So that's it then. The party's over. Finished. As dead as Monty Python's parrot. Among those of us who take an interest in politics, 24 September 2016 will always be remembered as the day a rag-tag army of Trots, Communists, fun revolutionaries, naive idealists and useful idiots killed perhaps the greatest engine of social reform this country has ever seen.

It wasn't meant to be like this. Labour moderates hoped that a leadership contest would replace Jeremy Corbyn with a (relatively) sensible, grown-up alternative. At the very least, they told themselves that a strong challenge might make Jezza think twice about some of his dafter policies.

Instead, they have contrived a catastrophic defeat for themselves and for their party. And there's no way back. With two resounding victories under his belt, Corbyn is now stronger than ever. Unassailable. And the platoons of Labour MPs who despise him and despair at where he is taking the party are helpless. They can only wait for the vengeance that is surely coming their way.

Oh yes. There's going to be a political bloodbath right enough. Don't believe all the soothing pap from the Corbyn camp about the importance of unity and the need to reach out to the moderates. The hard-left activists of Momentum – Jezza's praetorian guard – know that this is the greatest opportunity they have ever had to destroy moderacy. They won't let it slip.

We've already seen the deeply unpleasant side of these supposed progressives. The insidious anti-Semitism which the party leadership is too cowardly to address properly; the viciousness of online insults against supposed Blairites; the threats of violence, particularly against female backbenchers; and the ugly campaigns to unseat decent MPs, such as Peter Kyle in Hove.

None of this is remotely surprising. Anyone who knows the first thing about Labour politics has stories to tell about the nastiness of sectarian, left-wing fanatics. It's been a feature of the party political scene for decades. The only difference now is that the Left is now indisputably in the driving seat. And at all costs it intends to hang on to that control.

Labour leader Jeremy Corbyn speaks at a #KeepCorbyn event at the Brighton Dome on 13 September in Brighton, England. Getty Images

The first step will be to take over the running of Labour's headquarters – the one cog in the party machine that has not quite yet succumbed to the Corbynista revolution. But given the Leftist tide, it can only be a matter of time. Labour's general secretary Iain McNicol has incurred the bitter enmity of Momentum, which is determined to get rid of him. Momentum also intends to replace other senior staff with compliant, left-wing stooges.

Meanwhile the campaign to deselect Blairite MPs is up and running. Over the next couple of years we can expect a relentless cull of anyone who doesn't toe the party line – particularly as constituency boundaries are being redrawn, with fewer seats being contested at the next election. That offers the perfect excuse for the Corbynistas to strike.

And what can the hapless majority of Labour MPs do about it? Sadly, not very much. In private, they may talk bravely of breaking away to form a new party, but few really believe it's a realistic prospect. How would they pay for it? Who would be big enough and charismatic enough to lead it? And what on Earth would they call it, since the once-proud Labour name is now firmly in Jezza's possession?

Some see only one way out of their predicament. Might not Corbyn's disciples in the party begin to have second thoughts, if – when – he leads Labour to disastrous defeat at the next general election? Won't normal politics then be resumed, with somebody sensible in charge?

They're clutching at straws. Labour's rank-and-file members aren't interested in normal politics. They don't care that Corbyn is a lousy leader. They don't care that he's not terribly bright. Or that he makes a fool of himself with dishonest stunts on Virgin trains. They don't even care that he consorts with Hamas terrorists and calls them his "friends". They simply see an apparently nice bearded chap who isn't like other politicians.

So there's little chance that they'll have second thoughts about their hero. And even if he chooses to resign after an election defeat, there are plenty of comrades such as shadow chancellor John McDonnell – he who swears by Mao-Tse-Tung's Little Red Book – ready and willing to succeed him. By that time, the Left will have such an iron grip on the party that moderates – if they still exist – will be an irrelevant rump.

How could a once great party come to this? The sad truth is that mainstream politicians brought it on themselves.

Their smug, incompetent and occasionally corrupt reign in Scotland opened the door to rule by the Nationalists. Their refusal to listen to the concerns of the voters on issues such as immigration left them on the losing side in the EU referendum. And that's not to mention Ed Miliband, who sold the right to vote in the leadership contest for £3 a time. Or the boneheaded 35 Labour MPs who nominated Corbyn in the first place, even though they knew very well what kind of a character he is.

And now? Even in the face of political calamity, the best candidate they could find to take him on turns out to be Owen Smith. If this is indeed the end of the Labour Party after 116 often distinguished years – as I think it is – let's pin the blame where it belongs: not on silly Left-wingers who probably can't help themselves, but on sensible moderates who let them get away with it.