MEXICO CITY—The grainy video looks like something from a surveillance camera. Two men sit at a booth in a restaurant in southern Mexico. One man calmly slides a paper bag full of cash across the table to a man sitting across from him. Both keep talking, as if nothing has happened.
The video would be unremarkable if one of the men weren’t Pío López Obrador, the younger brother of Mexico’s president, Andrés Manuel López Obrador, a leftist nationalist who has framed his political career as a crusade to end endemic corruption.