SELMA, Ala. — By early this year, around the time a prosecutor called President Joe Biden a “well-meaning, elderly man with a poor memory,” Vice President Kamala Harris already knew something had to change. It was up to her, she had told allies, to finally distinguish herself in her job — something she had been struggling to do for more than two years — and reassure American voters that the Biden-Harris ticket was still a safe bet. She had been feeling sidelined in the early stages of the campaign, one adviser said, and she wanted a bigger role. She fled...