Late on Sunday morning, Vice President Kamala Harris summoned a small clutch of her closest advisers and allies to the Naval Observatory, where she lives and works, with little notice and even less information. President Biden had informed Ms. Harris earlier that morning that he was withdrawing from the race. The vice president had assembled her team so that the exact moment Mr. Biden formally quit, at 1:46 p.m. — one minute after the president had informed his own senior staff — they were ready to go. Time was of the essence. A sprawling call list of the most important...