Just because something is absurd doesn’t mean it can’t happen. This is the lesson of Jeremy Corbyn’s seemingly inevitable victory in the Labour leadership contest. At first, the prospect of Corbyn leading Her Majesty’s Loyal Opposition was seen to be so ridiculous that bookmakers put the chances of it at 200 to 1. Labour MPs were prepared to nominate him to broaden the ‘debate’. Now, almost everyone in the Labour party thinks we are days away from Corbyn’s coronation, and some bookies are already paying out. Even Tony Blair has accepted that Corbyn will triumph.

The temptation now is to declare that a Corbyn leadership can’t possibly last. The talk among senior Labour figures is not about his reign but his downfall; each has their own theory as to how and when he will be deposed. Surely, they say, the laws of political gravity would pull him back down so normality could be resumed. This view is dangerously naive. The forces propelling Corbyn to the top of the Labour party are very real, and will change British politics in profound ways — whether he wins or not.

The Corbyn delusion is driven by a few grains of truth. There are, as his supporters claim, voters to the left of Labour who might well be won over by a more left-wing leadership. There is also some public support for some of the policies of the old left — re-nationalisation of the railways, for instance, or whacking the rich. But the overall Corbyn effect would be disastrous for Labour. He is Michael Foot without the anti-fascist record.

Labour lost the last election because the voters didn’t trust the party with their money and the nation’s finances. Corbynomics (and his proposed ‘people’s quantitative easing’) is not the answer. Then there is Corbyn’s long list of dubious statements and associations. One of the Tories involved in doing the opposition research on Corbyn says gleefully, ‘There’s just so much. Calling Osama bin Laden’s death a “tragedy” is just the start.’ Indeed, what should worry Labour is how silent the Tories have been during this leadership contest. When I asked one if they would throw the book at him straight away if he wins, I was told no. The plan is to wait until he is firmly ensconced before doing so.

Indeed, the very sight of him will awaken folk memories in the British electorate. Corbyn may have ditched his vests, but one glance at him still gives a good sense of what his politics are. We might all laugh at Labour’s Peter Mandelson-inspired late 1980s makeover, but it was done for a reason. He wanted the party to look as if it was on the same wavelength as the rest of the country; Corbyn has no such concerns.

Even if Corbyn were quickly deposed, the public would question the judgment of the party that elected him. ‘The hangover from this is going to last an awfully long time,’ concedes one Labour strategist. The Tories will ask, do you want to entrust the country to a party that can elect Corbyn as leader? They would also warn that any Labour government will end up hostage to the far left.

Moreover, a Corbyn victory would pose an immediate dilemma for any ambitious Labour MP. Anyone who served under him would be tainted. They would be asked in every interview if they wanted Corbyn to be Prime Minister; if they said yes, then that clip would be used endlessly against them come their own time at the top. But getting rid of him would be just as hard. Labour MPs who openly opposed Corbyn would find themselves in a battle to hold on to their own constituencies. Internal warfare would ensue. It would be back to the 1980s in more ways than one for Labour. It is easy to see why so many Tories are rubbing their hands in glee. One source in No. 10 says, ‘We wouldn’t have dared script it like this, people just wouldn’t have believed it.’ Corbyn as leader would mean that the next election is the Tories’ to lose, and they would need to make an epoch–defining mistake to blow it. All of a sudden, the Tories have gone from fearing that they would never win outright again to being confident of at least a decade of majority rule. One secretary of state predicts that in 2020 the electorate ‘will form their judgment even more decisively than before.’

Some Tories are unnerved by the prospect of a Corbyn leadership, arguing that bad opposition leads to bad government, and some worry that an unelectable Labour party would lead to Cameroon complacency. What is perhaps most striking is the fear of the more radical Tories about what it means for their agenda. One of the more ideologically committed members of the cabinet frets that a Corbyn victory would lead to Cameron and Osborne tacking hard to the centre, abandoning Tory radicalism in the hope of hoovering up centrist voters disillusioned by Labour’s left turn.

In the 1980s, Margaret Thatcher took advantage of Labour’s lurch to the left to push through right-wing policies that would not have been possible in normal times. But Cameron and Osborne are far more traditional Tories than she ever was: their aim is to hold power rather than to implement an ideological agenda. They both remember all too clearly the party’s 13 years in opposition. Facing Corbyn, their instinct would be to grab as much of the centre ground as possible, and much of this work has already begun. The Tories now pride themselves on being the party of the national living wage; Cameron’s first intervention of the new political term was to threaten firms who failed to pay it with the full force of the law. It won’t stop there; plans are also afoot for a major push on equal pay for women.

The prospect of a Corbyn victory is already changing the dynamics of the next Tory leadership election (which we can expect in about three years’ time). Until recently, Boris Johnson’s supporters argued that the Tories needed something extra for the party to win outright. Boris, who had won twice in a Labour city and had the appeal of a celebrity as well as a politician, appeared to be that something. If Corbyn becomes leader, however, suddenly it would appear that anyone sensible could beat Labour. It is no coincidence that in the past few weeks, the odds on George Osborne’s leadership chances have been shortening almost as fast as Corbyn’s. The Chancellor is now, for the first time, the bookmakers’ favourite. He offers continuity, which is more appealing to the Tory tribe by the day. One of the Chancellor’s cabinet backers argues that ‘the contrast could not be more pointed’ between Corbyn, who has never held office of any sort, and Osborne, the steady hand on the Treasury tiller.

The place where the Corbyn effect is least predictable is Scotland. The SNP has been posing as the only genuinely left-wing party; a Corbyn victory complicates that strategy. There isn’t much space to the left of him. But if he were removed as leader before facing the electorate, the SNP could use this to claim that Westminster politics is dominated by a centre-right consensus that tolerates no opposition. And how would Corbyn handle a Scottish Labour party led by Kezia Dugdale, a centrist?

As the last general election so spectacularly demonstrated, pollsters and bookmakers can get it horribly wrong. Labour’s election is not over. Andy Burnham or Yvette Cooper may yet win — but if they do, the leadership that resulted would probably be a mere footnote in history: a holding period in which some of the more talented younger Labour figures could gain the experience and the stature necessary to lead. Should Corbyn win, the shockwaves will be felt for decades.

Corbyn’s supporters are right about one thing: you can change politics without winning a general election. If he becomes leader, his time in charge would change both the Labour and Conservative parties. It would confirm that May 2015 marked the beginning of a new era of Tory majority rule. A Labour party that is prepared to elect Corbyn as leader is a party that has consigned itself to not being in power for a very long time.