Mexico's conservative governing party and the leftist coalition that finished second in the July 1 presidential election said Thursday that they will demand an investigation of the finances of the Institutional Revolutionary Party, or PRI, whose candidate ostensibly won the contest.

The respective chairmen of the conservative National Action Party and the leftist PRD, Gustavo Madero and Jesus Zambrano, held a press conference in the capital to announce their plans to file a formal complaint with the Attorney General's Office.

The party chiefs also disclosed that they asked electoral authorities to complete their probe of PRI campaign finances before the Aug. 31 deadline for Mexico's TEPJF electoral court to certify the PRI's Enrique Peña Nieto as president-elect.

Peña Nieto won the election with 38.21 percent of the vote, while the PRD's Andres Manuel Lopez Obrador took second place with 31.59 percent, according to the final official tally.

But the leftist hopeful filed a motion last week with the TEPJF seeking to have the election overturned, pointing to reports of vote-buying by the PRI and other violations.

The accusation of money laundering does not necessarily imply that the PRI used money from organized crime, as the concept covers any "irregular" financial operations involving public or private funds, National Action's Madero said Thursday.

He may have been seeking to distance himself from comments made a day earlier by a member Lopez Obrador legal adviser Jaime Cardenas, who spoke of "triangulation" among front companies to supply the Peña Nieto campaign with money that came from PRI-controlled state governments "or from organized crime."

National Action, whose candidate, Josefina Vazquez Mota, finished third in the presidential election, approached electoral authorities on June 14 with a request to freeze the PRI's accounts with Monex bank on suspicion of violations of campaign finance law.

While election officials rejected the request, it later emerged that PRI operatives handed out prepaid debit cards from Monex in an apparent attempt to buy votes.

"We have the certainty that here is strong, resounding evidence of illegal handling of resources during this campaign," Madero said at Thursday's session with reporters.

PRD chairman Zambrano said Madero was showing political courage by joining the left in demanding an investigation of the PRI's financial activities.

The PRI, which governed Mexico from 1929 to 2000, lost the 2000 presidential election to National Action and finished third in 2006.

During its 71 years of largely unchallenged hegemony, the PRI relied mainly on patronage and control of organized labor and the mass media, though it was not above resorting to outright vote-rigging and even violence.

Despite their ideological differences, National Action and the PRD have a history of working together at the state and local levels to battle the PRI.

That relationship was put under strain when Lopez Obrador lost the 2006 contest to National Action's Felipe Calderon by 0.56 percent of the vote, a result the leftist candidate refused to accept. EFE