Former Mexico City Mayor Andres Manuel Lopez Obrador skipped Mexico's first presidential debate in April — and it cost him. The race's front-runner saw his once comfortable lead in opinion polls evaporate in a matter of weeks.

Now Lopez Obrador is running neck-and-neck with Felipe Calderon of outgoing President Vicente Fox's conservative National Action Party, making Tuesday night's second and final debate all the more important.

CountryWatch: Mexico

"The race is so tight that the debate has to have a major impact," said pollster Jorge Buendia of Ipsos Bimsa. "All eyes will be on Lopez Obrador. The test will be whether he can keep his calm and persuade voters he is not too radical."

Lopez Obrador, running with the leftist Democratic Revolution Party, said he skipped the first debate because he preferred to speak face-to-face with voters rather than spar with his rivals on national television.

In his absence, Calderon was forced to defend himself against a barrage of attacks from the race's third major candidate, Roberto Madrazo, but also managed to speak about some of his key proposals for the country. He was seen by many as the evening's winner.

Madrazo's Institutional Revolutionary Party ruled Mexico from when it was founded in 1929 until losing to Fox in 2000, but he trails in recent polls and is badly in need of a boost. Some say he could shift his attacks to Lopez Obrador in the second debate.

Calderon and Lopez Obrador, meanwhile, have been using television ads to tear into each other for weeks. In one, Calderon's campaign showed the silver-haired Lopez Obrador giving fiery speeches and then flashed images of populist Venezuelan President Hugo Chavez, an impassioned critic of the United States.

Another Calderon spot showed an old photo of Lopez Obrador with Zapatista rebel leader Subcomandante Marcos, who led an armed uprising in southern Mexico in the name of Indian rights and socialism in 1994. The images were overlaid with the words "danger," "crisis" and "liar."

"Calderon is playing on the fear of the middle classes," said writer Guadalupe Loaeza, a Lopez Obrador sympathizer. "He is playing on their fear of change, their fear of losing their privileges, their fear of the millions of brown-skinned poor people."

Last week, the Federal Electoral Institute ordered Calderon to withdraw the spots, ruling they made unsupported accusations. Chavez has denied supporting Lopez Obrador's campaign, while Marcos has called all the presidential candidates mediocre.

The electoral institute also has ordered Lopez Obrador to suspend ads linking Calderon to a scandal surrounding Mexico's bailout of banks during the country's 1994 financial crisis.

Lopez Obrador, a former Indian rights activist from the southeastern state of Tabasco, has often used confrontational rhetoric and organized mass marches. But since launching his campaign in January, he has adopted a more moderate tone, promising to be financially responsible and treat the United States with respect.

In a recent radio interview, he said he is closer to the moderate leftist presidents of Spain and Brazil than strident anti-American leftists Chavez and Bolivian leader Evo Morales.

However, Lopez Obrador has a history of losing his cool.

When Fox in April warned about the dangers of a populist government, Lopez Obrador told the president to "shut up chachalaca," referring to a noisy bird. Pollsters say the outburst was one of the most damaging moments in Lopez Obrador's campaign.

In Tuesday's debate, Calderon, Madrazo and two small-party candidates are likely to try and push Lopez Obrador's buttons.

"They will try and provoke him to make him look angry and aggressive," said political analyst Jose Antonio Crespo. "Let's see if Lopez Obrador can use self-restraint."