MEXICO CITY (Reuters) - Mexico’s ruling party could be defeated by the candidate of leftist leader Andres Manuel Lopez Obrador in a June state election seen as a litmus test for next year’s presidential race, a newspaper poll showed on Tuesday.

FILE PHOTO: Andres Manuel Lopez Obrador, leader of the National Regeneration Movement (MORENA) party, signs an autograph after giving a speech in Tlapanaloya, Mexico, January 25, 2017. Picture taken on January 25, 2017. REUTERS/Henry Romero/File Photo

On June 4, voters will choose a new governor in the State of Mexico. The most populous state in the country, it is a bastion of President Enrique Pena Nieto’s Institutional Revolutionary Party (PRI), which won by a landslide in the last election in 2011.

However, the poll by daily Reforma showed Delfina Gomez of Lopez Obrador’s party, the National Regeneration Movement (MORENA), holding a wafer-thin lead over PRI rival Alfredo del Mazo, a cousin of Pena Nieto.

The survey of 1,000 voters showed 29 percent backing Gomez and 28 percent del Mazo, with 22 percent favoring Josefina Vazquez Mota, a former presidential candidate for the center-right opposition National Action Party, or PAN.

A March poll by Reforma had shown del Mazo with a one-point lead over Gomez in the state, where the PRI has held power since the party’s inception.

The loss of the 16-million-strong State of Mexico and its financial resources would be a major blow to the PRI in its bid to stop Lopez Obrador from winning the presidency next year.

The runner-up in the last two presidential contests, Lopez Obrador has led a number of early polls for the 2018 race, profiting from widespread discontent over political corruption, gang violence and anemic economic growth.

However, MORENA suffered a blow on Monday when newspaper El Universal published a video showing one of its candidates for a mayoralty in eastern Mexico accepting a cash donation apparently worth 500,000 pesos for Lopez Obrador, which would be illegal.

The candidate immediately stepped down and Lopez Obrador dismissed the incident as a set-up aimed at discrediting him.

Reforma’s State of Mexico poll was taken between April 19 and 23, before the video emerged.

The PRI has bet on keeping the opposition divided to limit electoral losses in recent elections, but faces formidable opposition in the State of Mexico.

Del Mazo was shown losing by at least 10 points in hypothetical run-offs with his two main rivals in the Reforma poll.

The poll had a margin of error of plus or minus 3.9 percent, said Reforma, which came closer than most of the major polling outlets to showing Lopez Obrador’s true strength in the 2012 election, when his final vote tally surprised many.