Andres Manuel Lopez Obrador will represent the main leftist party in Mexico's 2012 presidential election, after claiming for years he was robbed of the 2006 presidency.

The charismatic 58-year-old former mayor of Mexico City was selected in a polling process Tuesday by two analysis firms, using the opinions of 6,000 Mexicans on the best candidate for the Democratic Revolution Party (PRD).

"We kept our promise that the leftist candidate would be the one who was best placed," Lopez Obrador said at a joint news conference with his rival and Mexico City Mayor Marcelo Ebrard.

Lopez Obrador led a long, unsuccessful fight to dispute the results of the last election in 2006, which he lost by less than one percentage point.

"The left divided will just fall into the precipice. I accept the results of this poll," Ebrard, who has long expressed presidential ambitions, said Tuesday.

The PRD candidate 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 1, 2012 presidential election.

Three people are vying to represent the conservative National Action Party (PAN) of President 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 in 2012 of the Institutional Revolutionary Party (PRI), which ruled Mexico for more than seven decades until 2000.

According to opinion polls, the current favorite 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, 58, 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.

The politician known as AMLO has campaigned across the country for grassroots support but lost many followers with his dogged claim to the "legitimate" presidency, after blocking the city's main avenue for weeks after the election.

The divided PRD meanwhile lost two state governorships this year, including Michoacan on Sunday, according to preliminary results.