MEXICO CITY, May 29 (Reuters) - Enrique Pena Nieto, the front-runner in Mexico's presidential race, has suffered his biggest drop in support during the campaign after demonstrators said his party was corrupt and oppressive, an opinion poll showed on Tuesday.

The poll by Consulta Mitofsky showed backing for Pena Nieto, candidate of the opposition Institutional Revolutionary Party (PRI), at 35.6 percent, still a big lead over Andres Manuel Lopez Obrador, the leftist contender and 2006 runner-up.

But the PRI candidate's support was down by 2.3 percentage points from a previous Mitofsky survey published on May 22.

That was the sharpest drop since the campaign for the July 1 elections began at the end of March. Pena Nieto's support in Mitofsky has been above 40 percent at times during the race.

Backing for Lopez Obrador rose 1.2 percentage points to 21.7 percent.

The latest poll by Mitofsky was taken between Friday and Sunday, the first conducted since thousands of protesters took to the streets of Mexico City on May 19 against the PRI, which ruled the country for 71 years until losing power in 2000.

Demonstrators said the PRI was corrupt and repressive, and some smaller protests, many of them organized in online social media, have followed since.

Lopez Obrador, who narrowly lost in 2006, has sought to capitalize on this opposition to Pena Nieto, who has led presidential polls for more than two years.

Josefina Vazquez Mota, candidate of President Felipe Calderon's National Action Party (PAN), stayed in third place with 20.4 percent, up by 0.3 percentage points.

Support for the PAN has been damaged by a brutal drug war and a sluggish job market, allowing the PRI to come back.

Especially toward the end of its lengthy hold on power, opponents accused the PRI of stealing public money, rigging elections and violently cracking down on dissenters.

But Pena Nieto argues the PRI has transformed into a democratic and honest party, which can halt Mexico's crime wave and create enough jobs for its growing population.

The latest survey was based on 1,000 face to face interviews with eligible voters. It had a margin of error of 3.1 percent. (Reporting By Ioan Grillo; Editing by Philip Barbara)

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