President Felipe Calderon's conservative National Action Party (PAN) will formally request that electoral officials probe allegations of vote buying in Mexico's July 1 presidential election.
Enrique Pena Nieto from the Institutional Revolutionary Party (PRI) won the ballot by a solid margin, but opponents claim he engaged in a massive scheme to purchase votes ahead of election day.
"We recognize the official results published up to now," PAN leader Gustavo Madero said late Monday, "but there is important evidence of severe inequality and of grave faults."
Madero, who spoke after a lengthy meeting with senior party leaders, said there was evidence that the PRI broke campaign finance spending limits, used off-the-books financing, and directly bought votes.
Pena Nieto, 45, won the election with 38 percent of the vote, followed by leftist Andres Manuel Lopez Obrador with 31 percent and the PAN's Josefina Vazquez Mota with 25 percent, according to the final official vote result.
Lopez Obrador said Monday he was gathering evidence to ask vote officials to declare the presidential election invalid, claiming the PRI bought five million ballots. Pena Nieto obtained 3.3 million more votes than Lopez Obrador.
Madero however made it clear that the PAN will not join the leftist legal case, and will not ask for the results to be invalidated.
Calderon on Monday said the alleged vote buying was "unacceptable" and something that "had to be immediately corrected" and punished.
The PRI, in power for seven decades ending in 2000, rejected the accusations. Breaking campaign spending limits and vote buying are punishable by fines under the law.
Mexico's Electoral Tribunal will accept challenges to the vote until Thursday, and has until September 6 to issue a verdict.
Lopez Obrador came in second in the 2006 presidential election, losing by less than one percentage point. He then led protests that virtually paralyzed Mexico City for more than a month, claiming fraud.
Tens of thousands of demonstrators marched through Mexico City on Saturday protesting Pena Nieto's victory.
Pena Nieto, a former state governor, is scheduled to take office in December. He will inherit a country engaged in a brutal drug war and an economy struggling to create jobs.