The PRI is expected to win an additional three states _ Jalisco, Chiapas and Morelos _ beyond the 20 states the party already controls.
Mexico does not have a runoff system, and the presidential candidate who wins the most votes Sunday will take office Dec. 1 for a six-year term.
Never have so many election observers flooded across Mexico for a vote. Leonardo Valdes Zurita, chief of the Federal Electoral Institute, said more than 28,000 observers were accredited, including monitors from 65 nations, and the hemispheres oldest regional group, the Organization of American States.
Most eyes were on Pena Nieto, the standard bearer of the PRI, whose triumph many Mexicans figured was a foregone conclusion. The party united around the former governor of the State of Mexico, who wed a soap opera star and received open cheerleading from the nations biggest network, Televisa.
Angered by that apparent bias, tens of thousands of young Mexicans took to the streets in May, an impromptu student movement that instilled some life into a campaign that failed to touch deeply on issues such as corruption and security.
The one-time margin of some 30 percent that Pena Nieto held over his rivals a year ago diminished sharply during the three-month formal campaign. But the candidate, handsome and genial, drew swoons on the campaign trail, with women clasping at his forearms at campaign rallies, leaving red welts.
Polls found that Lopez Obrador, a former Mexico City mayor who lost a 2006 presidential bid by only 0.56 percent, gained ground in recent weeks as the No. 2 candidate, raising questions about how he might respond if the vote is close.
Lopez Obrador and his supporters blocked Mexico Citys main boulevard for six weeks after the 2006 vote, claiming fraud.
In recent weeks, Lopez Obrador has assailed Pena Nieto as a puppet of behind-the-scenes PRI masters, such as former President Carlos Salinas de Gortari (1988-1994), whose privatization policies vastly enriched some tycoons.
This is no time for the country to go in reverse, Lopez Obrador said early Sunday after casting his ballot.
Whoever moves into Los Pinos, the presidential palace, Dec. 1, will find good and bad news on the security front. For the first time in years, the homicide rate is dipping. But the criminal landscape is unstable. Powerful criminal organizations, like Los Zetas and the Sinaloa Cartel, face an array of upstart crime groups.
Surprisingly, public security was not a major issue in the campaign. All candidates vowed to keep the army on the streets for the foreseeable future.
Rather, how to rev a limping economy and create jobs was a main concern of voters.
On that front, the winner may have an easier time. The lackluster U.S. economic recovery is not proving a drag on Mexico, which is forecast to grow 4 percent this year and 3.8 percent next year. Even so, Mexicos growth has been modest over much of the past two decades.