MEXICO CITY -- Mexico's presidential election was too close to call early this morning, with leftist Andres Manuel Lopez Obrador and conservative Felipe Calderon locked in a tight race, according to early exit polls.
The returns announced by a top official of the Mexican Election Institute echoed a half-dozen private exit polls in portraying a deadlock between Calderon, who promised free-market economics and a closer relationship with the United States, and Lopez Obrador, who promoted expanded social programs and a more assertive stance toward the U.S.
Luis Ugalde, president of the election institute, said that a "quick count" from precincts across the country that was designed to give a valid indication of the result was too close to show the winner.
Officials said they wouldn't be able to declare a winner until later in the week.
The vote, pitting two candidates at opposite ends of a wide ideological divide, was seen by many as a referendum on the open-market policies embraced by departing President Vicente Fox of the National Action Party, or PAN.
Fox backed PAN nominee Calderon as dozens of labor unions and leftist groups supported Lopez Obrador of the Democratic Revolution Party, or PRD. Roberto Madrazo, the candidate of the Institutional Revolutionary Party, or PRI, which ruled Mexico for much of the 20th century, was expected to finish a distant third.
Pollsters for the two major television networks here, Televisa and TV Azteca, said their exit polls showed the result still too close to call. The U.S.-based Univision network made an early call for Calderon, within minutes of the earliest poll closings. It later backed off the prediction.
More than 40 million people, or about 60 percent of the electorate, were expected to vote, according to officials with Mexico's Federal Election Institute. More than 130,000 polling places have been set up, from within yards of the U.S. border in Tijuana to Indian villages in Chiapas.
The campaign was one of the most acrimonious in Mexican history, with three leading candidates spending millions of dollars on television and radio commercials denigrating their opponents.
Calderon, 43, ran as the candidate who could best continue economic policies initiated by Fox, who is limited by Mexico's Constitution to a single six-year term.
Lopez Obrador, 52, the charismatic former mayor of Mexico City, held a slight lead in most polls. He promised to expand government subsidies to the needy and to stimulate Mexico's economy with public works projects and reductions in fuel prices.
Yesterday's vote was also for all members of Congress, including 500 members of the Chamber of Deputies and 128 members of the Senate.
Since 2003, when he held a referendum in which Mexico City residents voted overwhelmingly to keep him in office, Lopez Obrador has been widely considered to be the favorite in the presidential race. But he had to fight off an effort last year to have him impeached as mayor of Mexico City - which also would have prevented him from running for president.
The Fox administration sought to prosecute Lopez Obrador on an obscure charge related to the construction of a local hospital. Congress impeached him, stripping him of his immunity.
But the charges were dropped after hundreds of thousands of demonstrators took to the streets of Mexico City. Lopez Obrador returned to office, and his popularity soared.
Calderon, a former energy secretary under Fox who won his party's nomination in October, trailed far behind Lopez Obrador until March, when he launched what was arguably the most sophisticated media campaign Mexican politics has seen. In more than a dozen different commercials, the Calderon campaign portrayed Lopez Obrador as a demagogue and spendthrift who would bring back hyperinflation and currency devaluation.
By April, Calderon surged into a narrow lead in most polls.
But Lopez Obrador revived his campaign with a dramatic accusation at the end of the last presidential debate June 6. A software company owned in part by Calderon's brother-in-law, Lopez Obrador alleged, had won millions of dollars in government contracts without paying taxes.
The dispute that followed dominated news coverage for days and helped propel the former mayor back into the lead in most polls.
Hector Tobar writes for the Los Angeles Times. The Associated Press and Cox News Service contributed to this article.