The 2006 presidential elections pushed Mexico’s fledgling democracy from relative stability into a period of tension and uncertainty. Left-wing candidate Andrés Manuel López Obrador lost by a razor-thin margin to the conservative Felipe Calderón. He claimed fraud, took his followers to the streets in massive protest, and finally declared himself the “legitimate president” of Mexico. The article examines the contingent and structural sources of the post-electoral dispute. Allegations of vote-rigging were unfounded, yet credible, due to two principal factors: (1) a self-reinforcing spiral of distrust among political actors, and (2) discrete failures by the institutions of electoral governance and dispute settlement that provided easy pretexts for the loser’s discourse of fraud.