India, June 24 -- Outside 10 Downing Street, Keir Starmer stared into the cameras and made what some at Westminster had already imagined spoken out loud. He said that he would resign as leader of the Labour Party and remain as the caretaker prime minister until a successor was nominated.

The spirit was dark and sometimes stuttering. Less than two years after getting Labour to a landslide election, Starmer said, he wasn't the right man to lead the party from here to the next general election. The news rippled through Parliament because the fall was so abrupt.

"Every decision I have taken has been about putting the country on which I hold great feelings in mind. It is for this reason that I will resign as leader of the Labour Party."

The...