Despite such support, the left in Mexico has taken a bruising since its popularity surged to new highs on the eve of the 2006 election. Divisions have emerged within the Party of the Democratic Revolution (PRD) over the increasingly aggressive tactics of Mr. López Obrador.
He remains the left's most commanding leader, analysts say, but could also be its greatest liability as his radicalization alienates more party members.
The election this Sunday to choose new PRD leadership could be pivotal. On one side is Jesús Ortega, a candidate from the more moderate camp; on the other, Alejandro Encinas, one of the staunchest supporters of López Obrador.
The vote is essentially a referendum on the increasingly radical López Obrador and on the direction of the left in Mexico.
"The results [Sunday] are crucial because if Encinas wins, I don't think we'll see a change in the direction of the PRD," says Jorge Buendia, the director in Mexico for Ipsos-Bimsa, a polling firm. "If Ortega wins, he will have a less confrontational attitude towards the government."
While both candidates share the same basic vision for the party, Mr. Ortega appeals to the more moderate party members; Mr. Encinas, who came onto the political scene with the Communist Party, is considered more radical. Their main difference lies in their relation to López Obrador: Encinas is his clear collaborator in opposition to the government, while Ortega is seen as distancing himself with appeals for greater negotiation.
The PRD – which was formed in 1989, after a presidential election was widely believed to have been stolen from the leftist candidate, Cuauhtemoc Cardenas – peaked in popularity during the July 2006 election, which López Obrador lost by less than a percentage point. But party identification among voters has since plummeted. It fell from a high of 23 percent during the 2006 elections to just 11 percent last month, according to a survey by Ipsos-Bimsa.
Mr. Buendia says that much of the decline can be attributed to the radicalization of López Obrador – starting with a seven-week sit-in in the middle of Mexico City to denounce fraud during the electoral process. López Obrador still calls President Calderón a "puppet."
This weekend's vote will help indicate how much support the fiery politician retains, since he has publicly allied himself with Encinas.
Still, López Obrador remains the de facto leader of the left in Mexico. The only reason that support for the PRD has fallen so precipitously is that it reached such highs on the coattails of López Obrador's popularity, says Federico Estevez, a political science professor at the Autonomous Technological Institute of Mexico.
And like the flurry of new representatives in Ecapetec, hundreds of thousands of Mexicans are willing to support López Obrador's every effort. Alma Navarrete, who logs people's names and addresses before they become card-carrying members of his "legitimate government," says that 20,000 people in this municipality have become members of his government since she began volunteering more than a year ago.
Enthusiasm has surged now that López Obrador, who tours municipalities across the country regularly, has taken on a new cause: energy reform. As the government debates ways to increase oil production, the PRD has made itself the main obstacle to talk of any foreign investment. This resonates with voters.
"To privatize [Pemex, the state oil company], I don't think it's anything more than to enrich the wealthy and wealthy communities, while the poor become poorer," says Marta Elena Cabrera, who says she signed up as part of his government precisely to protect Pemex against private forces.
"For 25 to 35 percent of the country, López Obrador is the savior," says George Grayson, an expert on Mexican politics at the College of William and Mary in Virginia. "He is the one who stood up for them. He is the one who is voicing their concerns."
"The legitimate president of Mexico supports the poor," says Ana Bertha Santiago Castillo, who has signed up as one of López Obrador's "representatives."
That popular appeal is not to be underestimated, says Mr. Estevez, despite the elections this Sunday. Some analysts speculate that, if Encinas does not win the election, López Obrador – boosted by the sheer numbers of those who support his "legitimate government" – will form his own political party, says Alberto Aziz Nassif, a researcher at the Center for Research and Higher Education in Social Anthropology in Mexico City. López Obrador is also widely expected to rerun for president in 2012.
Estevez says that if moderates take PRD leadership, they will do their best to allow a significant role for the man who has grown Mexico's left to proportions it had not known.
They need him, he says. "Should the moderates win and somehow drive or counter López Obrador to the edge, then they lose."