Kamala Harris, the 49th vice president of the United States, announced her 2024 campaign for president on July 21, 2024. On that date, incumbent president Joe Biden withdrew his re-election campaign and immediately endorsed her to replace him in his place as the party's presidential nominee.
I fully understand that there are mechanics involved. States, and committees, influential party figures. Nothing is automatic, and people engage in maneuvering to make any nomination official. I understand that Biden could not "wave a magic wand" and just make Harris the nominee. But I think it is true that Biden basically said, "I'm dropping out and I endorse Kamala Harris as my repalcement" and that complicated everything for the Democrats.
I assume the people who pressured Biden to drop out, simultaneously pressured him to endorse Harris.