MEXICO CITY (AP) — Mexico's leftist presidential front-runner is combative and unbending, and his personality-based campaign proclaiming honesty and fiery nationalism could set up a unique and combustible relationship with his northern counterpart should he win.

Andres Manuel Lopez Obrador has long preached a Mexico-first stance, accusing its government of servility to Washington and lambasting the free trade that he says has devastated his country's farms and workers. President Donald Trump's call to reduce U.S. economic ties with Mexico meshes with his demand to make Mexico less reliant on A...