Early last September, at a restaurant a block from Mexico City’s National Palace, the leaders of the Movement for National Regeneration, known as Morena, gathered for a changing of the guard. President Andrés Manuel López Obrador, Morena’s founder and figurehead, handed a wooden stick adorned with colorful ribbons to his protégée, Claudia Sheinbaum, the former mayor of Mexico City. She had just won the primary to run as Morena’s presidential candidate in the June 2 election.