MEXICO CITY (Reuters) - Mexican leftist Andres Manuel Lopez Obrador, who leads the National Regeneration Movement (MORENA) party, is the front-runner ahead of next year s presidential election, according to a newspaper poll published on Monday.  The El Universal survey found that 23.3 percent of respondents would vote for MORENA in the 2018 vote, ahead of the conservative National Action Party, which had 20 percent support.  President Enrique Pena Nieto s ruling Institutional Revolutionary Party (PRI) was in third place, with 16.5 percent, the poll found. The major parties have yet to pick their candidates for the race, but Lopez Obrador, or AMLO as he is known locally, is the de facto candidate of MORENA, which he founded in 2014 after breaking with his longtime base, the center-left Party of the Democratic Revolution (PRD). A victory for the combative and nationalist-leaning Lopez Obrador could stoke tensions with the Trump administration just as the United States, Mexico and Canada seek to seal a renegotiation of the North American Free Trade Agreement. The silver-haired Lopez Obrador, 63, is one of the best-known politicians in Mexico, having spent years relentlessly attacking other parties for corruption. But he is also a divisive figure, feared by the country s business elite. A former mayor of Mexico City, Lopez Obrador finished runner-up in the last two presidential contests.   In various scenarios polled by El Universal, in which AMLO faced different candidates from the other parties, the leftist came out ahead, garnering between 28.6 and 31 percent support. Political analysts say that may be enough to win the July 2018 contest with the electorate increasingly divided. Only the PAN s Margarita Zavala, who is the wife of former President Felipe Calderon, came close, with 26.1-26.6 percent of respondents giving her their support.  Additionally, AMLO was the most positively viewed potential candidate, with 43.3 percent of those polled holding an upbeat view of the veteran politician.  An electoral pact linking the PAN, the PRD and the Citizen s Movement party is not well known in Mexico, the poll found, but among those with knowledge of the newly formed alliance, 46 percent thought it could beat MORENA next year.  The survey, which had a margin of error of 1.3 percent, polled 6,400 Mexicans in face-to-face interviews between Aug. 18 and Sept. 3. 