MEXICO CITY — Mexico’s largest political party is calling for an investigation into why criminal charges against Mexico City’s mayor were dropped.

Leaders of the Institutional Revolutionary Party, or PRI, are upset President Vicente Fox and the attorney general’s office dropped criminal charges against Mexico City Mayor Andres Manuel Lopez Obrador, El Universal reported Friday.

Obrador faced charges for defying a judge’s order in 2001 to halt construction of an access road to a hospital. But Fox fired Attorney General Rafael Macedo last week for his efforts to bring the mayor to trial.

Obrador’s opponents accused him of abusing his office to further his political ambitions. The popular mayor is considered a virtual lock to win the presidency in 2006.

The PRI may file contempt charges against new Attorney General Daniel Cabeza de Vaca for not charging Obrador, the newspaper said.