What separated the eight Labour MPs who left the party to form the shortlived Change UK from the ones who stayed? There are plenty of Labour MPs who believe that Jeremy Corbyn is unfit for office having presided over increased antisemitism within the Labour party and who want to stay in the European Union – but most have opted to stay in the Labour party. (As, indeed, have those who believe Corbyn is unfit for office and who want to leave the European Union).

What separates Louise Ellman, the Liverpool Riverside MP, who has tonight quit the Labour party, telling the Times’ Henry Zeffman that Jeremy Corbyn is unfit to be Prime Minister as, under him, Labour is “no longer a safe place for Jews” and he cannot be allowed to do the same to the country, with the Labour MPs who agree with her but are still in the Labour party?

Part of the answer is deselection, or the fear of it. As I wrote at the time, one of the important things that united the founding members of Change UK is that they all regarded the call to “stay and fight” as doomed because they regarded the fight as lost. As one of their number put it to me at the time: “Stay and fight for what? What does fighting look like? What does winning look like? Is it just Emily [Thornberry] takes over and we all pretend that the mural didn’t happen?”

That most Labour MPs are passing their trigger ballots means that few fear deselection now. That they also see that the Corbynite leadership is as willing to manage shortlists to get its candidates in means that Corbynsceptic MPs feel that to leave is to hand yet more power to a leadership they dislike and have fresh hopes of being able to dislodge. But in Liverpool Riverside, that dynamic didn’t apply: Ellman was one of the MPs facing the serious prospect of removal, which makes the painful decision to leave – when I interviewed Bill Rodgers, 35 years after he split from Labour, the act of leaving was still a painful memory for him – easier, at least subconciously. As one of the Change founders admitted to me recently, while their prospects for deselection were not a concious factor they were aware of at the time, that they thought they had a choice been self-deselection or deselection by their members almost certainly played a role in their thuinking.

But Ellman speaks for a broader group: in annoucing that she has no plans to join another party and hopes to rejoin Labour under a new leader underlines the dilemma she speaks for the Labour MPs who don't want a new party, but to reclaim the old one. So why is she leaving?

Well, because the other, and equally important division, is that Ellman thinks that Labour might win the next election. For many stay-and-fighters, the matter does not arise. As one of their number put it to me when I asked what their red line would be, they said “I don’t have one. I keep reading: oh, how can you make him Prime Minister? And well, the answer, obviously, is that we are going to lose the election. It’s a non-issue.”

The same line of thinking is present in the House of Lords – as Betty Boothroyd said, recently told Anoosh when she asked how she reconciled her opposition to the party’s current direction with her membership: ““He won’t be prime minister, dear. So that’s not a relevant question. Corbyn ain’t going to win”. While few Corbynsceptic MPs would say that publicly, that calculation is why Ellman continues to be the exception, rather than the rule.

There is just one problem: Corbyn defied bad polls to win seats in 2017, and as the British Electoral Survey reminded us, British voters are now too volatile to safely predict. Those underestimating Corbyn, whether inside or outside his party, may live to regret it.