Lopez Obrador disputed the result of 2006 elections, which he lost by less than one percentage point [Reuters]
Andres Manuel Lopez Obrador, the leftist former mayor of Mexico City, will again run for the Mexican presidency in 2012 after his narrow loss in the 2006 election sparked massive protests.
The 58-year-old beat the current mayor of the capital in a poll of 6,000 supporters of left-wing parties on Tuesday.
"We kept our promise that the leftist candidate would be the one who was best placed," Lopez Obrador said on Wednesday at a joint news conference with Marcelo Ebrard, his rival for the nomination.
"The left divided will just fall into the precipice. I accept the results of this poll," Ebrard, who has long expressed presidential ambitions, said.
Lopez Obrador, who represents the Democratic Revolution Party (PRD), led a long, unsuccessful fight to dispute the results of the last election, which he lost by less than one percentage point.
Lopez Obrador will also have support from three other leftist movements, after a polling process aiming to choose a unity candidate among rival factions.
The PRD was the first of three major parties to pick a candidate for the July 2012 presidential election.
Three people are vying to represent the conservative National Action Party (PAN) of incumbent Felipe Calderon, who is barred from standing again: former ministers Ernesto Cordero, Josefina Vazquez Mota and Santiago Creel.
The PAN candidate will be chosen in an internal vote at the start of 2012.
Many predict a return to power in 2012 for the Institutional Revolutionary Party (PRI), which ruled Mexico for more than seven decades until 2000.
According to opinion polls, the current favourite to win the presidency is PRI ex-state governor Enrique Pena Nieto, expected to be chosen to run by his party's supporters in February.
The campaign officially begins in March.
Lopez Obrador started his political career in his southeastern state of Tabasco with the PRI, which he left at the end of the 1980s. He was mayor of Mexico City from 2000 to 2006.