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Mexican President Andres Manuel Lopez Obrador greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Supporters of Mexican President Andrés Manuel López Obrador gather for a rally at the capital's main square, the Zócalo, to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Supporters of Mexican President Andrés Manuel López Obrador gather for a rally at the capital's main square, the Zócalo, to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

A supporter of Mexican President Andrés Manuel López Obrador takes part in a rally at the capital's main square, the Zócalo, to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

A supporter of Mexican President Andrés Manuel López Obrador takes part in a rally at the capital's main square, the Zócalo, to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador, accompanied by first lady Beatriz Gutierrez, greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador, accompanied by first lady Beatriz Gutierrez, greets supporters as he arrives to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador and first lady Beatriz Gutierrez wave as they walk out of the National Palace to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)

Mexican President Andres Manuel Lopez Obrador and first lady Beatriz Gutierrez wave as they walk out of the National Palace to attend a rally to commemorate the 85th anniversary of the 1938 expropriation of the country's oil industry, at the Zocalo in Mexico City, Saturday, March 18, 2008. (AP Photo/Marco Ugarte)