MEXICO CITY—Mexican leftist presidential candidate Andrés Manuel López Obrador, the front-runner ahead of July elections, would respect a renegotiated North American Free Trade Agreement if the U.S., Mexico and Canada reach a deal before the vote, according to a top aide.

Talks to redraw Nafta are at a critical point, with negotiators from the three countries in permanent session in Washington, D.C. Differences remain, particularly on how to increase regional content in the auto industry, but some observers say an agreement...