Polls show the once-reigning Institutional Revolutionary Party, or PRI, comfortably in the lead with its fresh-faced candidate, former Gov. Enrique Peña Nieto. Student demonstrations are planned nationwide to coincide with the debate, potentially raising the stakes. The grass-roots social media movement known as I Am 132 is gaining reach and influence as it has gone after the PRI and big Mexican media companies, especially Televisa, the nation's biggest TV network and the target of accusations that it has sought to manipulate public opinion in Peña Nieto's favor.

Four candidates gather Sunday night in Guadalajara for the second and last officially scheduled debate of the campaign. The session offers another chance for one of the trailing contenders to alter the shape of the race.

MEXICO CITY -- It's looking more and more like a two-way race in Mexico's July 1 presidential election. That is, a contest between Mexico's former ruling party and everyone opposing its return.

Enrique Peña Nieto

The PRI (Partido Revolucionario Institucional in Spanish, and pronounced "pree") ruled Mexico for much of the 20th century until it was ousted by voters in 2000 after decades of corruption, scandal and economic crises. This year, it is poised to return to power.

Peña Nieto, former governor of the central state of Mexico, faces two main rivals: Andres Manuel Lopez Obrador, representing a leftist coalition, and Josefina Vazquez Mota, a conservative. The parties are poles apart ideologically but united in their disdain for the PRI, with histories of struggle against its authoritarian leanings.

The Peña Nieto campaign has been forced to confront corruption allegations against former PRI state governors and accusations that the party shells out cash for favorable polls and media coverage. The party and Peña Nieto's campaign on Friday were denying a newly published report that his office appeared to have paid millions to Televisa after taking the governor's office in 2005, for favorable coverage on TV and in Televisa publications.

Nevertheless, the PRI's campaign has been run smoothly and holds a double-digit lead in most polls.

Its main opponents -- Lopez Obrador's leftist coalition led by the Democratic Revolution Party, or PRD, and Vazquez Mota's National Action Party, or PAN -- have long ruled out forming a broader alliance to challenge the PRI in the presidential vote. But a grass-roots student-led movement opposed to the PRI has provided a jolt to the race and is a likely factor in Peña Nieto's slight dip in support in several polls.

Though his lead remains strong, analysts note that as many as a quarter of voters say they remain undecided.

Andres Manuel Lopez Obrador

Lopez Obrador, who barely lost in the 2006 race and never accepted the results, is running again and has moved into second place in most polls.

On Wednesday night, during a tense conversation with panelists of Televisa's "Third Degree" program, Lopez Obrador said calmly that he was sure to "win again" and that his campaign's tracking polls showed he had moved two points ahead of Peña Nieto (link in Spanish).

Even if that proves to be a bluff, recent developments are a reminder that Lopez Obrador, a former mayor of Mexico City, commands a formidable base and that he has been boosted by the I Am 132 movement.

Lopez Obrador's opponents are eager to remind voters of his less than flattering image as a polarizing figure. Attack ads against him were rolled out this week, including one by the PAN that doctors a fragment of a recent Lopez Obrador speech to make it seem as if he supports armed revolutions (link in Spanish).

Lopez Obrador called the new ads the start of a "dirty war."

Josefina Vazquez Mota

The PAN also has some supporters in the I Am 132 movement. But Vazquez Mota, a former Cabinet member under the two PAN presidencies since 2000, has lost traction overall, slipping to third in the polls.

The first woman to bid for Mexico's presidency from a major party, Vazquez Mota has sought to convince voters she would be a "different" president, but she has so far been unable to connect broadly with female voters.

Perhaps worse, she's taken hits from senior figures in her own party, some of whom appear to be breaking for Peña Nieto.

On Sunday, former PAN President Vicente Fox suggested he would support Peña Nieto and that other party members should do the same. Vazquez Mota's campaign denounced Fox vehemently for the apparent betrayal. Fox, after all, is the man whose victory removed the PRI from power.

A former PAN president also said he would be supporting Peña Nieto.

The Student Factor

The presidential campaign took on a new player May 11, when Peña Nieto was surprised by a tempest of jeers from students during an appearance at the private Ibero-American University in Mexico City. He and PRI leaders then came under fire when they suggested initially that the protest was staged or orchestrated by a rival party.

In reply, 131 Ibero students who participated in the protest recorded themselves holding up their university ID cards or repeating their student ID numbers. The video went viral on social networks and spawned a movement taking its name from a Twitter hashtag, #YoSoy132, or "I am 132."

Counterparts at some 35 universities across Mexico have joined. The students have held demonstrations calling for more transparency in the news media and protesting the PRI's history of corruption and repression. They have also taken aim at the soaring drug-related violence during the PAN's tenure.

The I Am 132 phenomenon has already had an impact on the race. Under pressure, the two dominant networks, Televisa and TV Azteca, agreed to show Sunday's debate on their main channels, after relegating the first debate, on May 6, to secondary channels.

Last week, the students held a "general assembly" on the campus of the National Autonomous University of Mexico and, after hours of debate, agreed on a 15-point platform, declaring the movement officially nonpartisan (link in Spanish).

Students have called for a third debate between the presidential candidates, to be organized and moderated by the I Am 132 movement. Lopez Obrador, Vazquez Mota and a fourth candidate, Gabriel Quadri, of the fringe New Alliance Party, have agreed to attend. Peña Nieto has not.

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-- Daniel Hernandez

Photo: A Mexican university student member of the I Am 132 movement holds a banner in Mexico City on May 31. Credit: Omar Torres/AFP/Getty Images