MEXICO CITY — During several lively campaign rallies on a recent day in Mexico City, the leftist front-runner in the Mexican presidential race told his supporters what they wanted to hear, time and again.

Andrés Manuel López Obrador, a former mayor of Mexico City, railed against corruption and what he called “the mafia of power.” He vowed to combat violence and impunity. He decried inequality and promised wage increases for the working class. He pledged to increase investment in social services for the young and the old.

But there was one issue notably absent from his populist oratory: international relations, specifically those with the neighbor to the north.

Mr. López Obrador’s only mention of President Trump — who has spent the past two years hectoring Mexico, stripping the bilateral relationship of much of its hard-fought good will — was a throwaway joke about selling him the Mexican presidential plane in an effort to pare back executive branch luxuries.