MEXICO CITY — The party that ruled Mexico with an iron grip for most of the last century has sailed back into power, promising a government that will be modern, responsible and open to criticism.

Though Institutional Revolutionary Party candidate Enrique Pena Nieto's margin of victory was clear in the preliminary count from Sunday's election, it was not the mandate the party had anticipated from pre-election polls that had at times shown the youthful, 45-year-old with support of more than half of Mexico's voters.

Instead, he won 38 percent support, about 7 points more than his nearest rival, according to a representative count of the ballots, and he went to work immediately to win over the two-thirds who didn't vote for him, many of whom rejected his claim that he represented a reformed and repentant party.