Updated

The conservative National Action Party joined Mexico's main leftist party Thursday in accusing the winner of the country's July 1 presidential election of campaign wrongdoing, saying it has "strong and conclusive" evidence of the use of illicit funds.

National Action leader Gustavo Madero said his party is demanding that electoral authorities investigate the purported use of pre-paid debit cards by apparent winner Enrique Pena Nieto's campaign to disburse an estimated 108 million pesos ($8.2 million) in funds. That alone would be about a third of all the money the candidate was legally allowed to use in the race.

Pena Nieto of the old ruling Institutional Revolutionary Party, or PRI, won the election with about a 6.6 percentage-point lead over the second-place finisher, leftist Andres Manuel Lopez Obrador.

National Action and the Lopez Obrador's Democratic Revolution Party normally don't agree on much, so Thursday's joint news conference between Madero and Democratic Revolution leader Jesus Zambrano was a rare occurrence.

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.

A secretary at that lawyer's office who refused to give her name said that she had never heard of one of the companies that bought the debit cards, and which listed the office as its address on receipts obtained by Lopez Obrador's party. A relative of that lawyer allegedly played a role in Pena Nieto's campaign, something the PRI denied.

Strange circumstances seemed to surround the debit card purchases; the address listed on the receipts for another big purchaser of the cards, an import, export and trading firm, listed as its address an office which is occupied by a dentist. The dentist's receptionist declined to comment.

"Every day it is becoming clearer ... the link between this money and PRI activists and Enrique Pena Nieto," Madero said.

No big-name PRI member is listed on the receipts, which Lopez Obrador's campaign made public Wednesday.

"The PRI categorically denies the use of illicit funds in the presidential campaign, or having exceeded campaign spending limits," PRI party leader Pedro Joaquin Coldwell said Thursday.

PRI officials denied the campaign had any contracts with Monex, the Mexican bank where the suspect cards were allegedly bought. But they said that about 66 million pesos ($5 million) in routine expenses for party workers had been paid through debt cards contracted through another company.

The country's top electoral watchdog agency said in a statement Thursday that it had not yet found any proof of illegal acts committed by any party, but stressed that investigations were continuing.

On Thursday, leaders of a student movement called "I Am 132" that sprang up before the July 1 elections to oppose campaign violations and the PRI's return presented a resolution that warns of "a serious risk of social explosions" if the electoral courts uphold the election results.

The movement's leadership commission said the resolution must be approved by its national convention at the end of July, but commission member Edgar Tafoya expressed confidence that members will approve the position calling for marches and petition drives to press its demands.

More radical farm and labor groups say they are planning to block highways and government offices to prevent Pena Nieto from taking office Dec. 1. Tafoya said the student convention will consider those plans.

The National Action Party, whose candidate came in third in the presidential race, has recognized its defeat and says it won't ask to have the elections results overturned in court. President Felipe Calderon, who belongs to National Action, has met with Pena Nieto and promised a smooth transition if the courts uphold his victory.

But Lopez Obrador's party has filed a challenge with an electoral court seeking to have the vote annulled.

Madero also said that before declaring the elections valid, electoral authorities must investigate whether front companies bought the debit cards to hide the origin of the money. He also called on electoral authorities to investigate any cash payments made by state governments during the campaign.