Mexico presidential candidate under pressure over Televisa media scandal Media bias has become a central issue in Mexico's presidential election, which Enrique Peña Nieto is currently favourite to win. Photograph: Eduardo Verdugo/AP Mexico's leftwing presidential candidate, Andrés Manuel López Obrador, has called on Enrique Peña Nieto, the current favourite to win the election on 1 July, to come clean about the alleged purchase of favourable coverage on Mexico's biggest television network. His comments came a day after the Guardian published documents implicating the Televisa network in the sale of news and entertainment content to promote Peña Nieto's national profile when he was the governor of Mexico state and preparing his presidential bid. "They should hand over all the information, the contracts, that they haven't wanted to show," López Obrador told reporters. Of course they have them, and we need to see how much they paid, for what kind of message, and if they include all the promotion of Peña Nieto on the television. López Obrador, who represents a coalition of leftist parties called the Progressive Movement - and who in the past has also been criticised for failing to release details of his own publicity budget - said he wanted to study the documents before saying anything more. López Obrador did not mention the PowerPoint presentation mentioned in the Guardian story that detailed an apparent strategy within Televisa to destroy his first bid for the presidency in the 2006 election. Several leading Mexican newspapers and radio shows led on the Guardian report in their Friday editions, and the story has become a trending topic on Twitter in the country. The allegations are particularly sensitive in the current electoral climate, in which student demonstrators accusing Televisa of favouring Peña Nieto have turned alleged media bias into a central issue of the campaign. Televisa did not feature the allegations in its main nightly news show on Thursday despite the fact that it had already demanded a public apology from the Guardian. In a statement the network - which is the largest in the Spanish-speaking world - lamented what it called the use of "apocryphal material that has been repeatedly published and denied." The only document in the Guardian's report which had previously been published by the Mexican media was one of several versions of a promotional budget apparently developed for Peña Nieto towards the end of 2005 by a Televisa-linked company called Radar Servicios Especializados. The morning news programme on MVS Radio included an interview with a former Televisa employee who revealed an internal electronic chat about that document immediately after its publication in the weekly magazine Proceso that same year. According to the former employee, Laura Barranco, a news anchor at the channel, Carlos Loret de Mola, who has been a vocal defender of Televisa in recent weeks, wrote: "Everything, absolutely everything, is true." The Guardian has also been able to trace evidence which appears to corroborate another file, referring to a promotional campaign for the then president, Vicente Fox, also in 2005. The document states that, while the cost of the campaign was 60m pesos (£2.8m), the presidential office has been directly billed for only 3m pesos pending instructions on how to arrange payment of the rest. Official accounts for publicity spending by the presidential office for that year include a payment of 3m pesos to Radar.