The second- and third-place candidates, the PRD’s Lopez Obrador, and Josefina Vazquez Mota of the PAN, or National Action Party, garnered a combined 57 percent of the vote. So it may be difficult for Peña Nieto to govern without the support of the other political parties in the Mexican Congress.

"It's a victory with some limits to it," said Eric L. Olson, associate director of the Mexico Institute at Woodrow Wilson International Center for Scholars in Washington, D.C.

Peña Nieto will have to convince those who did not vote for him that the PRI, often compared to the old Soviet Politburo, is no longer the corrupt party that imperiously ruled Mexico for 71 years, and which made deals with drug traffickers and monied interests.

"He will have to show that the PRI he will lead will be democratic and honest," Olson said. "There are a lot of doubts about that."