López Obrador, a former mayor of Mexico City, represents an emphatic rejection of the traditional political parties and politicians whom he regularly calls the "mafia of power". In recent decades, Mexico has been led by technocrats and pro-American politicians, while López Obrador's role models are Mexican independence and revolutionary leaders who stood up to more powerful foreign countries.

He was competing against Ricardo Anaya, an ambitious 39-year-old former president of the right-leaning National Action Party (PAN); and Meade, a 49-year-old Yale-trained economist,  representing the governing Institutional Revolutionary Party (PRI). Both have lagged in the polls.

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López Obrador's opponents have sought to portray him as a dangerous populist who will lead Mexico back to failed economic models involving subsidies and state intervention, while provoking more tension with Trump's administration.

But the unpopularity of President Enrique Peña Nieto and the PRI - which has ruled Mexico for most of the past century - hobbled the candidate from his party and prompted voters to search for an alternative to traditional candidates. (Peña Nieto is not running; Mexican presidents are limited to one term.)

López Obrador grew up in a middle-class family in the Gulf Coast state of Tabasco and began his political career helping indigenous villagers with public works projects, which exposed him to Mexico's glaring inequality. He broke away from the PRI in the late 1980s and joined a leftist opposition party. López Obrador grew famous as a protest leader against voter fraud and the abuses of the state-owned oil industry.

He was elected mayor of Mexico City in 2000 - his only electoral victory - and boosted social spending for single mothers, the handicapped and the elderly. Major projects, such as an elevated highway through the city, and the revitalisation of downtown neighborhoods also boosted his popularity.

After two failed presidential bids, López Obrador has tempered his message this year. While he still emphasizes the fight against extreme poverty, saying it will lead to less violence and a stronger economy, he has portrayed himself as more pro-business and pro-American than in the past. But his critics worry that he would roll back a recent policy to allow private investment in the oil industry and cancel a multibillion-dollar airport project in Mexico City.

Washington Post, Reuters