Theresa May sat quietly sipping tea in the corner of a small room at the Conservative Party conference in Manchester last October. The British prime minister had just delivered a speech meant to restore her authority over a ruling party that was sliding in the polls and riven by the country's decision to leave the European Union.

It had gone disastrously. A prankster handed her a fake notice of dismissal, she struggled with a persistent cough and even the slogan behind her, "Building a country that works for everyone," failed when several letters fell to the floor, prompting giggles from the audience.

Her grip on power had never looked weaker.