Fiery leader: Mr. López Obrador has advocated for the poor. On Feb. 24, he protested the privatization of Mexico's oil company. Alexandre Meneghini/AP/FIle

Ecatepec, Mexico - In a makeshift office in this industrial town outside Mexico City, residents stream in to sign up as representatives in the "legitimate government" of Andrés Manuel López Obrador.

The fiery leftist leader, who is still contesting his loss in Mexico's 2006 presidential election to the conservative Felipe Calderón, has signed up well over a million residents.

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.

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