Sheinbaum, 61, is poised to make history as Mexico’s first female president and first Jewish head of state. Polls a week before Mexico’s election show her enjoying a wide lead over the next candidate, the conservative entrepreneur Xóchitl Gálvez. Still, after nearly a quarter-century in the public eye, she remains an enigma, known mainly as the low-key protégé of President Andrés Manuel López Obrador, the charismatic leader known as AMLO. AMLO doesn’t speak English and dislikes traveling abroad. Sheinbaum did postdoctoral research at the University of California at Berkeley; her sister and daughter live in the United States. The president,...