Both parties claim there was widespread overspending and vote buying by Pena Nieto, a charge he denies. They say they don't know where the money for Pena Nieto's campaign came from, but claimed the way it was apparently channeled through front companies suggested money laundering or other illicit activities.

Lopez Obrador had previously accused the party of giving out thousands of gift cards redeemable at a supermarket chain to people ahead of the vote. Shoppers nearly stripped some shelves at a Soriana store in a poor Mexico City district in the days following the July 1 election.

National Action first discovered the pre-paid debit cards, which allegedly were used to pay PRI district representatives thousands of pesos. Lopez Obrador's party later uncovered dozens of receipts for mass purchases of the cards. Those receipts listed addresses for apparent front companies, but one of the addresses on the receipts is the same as the office of a prominent former PRI official and lawyer.