How much power is too much power? On Sunday, the governing coalition around the National Regeneration Movement — also known as Morena — of President Andrés Manuel López Obrador trounced the opposition alliance in thousands of federal, state and local races in Mexico’s biggest election ever. Claudia Sheinbaum, Morena’s presidential candidate, secured at least 58.8 percent of the vote, unheard of since the advent of multiparty democracy at the turn of the century, beating her main rival,...