You have got to admire David Miliband’s sunny optimism about the Labour Party. “Labour is further from power than at any stage in my lifetime,” he said today. He was born in 1965. The party is plainly further from power than at any time since the Ramsay MacDonald split in 1931. It is this kind of attempt to gloss over defeat in the Copeland by-election that makes people call Labour politicians deluded.

In the 1931 election, Labour was returned with 52 seats. The National Government had a total of 518 seats, most of them Conservative. That sort of thing obviously couldn’t happen this time. It is not as if the Labour leadership has defected to form a government with the Tories in the middle of the Depression. But as soon as I write that something “obviously couldn’t happen”, I ought to be on my guard.

As David Herdson writes on the Political Betting website, people thought Labour couldn’t be wiped out in Scotland at the last election. They just assumed that politics changes gradually and that there was a floor to Labour’s support. If there was a floor, however, it turned out that it was below the level at which Labour could retain more than a single seat.

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Until this week, that “inertia of thinking” prevailed in English politics. It still seems unlikely that Labour could be displaced by the Lib Dems, or by a new centrist party, or by Ukip, in descending order of probability. But the Copeland by-election raises the possibility of something else: that the governing party could just hoover up Labour seats directly. Precedents from history suggest themselves. Labour’s landslide in 1997, which reduced the Conservatives to 165 seats. But why should Labour find its floor there? William Hague was a more credible leader than Jeremy Corbyn. What is to stop the Labour lift descending all the way to 1931?

The easy answer is a change of leader. Labour’s problems go much deeper than Corbyn’s weak leadership, but at least that is simple to fix and almost any change would improve the party’s chances greatly. My view is that Copeland has shifted the probabilities. Before it, I thought Corbyn would continue as leader until the 2020 general election; now, I don’t think he will. But the change is not imminent.

Corbyn himself cannot want to continue. He is, visibly, not enjoying leadership. He keeps going out of duty to the so-called left-wing cause, because he and his cabal know that if he resigned, they would be handing the leadership back to what they regard as the right of the party (that is, the true left wing who want to get into government to improve the lives of working people).

Jeremy Corbyn ducks resignation questions after byelection humiliation

He knows what would happen if he escaped from his handlers and found a TV camera to resign to. First, his office would put out a statement saying that his words had been quoted out of context, and the phrase, “it is with a heavy heart that I step down from the leadership”, was a reference to his role in the All-Party Parliamentary Group on Cycling. Second, Tom Watson would become acting leader until a leadership election could be organised.

In that election, candidates would need nominations from 38 MPs and MEPs to stand. (Pedantry special: if Labour loses another seat in a by-election the threshold will go down to 37; after we leave the EU and have no MEPs it will go down to 34.) None of the so-called left could secure enough nominations. Not John McDonnell, Rebecca Long-Bailey or even Clive Lewis, who attracted some attention when he resigned from the shadow cabinet to vote against the EU (Notification of Withdrawal) Bill.

That is why McDonnell is pushing the so-called “McDonnell amendment” to Labour’s leadership election rules, to cut the nomination threshold to 5 per cent, which would be 13 MPs and MEPs. But it doesn’t seem likely that the amendment would go through the National Executive, which is finely balanced between Corbyn supporters and opponents, or through party conference, even if Len McCluskey holds on as Unite leader and wields the union’s giant block vote.

In the meantime, then, Corbyn will stay on. Political parties can go on for years like this. Andrew Hawkins, chairman of The Independent’s polling company ComRes, wrote this week: “In the mid-1990s I headed up the political research unit for a polling company providing data to a Conservative Party that knew it was heading for disaster. I realised that in such circumstances, fuelled by self-delusion and self-preservation, political parties are almost always incapable of changing their trajectory.”