In his new biography of David Butler, who probably knows more about British politics than any other living person, Michael Crick provides us with a telling anecdote. As a don in his twenties, Butler was twice summoned by Winston Churchill, then leading the Opposition in the last months of Clement Attlee’s post-war government, to discuss the new-fangled science of opinion polling. In 1950, the word “psephology” had just been invented. “Tell me young man,” Churchill asked Butler. “Do you think I have become a liability to my party?”

Theresa May must be mulling precisely the same question, particularly since her catastrophically misjudged decision last year to call a general election, which has imperiled the survival of her Conservative government.