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The PRI was ousted in 2000 after 71 years in power. Protesters argue that a July 1 victory by Mr. Pena Nieto will signal a return to the party’s once opaque politics. Some have gone so far as to accuse the PRI of cutting back-door deals with drug cartels to keep violence down.

Mr. Pena Nieto has pushed a contrary message during the campaign, presenting himself as the jewel of a 12-year regrouping by a PRI bent on increasing Mexico’s economic growth and combating the nation’s rampant organized crime.

The Calderon administration has been plagued by years of relatively slow growth, and Mexican voters may be weary of soaring crime and murder rates that have coincided his hard-knuckle crackdown on drug cartels.

As a result, Mr. Pena Nieto’s message appears to have resonated.

“The real question now is whether Lopez Obrador could beat out Pena at the very end,” said Federico Estevez, a political scientist at the Autonomous Technological Institute of Mexico. “That’s possible, but not probable right now.”