The hotly contested race turned negative in the run-up to Sunday's vote.
Six years ago, Vicente Fox won a historic presidential election here and ushered in a new era of true multiparty politics. Out went the days of rigged elections, hand-picked leaders, and rubber-stamp congresses, and in came a period of more responsive politicians, increasing transparency, vigorous political debate – and vicious negative campaigning.
"[This year's presidential race] has been the most competitive, arguably the most interesting race in our history ... but also the dirtiest," says Julio Madrazo of the CMM consultant group in Mexico City.
The outcome of Sunday's vote will affect whether Mexico will join Latin America's leftward trend or continue to strengthen US ties and focus on free-market reforms.
Andrés Manuel López Obrador, a leftist who portrays himself as a savior of the poor, holds a very slim lead in opinion polls over conservative Felipe Calderón, a proponent of fiscal conservatism and free trade. Running third in polls is Roberto Madrazo, of the Institutional Revolutionary Party (PRI) that governed Mexico from 1929 to 2000. Two other minor candidates have single-digit support each.
But for many Mexicans, notes Mark Schneider, a Latin America expert at the Brussels-based International Crisis Group, it's the negative campaigning, and not the substance that will ultimately drive the vote.
"There is no question that Mexican democracy is maturing with a truly competitive multiparty system," he says. "But unfortunately the political process has not been able to avoid the pitfalls that a media-dominated electoral environment throw up in the way of serious debate over policies and issues."
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