After 100 years as a significant national movement, Labour is quitting the stage. Jeremy Corbyn’s re-election with an increased majority makes a formal split in the People’s Party all but inevitable.

If you’ve never worked in politics, you might imagine that parties split over issues of high principle: disarmament, say, or nationalisation. In fact, splits almost always happen for grubbier reasons. The last Labour rupture, in 1981, wasn’t to do with unilateralism or the EEC; it was to do with a plan to make incumbent MPs subject to reselection.

Today, just as 35 yeas ago, sitting Labour MPs know that they will struggle to keep their jobs if their local parties, now swollen with new Momentum activists, get to nominate them. Those Momentum activists, buoyed by their victory, have no intention of letting another year be lost to, as they see it, sniping from self-seeking Blairite MPs.

The proposed reduction in the number of MPs from 650 to 600 means that most constituencies will now be technically new, making reselections almost unavoidable. The majority of Labour MPs, having twice now defied Corbyn and lost, can see that they are about to lose their incomes and their pensions. Being a former MP doesn’t do much for your employment prospects.

The people saying “Why is Labour damaging its own interests like this?” misunderstand what a political party is. Labour is not a single organism with a unified survival instinct. It is made up, as all parties are, of individuals with diverging motives. To understand what is going on, you need to make a Namierite analysis or, if you prefer, a Freakonomics analysis: it’s all about working out the personal incentives.

For most Labour MPs, facing deselection under Corbyn, the incentive is now to launch a separate party. For most Momentum activists, the incentive is to replace disloyal MPs with far-Left loyalists. There will be an almighty row over who gets to keep the Labour brand, but it is hard to see how anything short of Jeremy Corbyn retiring on health grounds can now prevent formal schism.

This time, Labour is an even worse state than in 1981. Labour MPs, Labour activists and Labour voters occupy three circles that barely intersect. The MPs, pro-EU and pro-immigration, are every bit as remote from traditional Labour voters as the Corbynistas, pro-IRA and pro-Hamas.

We Conservatives will soon face an empty field, a prospect that fills me with gloom. Without meaningful opposition, governing parties over time become remote, self-serving, faction-ridden and sleazy. It’s a bad business, this, and no mistake.