The United States is not the only North American democracy at risk from a president’s belief that he is a victim of election-rigging. In Mexico, left-wing populist Andrés Manuel López Obrador lost the 2006 presidential election by less than one percentage point, cried fraud and refused to concede even after tribunals unanimously rejected his claims, and mobilized supporters to blockade a busy thoroughfare in the nation’s capital. Though Mr. López Obrador ultimately relented and presidents from other parties governed through 2018, he remained obsessed with 2006. Now that he is president — having won an undisputed election in 2018 — Mr. López Obrador is bent on remaking the electoral system he still blames for cheating him more than 16 years ago.