Enrique Pena Nieto, the candidate seeking to return Mexico's former ruling party to the nation's highest office after 12 years, was accused of lying about his record as governor of the state of Mexico and maintaining ties to unsavory elements of his Institutional Revolutionary Party, or PRI.
He came under particularly relentless attack from Andres Manuel Lopez Obrador, the third-place candidate of the leftist Democratic Revolution Party. Lopez Obrador flourished pictures of Pena Nieto with disliked political figures from the past PRI administrations, attempting to paint the telegenic 45-year-old candidate as a tool for the return of the autocratic and corrupt party that ruled Mexico for seven decades before it was turned out of power.
"Who is Enrique Pena Nieto, really?" Lopez Obrador asked, as he held up photos of Pena Nieto grinning alongside one of a political mentor accused of stealing public funds, and a former president the leftist described as one of the clique of power-brokers behind the PRI candidate.
Pena Nieto has a reputation for scrupulously preparing but making errors while improvising. He has been criticized for limiting his public exposure and canceling appearances at academic forums, part of a strategy of avoiding confrontations with his rivals.
In the rare face-to-face confrontation, Lopez Obrador and the second-place candidate, Josefina Vazquez Mota of the ruling National Action Party, appeared to be hoping to force a damaging gaffe from the man who leads the race by as many as 20 points in most polls.
Mexico's first major female presidential candidate showed the cameras a blown-up cover of an Economist magazine profile of Pena Nieto that she said showed that he had lied about reducing homicide figures during his term as governor of the state of Mexico.
"There are two ways of lying," she said. "One, not telling the truth and the other, making up statistics."
But Pena Nieto avoided any significant error, striking back at his rivals with responses that grew increasingly aggressive and heated as he described cases of corruption among Lopez Obrador's aides as Mexico City mayor, and accused Vazquez Mota of absenteeism as a federal legislator.
"They seem to have come to an agreement," he said of his two main opponents. "They're coming with knives sharpened."
Speaking moments after the debate, political analysts said they were surprised by the heatedness of some of the exchanges, but the rigid structure of the debate, with two minutes for each response and a minute and a half for each rebuttal, had allowed candidates to stick almost entirely to prepared material and avoid any serious mistakes, particularly Pena Nieto.
"Many people thought he was going to slip up," said Jose Antonio Crespo, an analyst at the Center for Economic Studies. "His performance, without being spectacular, was sufficiently better than expected, and he can come away without such bad marks."
With questions agreed-upon beforehand, Pena Nieto stuck to his themes of change and competence, and parried Vazquez Mota's critiques as based upon incorrect information, a frequent refrain from his team in the first month of the campaign.
Pena Nieto opened the debate by describing Mexico's economic performance as the worst in 80 years, saying "there aren't enough jobs, and the ones that exist don't pay well."
Vazquez Mota went on the attack against the former governor's record minutes into the campaign, saying the state of Mexico had one of the country's worst economic records under his administration.
"It doesn't cease to surprise me that he talks about objectives that he was never able to achieve in the state of Mexico," she said.
She emphasized her role as the only woman in the race, and sought to de-emphasize her ties to the party that has governed for the last 12 years.
"I want to be president because I have the sensitivity, as a woman, to listen," she said. "I'm a different candidate ... different because I don't belong to powerful, privileged groups, because I'm honest."
Another presidential debate is scheduled for June 10.
Lopez Obrador portrayed both parties as bastions of an unjust socio-economic order in which billionaire businessmen enjoyed luxuries amid widespread poverty. Pena Nieto's rivals have argued that his party has not changed its autocratic and corrupt ways during its 12 years on the sidelines.
"This dominant group has privatized the government," Lopez Obrador said. "Do you think things will get better if the PRI comes back? Let's take a totally new path."
A survey by polling company Buendia & Laredo released Sunday in El Universal had Pena Nieto with 39.2 percent support, Vazquez Mota with 22.1 percent, Lopez Obrador with 17.5 percent and Gabriel Quadri of the New Alliance with 1.1 percent. The remaining people were undecided.
The poll surveyed 1,000 adults across Mexico and had a margin of error of plus or minus 3.5 percentage points.