James Callaghan was making an excuse for himself when he said that “there are times, perhaps once every 30 years, when there is a sea-change in politics”. What he meant was that he had blown it. If he had called an election in the autumn of 1978, he could have won it.

It is true that sometimes there are big changes in public opinion, but they usually happen for a reason; in 1979, for example, it was because of the strikes during the preceding winter.

And there is no doubt that it becomes harder to sustain support over time. But the idea that there is some mysterious force working through the people, who simply decide one day that enough is enough, is an easy way out for a politician who has made a mistake.

What is more sea-changey is the discipline and morale of parties. Sometimes parties just give up. The most dramatic example was in 1931, when the Labour Party just didn’t know what to do about the Depression because Keynesianism hadn’t quite been invented. Ramsay MacDonald decided that public spending must be cut, and most Labour MPs wouldn’t vote for it, so he formed a national government largely consisting of Conservatives.

But it happened to the Conservatives, too, after the devaluation of the pound in 1992. That had an effect on public opinion, but it wasn’t a sea change. The reasons the Tories were crushed in a landslide five years later were largely to do with the state of the parties. The Tories were divided over Europe and the Maastricht Treaty, while there was no alternative to John Major as leader – although Michael Heseltine might have held back some of the collapsing rubble in 1997. And Labour, by now endowed with a burning will to attain power, was lucky to find itself with an exceptional leader.

A decade later, it was Labour’s turn to give up. When I spoke recently to someone who served at the heart of Gordon Brown’s government, they said that, from the time of “the election that never was” in 2007 to the 2010 election, “we never thought that we were going to win”. Depriving the Conservatives of a majority was a better result than any of them expected.

The hung parliament of 2010 could have opened up opportunities for determined and creative Labour leaders, but by then too many Labour MPs just wanted to go and lie down in a darkened room. David Miliband and Alan Johnson came over all Callaghanesque, saying that the people had voted against Labour and there was nothing they could do.

They didn’t even try to put together an alternative to a Con-LibDem coalition; neither did the party’s official negotiators try hard enough. They were undermined by Brown, who should have resigned straight away to make way for a new leader, and by Labour MPs, who said they would vote down any attempt to change the voting system to please the Liberal Democrats.

It wasn’t even noticed at the time, but something similar happened in 2017, when the Conservatives were again deprived of a majority. Instead of a Labour Party hungry for power, prepared to do a deal with the Democratic Unionist Party, we had an opposition led by a person of “principle” to whom I don’t think it even occurred that such a deal might be possible.

Contrast Labour’s pessimism in the 2007-10 period with the spirit of those around Rishi Sunak now: they know that the odds are against them at the next election, but they think they have a chance of winning. I don’t think this optimism is feigned, or mere whistling to keep up their spirits. They genuinely think that, if they are lucky with the economy, if they throw everything at trying to fix the NHS, and if they can do something about the Channel crossings, they have a chance.