MEXICO CITY — Andrés Manuel López Obrador promised a revolution. Never before had a leftist won the Mexican presidency in a democratic election. At his 2018 inauguration, he declared that he wasn’t just ushering in a new government — he would be bringing a “political regime change.”

       He said he would attack corruption and impunity, reverse decades of neoliberal policies that favored the wealthy, and end a war on drugs that had turned Mexico into a cauldron of violence.