( CNN ) -- The runner-up in Mexico 's presidential election announced Thursday that he was filing a legal challenge to invalidate the vote . Andres Manuel Lopez Obrador of the Democratic Revolution Party has said that presumptive President-elect Enrique Pena Nieto and his party bought millions of votes in the election , an accusation that party officials have denied . Lopez Obrador has said a partial recount was not enough to erase his doubts about the vote . `` To proceed in another way would be to give up our fundamental rights , '' Lopez Obrador told reporters at a news conference . A tally of returns released by Mexico 's Federal Election Institute last week confirmed that Pena Nieto of the Institutional Revolutionary Party , or PRI , received 38.21 % of the votes . Lopez Obrador garnered 31.59 % of the votes , election officials said . The tally has been completed , but the results remain unofficial until the country 's electoral tribunal ratifies them . The tribunal will have until September 6 to complete its investigation and ratify -- or reverse -- the election results . The new president will be sworn in on December 1 . Lopez Obrador has criticized the election and refused to concede repeatedly over the past week , echoing comments he made in 2006 when election authorities said the leftist candidate narrowly lost the presidential vote to Felipe Calderon . After that election , the former Mexico City mayor claimed election fraud and never conceded . Back then , Lopez Obrador called himself `` the legitimate president of Mexico , '' and his supporters protested nationwide . In Mexico City , they staged sit-ins and blockades . Officials have called this year 's election the most transparent in Mexico 's history . It was the first election in which scanned copies of district-by-district election returns were posted on the Internet . But accusations have arisen of electoral manipulation by the PRI . Opponents of the PRI said they have video and photo evidence of the party buying votes through thousands of cards that could be redeemed for products at a chain of supermarkets . The PRI has called the claims a farce and accused political opponents of staging the videos and photographs purported to show vote-buying . CNN 's Samira Said and Nick Parker contributed to this report .
Andres Enrique Pena Nieto is refusing to concede in the presidential election . He says he is filing a legal challenge because of allegations of vote-rigging . Election officials : Manuel Lopez Obrador won 31.59 % of the votes , to Manuel Lopez Obrador 38.21 % . The results remain unofficial until the country 's electoral tribunal ratifies them .