When Gordon Brown was about to take over as Labour leader and prime minister, I bumped into Nick Brown, who was about to become chief whip, and we had a brief discussion about the chances of anyone standing against his candidate. “You can’t beat somebody with nobody,” he said. It is an American saying dating from around 1900.

He was right. The Blairites didn’t have a candidate. That remained the case during Brown’s time in No 10. Neither Alan Johnson, the home secretary, nor David Miliband, the foreign secretary, was willing to push themselves forward. In fact, the “cooked goose plot” of January 2010, hatched by Harriet Harman and Patricia Hewitt, came closer to toppling Brown than anyone realised at the time, but its critical weakness was that it was a contest between somebody and nobody.

What is surprising about the Conservative Party’s leadership rules is that they turn the old American adage on its head. They are explicitly designed to allow a sitting prime minister to be defeated by “nobody”, in that the important stage of a challenge is a vote of confidence in the leader among Tory MPs. This is a thumbs up or a thumbs down on the incumbent, with the alternative unspecified.

I asked Hewitt recently if she thought Gordon Brown would have been brought down if Labour had had the same rules as the Conservatives — 50 or so confidential letters to trigger a vote of confidence by secret ballot — and she said yes.

In that case, there would have been two somebodies waiting offstage, in that Labour MPs would have been acting in the knowledge that either Alan Johnson or David Miliband would have been a capable prime minister.

Just as in 2019, Tory MPs knew that the alternative to Theresa May was probably Boris Johnson. (She won her vote of confidence, but then resigned anyway when she failed to get her Brexit deal through parliament).

In January this year, Boris Johnson was vulnerable partly because Rishi Sunak was a “somebody” to whom the Tory party could turn, even if a vote against Johnson on a confidence motion would not have been explicitly a vote for him.

For some time, ever since Sunak burst on to the scene as a fully formed prime-minister-in-waiting as the new chancellor at the start of the pandemic, the structure of British politics had been shaped by his “somebodyship”.

All that time, the Tories seemed to be in an unusually strong position, in that they had a prime minister with some unusual election-winning abilities, but if he turned out to be no good, they had a spare of a more conventional kind, who was insanely popular. Now all that has changed. All that seemed so solid has melted into air. Which reminds us that prediction is a hazardous business, but I predict that Sunak will come bottom of the league table of cabinet popularity in the next Conservative Home poll of party members — Priti Patel’s Rwandan adventure will lift her from last place, leaving the chancellor exposed.

Sunak is paying the price for his disastrous mistakes. He was popular because he did the pragmatic, centrist thing during the pandemic, using “the overwhelming might of the British state” to protect citizens from the economic consequences of lockdowns. But then he veered off the Blairite path, and presented a spring statement that was all about seeing off the leadership threat from Liz Truss, the foreign secretary, instead of delivering the people’s priorities. Tony Blair warned his successors that “if we departed a millimetre from New Labour we were going to be in trouble” and so it came to pass. Announcing a penny cut in the basic rate of income tax two years in advance was a mistake and Johnson was too weak to stop him.

Since then, Sunak’s troubles have multiplied and the prime minister’s strength has returned. The structure of politics has realigned.