| MEXICO CITY
MEXICO CITY Mexico's presidential contenders lock horns for a final televised debate on Sunday night with front-runner Enrique Pena Nieto under mounting pressure from student-led opposition.
Pena Nieto, candidate of the opposition Institutional Revolutionary Party (PRI), has led polls for the July 1 election for over two years, but in the past month the race has tightened due to growing misgivings about the possible return of the PRI.
Lifted by a wave of student opposition to Pena Nieto and the PRI, leftist hopeful Andres Manuel Lopez Obrador has surged into second place in most polls. He insists he is on track to claim the presidency after narrowly losing the 2006 election.
Ahead of the debate, thousands of young people staged a demonstration against Pena Nieto in Mexico City, following another protest in the capital on May 19.
Organizing through online social media, the protesters have awakened memories of the PRI's checkered past, playing on the party's reputation for corruption and authoritarianism when it ruled Mexico from 1929 to 2000.
Less than a month ago, the PRI seemed certain of victory but since opposition to his party's return began to increase, Pena Nieto's poll ratings have wobbled. Nonetheless, he still holds a double-digit lead in most surveys.
A voter survey by pollster BGC published on Thursday by newspaper Excelsior gave Pena Nieto support of 42 percent, a lead of 14 points over both Lopez Obrador and Josefina Vazquez Mota, candidate of the ruling National Action Party (PAN).
"The debate could be a game changer," said George W. Grayson, an expert on Mexico at the College of William & Mary in Virginia. "It's hard to tell because we've never seen Pena Nieto in a strong confrontational situation."
However, the potential to inflict damage on Pena Nieto would likely be limited by time constraints and because Vazquez Mota needed to attack Lopez Obrador too, Grayson said.
"I think he's going to hold his own," he said.
The PAN, which ousted the PRI in 2000, has struggled under President Felipe Calderon to cope with drug-related violence and a weak job market, allowing Mexico's old rulers to regroup.
The youth-led opposition to the centrist PRI has stressed the party's links to entrenched interests like Mexico's dominant broadcaster Televisa, pointing to media reports alleging that Pena Nieto owed his rise to deals with the media group.
The latest allegations surfaced on Thursday in British daily The Guardian, which said it had acquired documents that appeared to show that Televisa had helped to raise Pena Nieto's national profile and had worked to sabotage Lopez Obrador's 2006 bid.
Televisa rejected the claims and demanded an apology from the newspaper, while Pena Nieto dismissed the story as "totally false" and a "rehash" of old allegations.
The Guardian story generated massive interest in online social media in Mexico and the left-leaning daily La Jornada devoted several pages of its Saturday edition to it.
Fernando Dworak, a Mexico City-based political analyst, said the report was unlikely to have a big impact as The Guardian had been unable to confirm the documents were genuine and because a significant part of the claims had been known since 2005.
"Lopez Obrador presented part of what was in The Guardian in the first debate. And it didn't have an effect," he said.
What has changed since the first debate on May 6 is the visibility of opposition to Pena Nieto and the PRI.
On May 11, students at Mexico City's private Ibero-American University heckled and booed the PRI candidate for his record as governor of the State of Mexico between 2005 and 2011.
Afterward some Pena Nieto supporters questioned whether the Ibero demonstrators really had been students, helping to galvanize his youth critics. Leading the online opposition that emerged was a movement called Yosoy132 ("I am 132").
Explicitly anti-PRI, the group took its name from 131 people who identified themselves on YouTube as students at the protest, and it has since helped to organize marches against Pena Nieto.
The student leaders say they are independent, although Lopez Obrador has championed their cause and newspaper El Universal published a photo of his son in a Yosoy132 T-shirt on Sunday.
Activists at Sunday's march slammed Televisa and Pena Nieto, attacking him for the police brutality that took place under his watch when the State of Mexico government put down an outbreak of civil unrest in the town of San Salvador Atenco in 2006.
"We don't want Pena Nieto. It would be a return to the past," said Viviana Diaz, a 34-year-old education worker. "I want a better life for him," she added, pointing to her son.
University professor Rodolfo Dominguez carried a sign saying "Mexican Spring" and said he was proud of Mexico's youth.
"What you're seeing now is unprecedented in this country three weeks before a presidential election," the 61-year-old said. "I think it'll lead to a change in who people support."
Others commemorated the anniversary of the notorious killing of student protesters in the capital by a paramilitary group known as the Halcones (Falcons) on June 10, 1971.
Estimates vary on how many died in the attacks by the Halcones, who prosecutors said were created by the PRI government. Investigators have put the figure in the dozens.
(Additional reporting by Gabriel Stargardter; Editing by Bill Trott and Eric Walsh)