PRD's dour but well-known Cuauhtemoc Cardenas, a twice-defeated runner, now mayor of Mexico city.

ECONOMIST: The institutional evolutionary party

PRD's Mr Cardenas has been slipping in the polls, thanks to his unimpressive running of Mexico city.

ECONOMIST: The institutional evolutionary party

Its social conservatism has limited its appeal in cosmopolitan Mexico City, where the mayor, the PRD's Marcelo Ebrard, has legalised gay adoption.

ECONOMIST: Mexico's ruling party

What was happening yesterday is representatives from Andres Manuel Lopez Obrador's PRD party were objecting to practically every, you know, packet of votes that were coming in in heavily PAN areas.

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He says that Mexico's deep inequalities mean that the PRD cannot just be a moderate European centre-left party, but nor can it be so radical as to become a movement outside the country's institutions.

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Whereas the PRI is planning to field Beatriz Paredes, a well-known figure with close ties to her party's machine in the capital, the PRD has struggled to unite behind a strong candidate.

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Although the party has yet to prove it has the ability to beat the PLD in 2010 legislative elections, it is likely to benefit from the electorate's disenchantment and perhaps improve upon the 40% share of the vote that PRD candidate, Miguel Vargas Maldonado, received in the 2008 presidential election.

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PRD, this means its handful of governors, chief among them Andres Manuel Lopez Obrador, Mexico city's mayor, who is Mr Cardenas's chief rival in the party.

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