MEXICO CITY, Mexico (Reuters) -- Mexico's presidential candidates ended a monthlong truce on Thursday and opened a rough election campaign that could see the country join others across Latin America in turning away from the United States.

Leftist front-runner Andres Manuel Lopez Obrador was given a boost as three opinion polls all showed him with a clear lead over his two main rivals in the July 2 election.

The popular former mayor of Mexico City has promised heavy spending on social programs to help Mexico's poor, although business leaders and investors worry he will run up a budget deficit and wreck Mexico's financial stability.

Lopez Obrador kicked off the campaign with his own television show -- a first in Mexican politics -- and a warning that powerful enemies are determined to stop him.

"We are facing very powerful interest groups that will try all means to prevent a true transformation of our country," said the austere 52-year-old, who rises at 5 a.m. every morning and lives in a modest Mexico City apartment.

Leftists have taken power in several Latin American nations in recent years, and are increasingly critical of U.S. policy. A victory in Mexico would be a major coup and could increase tension with Washington.

Lopez Obrador's rivals are Felipe Calderon of the ruling National Action Party, or PAN, and Roberto Madrazo of the Institutional Revolutionary Party, or PRI, which ruled Mexico for most of the 20th century.

The leftist insisted on Thursday he would fight to lift millions of people out of extreme poverty but denied allegations he is anti-business.

"Helping the poor does not mean we are against the rich. I am not against a man who builds a patrimony with effort and work and within the law," he said.

"I am against those who have looted the country. They are the ones who don't like the look of us and they are the ones who are pushing the campaign to discredit us."

He then headed to a rally in the remote town of Metlatonoc, considered the poorest in all of Mexico.

Lopez Obrador and Madrazo have been bitter rivals for many years and analysts say a tight three-horse election race could raise fears of violence and rattle financial markets.

The PRI ruled Mexico for 71 years until it was knocked from power at the last elections in 2000. It continues to dominate in many rural areas and its well-oiled campaign machine could help it close the gap in the next few months.

Madrazo has, however, been hit by fighting with rivals inside his own party and allegations that he is linked to the PRI's authoritarian and corrupt "old guard."

Dozens of teachers' union members, whose leader clashed with Madrazo throughout last year, hurled chairs before he arrived at his opening campaign rally on Thursday.

Under Mexico's constitution, President Vicente Fox is barred from seeking re-election. He is a popular leader but his government is widely seen as having failed to deliver the economic growth and millions of new jobs that Fox promised.

Conservative ruling party Calderon is trying to win over Mexico's poor with promises of a hard line against common criminals, kidnapping gangs and drug traffickers.

"My hand won't shake in acting firmly and with courage to stop the crime that has Mexicans kidnapped," he said as he launched his campaign in a working-class district of Mexico City on Thursday.

Lopez Obrador won wide support as Mexico City's mayor for ambitious public works programs and social spending, including pensions for all elderly residents.

Copyright 2006 The Associated Press.