Mexican leftist presidential candidate Andres Manuel Lopez Obrador said Monday he will mount court challenges against the results of the July 1 elections, claiming vote-buying and campaign overspending by the winner of official vote counts, Enrique Peña Nieto.

The announcement comes amid rising calls to investigate what appears to have been the distribution of thousands of pre-paid gift cards to voters prior to the elections, and allegations by Lopez Obrador's supporters that some state government officials passed funds to Peña Nieto's campaign effort.

Lopez Obrador finished about 6.6 percentage points behind Peña Nieto of the old ruling Institutional Revolutionary Party, or PRI.

Even President Felipe Calderon, who suffered his own post-electoral dispute with Lopez Obrador following the 2006 elections, called for an investigation into the vote-buying allegations and electoral reforms to prevent such practices in the future.

"The issue of the (gift) cards, the accounts and all, is an issue that I don't know if it will be sufficient to overturn an election with these margins, but it should be resolved," Calderon said.

"We cannot just accept this and say 'a little bit doesn't hurt'," Calderon told a local radio station. Referring to excessive campaign spending, Calderon said "every day more evidence is coming out, it's worrisome."

"I think electoral authorities should have a change of heart and punish this," Calderon said. Referring to possible measures against campaign overspending for future elections, he said "I think this is the big reform that Mexico still lacks."

And Calderon suggested for the first time the issue could play a role in Peña Nieto's transition period; he is scheduled to take office Dec. 1. He won office largely on promises the PRI had left behind the vote-buying and repressive tactics it used to hold on to Mexico's presidency from 1929 to 2000.

"I think this could be a smooth transition, if this is resolved correctly," Calderon said of the allegations.

However, it is unclear whether Calderon's conservative National Action Party will join Lopez Obrador in the court challenges.

National Action quoted party leader Gustavo A. Madero as saying over the weekend that if electoral authorities uphold the results, it would give "legality, but not legitimacy to the presidential election, due to the strong questions about the way millions of votes were obtained."

"We Mexicans were witness to a sophisticated operation of vote-buying, in which Institutional Revolutionary Party governors could be implicated," he said.

However, Madero did not say whether his party would supporter join in the court challenges, noting only that National Action was open to discussing the matter. The party's candidate, who came in third, has already accepted the vote results.

Lopez Obrador told a Monday press conference that "we have evidence to say, and at the right time, to prove, that about five million votes were bought."

In addition to previous charges that the PRI handed out thousands of pre-paid gift cards to voters in slums on the outskirts of Mexico City, Lopez Obrador's supporters showed a video Monday in which checks from the northern state of Zacatecas can be seen made out to a prominent local member of the PRI. The video also shows receipts and spending reports showing expenditures for what appear to be campaign events, rallies and travel expenses.

Asked about the new allegations Monday, PRI party spokesman Eduardo Sanchez said Eduardo Sanchez said "Lopez Obrador is seeking to justify his defeat by saying five million Mexicans sold their votes."

While final vote counts and recounts were finished Sunday for both the presidential and congressional races, it is still not clear exactly how many legislative seats each party won, because in Mexico's electoral system, some legislators are elected directly and others are assigned their seats according to the percentage of total votes won by their party, a calculation based on a complex formula. The results of that calculation aren't expected for weeks.

Madero said he did not believe the PRI had won a majority in congress, while Calderon said the PRI appeared to be "nearing 50 percent."

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