Aided by fury at Mexico\'s brutal drug violence, youthful leader Enrique Pena Nieto has delivered a stunning return to power for the country\'s once-reviled Institutional Revolutionary Party. Pena Nieto declared victory in the presidential election late Sunday, after first official results showed him with 38 percent of the vote, ending more than a decade in the political wilderness for the PRI. The winner\'s nearest rival, leftist Andres Manuel Lopez Obrador from the Party of the Democratic Revolution (PRD), took 31 percent but refused to concede the race, claiming to have data showing different results. The PRI was synonymous with the Mexican state as it governed for seven decades until 2000 using a mixture of patronage and selective repression -- isolating political foes through bought elections and skewed media coverage.  Current President Felipe Calderon\'s ruling right-wing National Action Party (PAN) was third, hemorrhaging support due to the brutal drug war that has claimed more than 50,000 lives since he came to power in 2006. \"If these results are confirmed... Pena Nieto will be named president elect, and starting December 1, will be the next president of the republic,\" Calderon said minutes after the early official results were out. \"I want to sincerely congratulate him.\" Elated by his poll triumph, the dapper, perfectly-coifed Pena Nieto, 45, entered the PRI headquarters in Mexico City to a hero\'s welcome after the results were announced by the independent Federal Electoral Institute (IFE). \"Presidente! Presidente!\" the crowd chanted, with the victor emerging to address supporters in a tone that was magnanimous toward his opponents. \"I assume the mandate that Mexicans have given me,\" Pena Nieto said, emphasizing that Mexico voted for a \"change of course.\"  \"The country now demands... the unity of all Mexicans,\" he said. The winner thanked the other candidates by name, including Calderon, for what he said was their contribution to democracy. An ex-governor of populous Mexico state, just west of the capital, Pena Nieto is married to glamorous telenovela star Angelica Rivera and benefited from family connections with powerful old guard PRI politicos, as well as a savvy media team that carefully stage-managed his appearances. IFE president Leonardo Valdes said the first official results were based on returns from 7,500 polling stations and have a 0.5 percent margin of error. In 2006, when Lopez Obrador ran for president and lost by less than one percent, he organized protests that paralyzed Mexico City for more than a month, and swore in as the \"legitimate\" president and even appointed a cabinet. Far behind in the initial results was Josefina Vazquez Mota from Calderon\'s PAN party with 25 percent, feeling the backlash from the incumbent\'s effort to smash the drug cartels. PAN party head Gustavo Madero on Monday acknowledged the debacle, which included losing the governor\'s mansions in key states like their stronghold of Jalisco. \"It was a capital defeat,\" he told local media. Calderon\'s military crackdown has turned parts of the country into war zones and despite presiding over a period of steady economic growth, he leaves as an unpopular president with a dubious legacy. The economy grew under Calderon, but so did poverty: 47 percent of the nation\'s 112 million residents are poor, according to figures from the government, Latin America\'s second biggest economy. Nearly one million Mexicans -- including election workers, volunteer citizens and party representatives -- as well as 700 international observers were at polling stations overseeing the vote. Election officials worked hard to convince skeptics that the ballot would be clean but faced a raft of complaints in the lead-up to the vote. The head of an observer team from a regional bloc, the Organization of American States (OAS), congratulated Mexico for the \"calmness, respect and order that prevailed\" in Sunday\'s vote. \"Mexico enjoys today a robust and trustworthy electoral system, with an important amount of control that... should show the reliability of the electoral system,\" said the lead observer, former Colombian president Cesar Gaviria. Media projections indicate that the PRI will have a majority in both chambers of Congress. In the Senate, the PAN has the second largest number of legislators, while the PRD is second in the chamber of deputies.