Andres Manuel Lopez Obrador said at a news conference there is evidence that money was shuffled between apparent front companies to buy pre-paid debit cards that the leftist candidate charges were used to sway millions of votes toward the winner of the race, Enrique Pena Nieto.
"I have evidence to say that funds of illicit origin were used in the Pena Nieto campaign," Lopez Obrador said.
His representative, Jaime Cardenas, described an alleged chain of funds in which four apparent front companies funneled money into pre-paid debit cards originally contracted under the names of other companies. He said that the addresses of some of the companies were false and that two of the firms were registered to men who listed their occupations as laborers.
Asked whether he was suggesting that drug money may have been involved, Cardenas said: "We don't have any conclusive proof of that. That's what the authorities have to investigate."
The company that issued the debit cards, which Lopez Obrador alleges were worth as much as a total of 108 million pesos ($8.2 million), has denied it supported any specific candidate. It did not respond to requests for comment Wednesday on whether front companies may have contracted for such cards.
Pena Nieto's Institutional Revolutionary Party, the PRI, denied Lopez Obrador's accusations and called them "an open defamation," part of a strategy by the leftist to weaken Mexico's institutions.
"All this new lie does is escalate the irrationality of his arguments, with which he tries to stay in the public spotlight and block a generational renewal of the leadership of the left," said a news release.
On Tuesday, Mexico's second-highest electoral court announced that it formally received the legal challenges seeking to annul the presidential election.
The challenges filed by Lopez Obrador appear to face an uphill struggle given the 6.6-percentage-point margin of victory for Pena Nieto.
Lopez Obrador claims Pena Nieto's campaign engaged in overspending and vote buying. The court said he submitted 58 boxes of evidence as part of the challenge.
Pena Nieto said Wednesday that his victory was legitimate and said he felt optimistic despite the protests and legal challenges against his victory.
"I hope that the political parties will assume a democratic attitude of respect for the results of these elections," Pena Nieto said. "I see a favorable scenario, I feel optimistic about achieving agreements" with other parties.
Pena Nieto met with President Felipe Calderon at the presidential residence late Tuesday.
Calderon's office said the president promised to start "an orderly transition process" after the court rules on the challenges. Members of Calderon's conservative National Action Party have also accused Pena Nieto of vote buying and overspending, but are not seeking to overturn the election results.
Pena Nieto's PRI party held Mexico's presidency without interruption for 71 years, until it lost to a National Action candidate in the 2000 election.