MEXICO CITY (Reuters) - Mexican presidential contender Andres Manuel Lopez Obrador has stretched his lead over rivals heading into the July 1 election, a voter survey by polling firm Mitofsky showed late on Thursday.

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Lopez Obrador, a left-wing former Mexico City mayor, got 29.5 percent support in the poll conducted from March 16-18, up from 27.1 percent in a previous Mitofsky survey last month.

Running second was Ricardo Anaya, former chairman of the conservative National Action Party, who heads a right-left alliance. His support slipped 1.1 percentage points to 21.2 percent.

In third place was Jose Antonio Meade of the ruling Institutional Revolutionary Party (PRI), who fell back by 1.6 percentage points to 16.4 percent, the poll showed.

The dip in support for Anaya comes after several weeks of accusations by opponents that he had engaged in financial wrongdoing linked to property deals in his home state of Queretaro in central Mexico. Anaya denies this.

PRI supporters hope that Anaya’s troubles will benefit Meade, whose campaign has been bogged down in widespread discontent with the ruling party over corruption scandals, rising lawlessness and sluggish economic growth.

Running fourth, with 4.8 percent backing, was independent candidate Margarita Zavala, the onetime first lady and wife of former Mexican President Felipe Calderon.

Just over 28 percent of those surveyed had no preference.

Lopez Obrador, the runner-up in 2006 and 2012, has led the polls for months, and a string of recent surveys have shown him widening his lead over his two main challengers.

He has toured Mexico relentlessly for years railing against corruption. However, his economic propositions are viewed with concern by influential sections of the business community.

If elected, Lopez Obrador has vowed to scrap a new, $13 billion airport in Mexico City already under construction, arguing it is too expensive and tainted by corruption.

Earlier on Thursday, he pilloried a top Mexican corporate lobby that is defending the new airport, adding to strains with business leaders.

Lopez Obrador has also criticized the government’s opening of the oil and gas sector, prompting President Enrique Pena Nieto to warn Mexico could put billions of dollars in investment at risk if the reform is rolled back.

The Mitofsky poll surveyed 1,000 registered voters in face-to-face interviews, and had a margin of error of 3.1 percentage points.

The election campaign formally begins on March 30.