Mexico’s old rulers have regained power following 12 years in opposition but likely will have to forge alliances with other parties to push through reforms after winning the presidency by a much narrower margin than polls had forecast.Enrique Pena Nieto of the Institutional Revolutionary Party, or PRI, declared himself the winner of Sunday’s presidential election after a quick count by Mexico’s electoral authorities gave him a clear lead.Promising to reinvigorate the economy and reduce rampant drug violence, the telegenic 45-year-old will take office in December for a six-year term as president, restoring the party that dominated Mexican politics for most of the past century, at times ruthlessly.Opinion polls in the last few days before the election had forecast Pena Nieto winning by a margin of between 10 to 15 percentage points, but with 85 percent of returns in, he was only 5.4 percentage points ahead of his leftist rival, Andres Manuel Lopez Obrador.Pena Nieto had 37.6 percent support compared to 32.2 percent for Lopez Obrador and 25.4 percent for ruling party candidate Josefina Vazquez Mota."Mexicans have given our party another chance. We are going to honour it with results,” a visibly moved Pena Nieto told followers packed inside the PRI headquarters in Mexico City, where confetti rained down on jubilant supporters.Although Lopez Obrador said on Sunday night it was too early to concede defeat, a senior electoral official said the PRI candidate’s lead was "irreversible” and outgoing President Felipe Calderon congratulated Pena Nieto on his triumph.The conservative Calderon’s ruling National Action Party (PAN) suffered a crushing defeat, hurt by his failure to bolster economic growth and curb the fierce violence of a drug war that has killed tens of thousands of people and battered Mexico’s image.