Andrés Manuel López Obrador and his Morena party won big in this month’s Mexican presidential elections by promising to drain the swamp. But they’re already engulfed in a scandal rivaling the misdeeds that put the Institutional Revolutionary Party (PRI) in such bad odor.

It’s the job of Mexico’s electoral watchdog, known as the National Electoral Institute or INE, to keep political parties from buying votes. On July 18 INE announced that Morena broke the rules when it set up a charitable fund ostensibly to support victims...