eternal backbencher since he first arrived in parliament as a disciple of Tony Benn and an enthusiast for the calamitous 1983 manifesto. Because it was so self-evident to his colleagues that the Corbyn candidacy was a joke they did not wake up to its potency until too late. Yet there had been warning signs of the potential for this to happen in the surge of populist left movements elsewhere in Europe, thriving on discontent with austerity and rage against elites. Closer to home, the rise at Labour’s expense of the Scottish Nationalists ought also to have sounded alarms. “Scotland should have told us about the power of emotion and identity in politics,” admits one member of the shadow cabinet.

Allied with those failures of inspiration and imagination has been a disastrous lack of mobilisation. One candid senior Labour MP on the team of one of Mr Corbyn’s leadership rivals says: “We completely fucked up organisationally.” The biggest mistake made by all three of the mainstream contestants was not to grasp the implications of the change to Labour’s system of leadership election. When that went through, in early 2014, something strange happened. Len McCluskey and the other chiefs of the big unions, who had initially been highly hostile to scrapping the electoral college, suddenly dropped their opposition and came along very quietly. I think it is now becoming clearer why. They had started to work out what the changes could mean and how they might be exploited with a systematic drive to sign up supporters for their niche of left politics. About half of the union members with votes in the contest have been signed up by Unite, whose executive were early endorsers of Mr Corbyn.

At the time of the rule change, there was not much comment about the potential effects of allowing registered “supporters” to buy a vote in exchange for the price of a pint. A couple of people on Ed Miliband’s senior staff did think through some of the implications. They argued with him that they should start to actively recruit £3 supporters to try to ensure that the party drew in people who were reasonably representative of the sort of mainstream voters Labour needed to win general elections. That idea was resisted by other members of the Miliband team. “We’ll only get hippies,” sneered one naysayer. So that initiative came to nothing and the composition of the selectorate continued to slide left, a process which has dramatically accelerated during the contest.

Labour modernisers, who had argued for years for one-member-one-vote, gave little thought to the change. There was no effort to recruit centre-left voters as Labour supporters. The machine behind Mr Corbyn did think about how the new system might change the dynamics of a contest and all credit to them for seeing a path to victory by allying old-style trade union organisation with the newer power of social media.

The MP for Islington North was alone among the candidates in putting on his campaign website a link to the £3 sign-up. The teams of all three of his rivals made a terrible mistake in not spotting how this would radically influence the race. In mitigation, one member of the shadow cabinet pleads that they were all too distracted by the shock of the general election defeat.

“While we had our heads under the bonnet of the car trying to work out why we lost the election, these people jumped into the car and drove it off.”

Which leaves stunned Labour moderates stranded on the kerbside, aghast at where the car is now heading, unsure when or how they will get it back and fearful that it might be a write-off by the time they do.