MEXICO CITY (AP) — Mexico's four presidential candidates are squaring off in their third and final debate before the country's July 1 elections.

The debate in the colonial city of Merida centers on economic growth, poverty, health, education and technology, based on questions culled from social media.

Front-runner Andres Manuel Lopez Obrador holds a commanding lead of as much as 20 percent in some polls over his main rivals, Jose Antonio Meade of the ruling Institutional Revolutionary Party and Ricardo Anaya who represents a left-right coalition.

Anaya opened the debate Tuesday with a salvo aimed at President Enrique Pena Nieto, pledging that "there will be a commission to directly investigate the president" for alleged corruption scandals.

Independent candidate Jaime Rodriguez rounds out the field.