MEXICO CITY (Reuters) -- The leftist favored to win Mexico's presidential election in July accused President Vicente Fox on Sunday of being too soft in opposing Washington's tough new efforts to prevent illegal immigration.

A plan by the U.S. Congress to build a fence on parts of the U.S.-Mexican border has upset many Mexicans and anger was compounded late last month when a Border Patrol agent in the San Diego area fatally shot a young Mexican man who had tried to enter the United States illegally.

Mexico has demanded an investigation into the shooting and Fox described the fence proposal as "shameful" but Andres Manuel Lopez Obrador, a former mayor of Mexico City who leads opinion polls, said that was not enough.

He said Fox's free-market policies caused slow economic growth, and aggravated mass emigration from Mexico.

"It's infuriating to see how President Fox, because he is dedicated to maintaining economic policies that only benefit the elite, does not have the moral or political authority to confront the disgrace of a wall on the border or to protest the deaths of migrants," he said.

Lopez Obrador, speaking to several thousand cheering supporters after formally registering as a candidate, said 2 million Mexicans had emigrated in Fox's five years in power.

The leftist, from the Party of the Democratic Revolution, tops most opinion polls by some 10 percentage points but analysts say his lead might be cut as other candidates crank up their campaigns.

His main rivals are Felipe Calderon of Fox's National Action Party and Roberto Madrazo from the once-dominant Institutional Revolutionary Party.

Despite his criticism of Fox, Lopez Obrador said he would take care not to alienate Mexico's neighbors if he became president.

"I want to point out now that we will carry out a measured foreign policy, without being antagonistic," Lopez Obrador said.

Drifted apart

The United States and Mexico have drifted apart since the Sept. 11 attacks as Washington focused more on its war on terrorism than on Latin America, and security fears on the U.S.-Mexican border began to cause rifts.

Mexico was the first country President George W. Bush visited after taking office in 2001 and Fox, a fellow rancher and former Coca-Cola executive, was confident Washington would soon agree to Mexican proposals to relax U.S. immigration rules. The U.S. Congress is now taking up immigration reform and while it may still allow a limited guest worker program for mostly Mexican immigrants, lawmakers have also proposed a border fence with security cameras and are considering making illegal immigration a felony.

Analysts say Mexico's presidential candidates are likely to be sharply critical of Washington as campaigning heats up in the coming months. Under Mexico's constitution, presidents may not run for reelection.

Bouyed by the rise of the left in other Latin American countries like Brazil, Argentina and most recently Bolivia, Lopez Obrador says he would give priority to Mexico's poor if he won the top job and promised a return to more state control of the economy.