A broad left-center alliance backed Cárdenas in 1988 and many believe he was cheated of the presidency by fraud in that election. After the vote, most of the coalition transformed itself into the PRD.

But as PRD candidate, Cárdenas finished third in a less-controversial election in 1994.

And in 2000, conservative candidate Vicente Fox of the National Action Party (PAN) took advantage of electoral reforms inspired by the 1988 debacle to end 71 years of control by the Institutional Revolutionary Party, or PRI, while Cárdenas finished third again.

Polls this year show the PRD has a chance to win in 2006, but only behind Mexico City Mayor Andrés Manuel López Obrador, who has not yet declared himself a candidate. Cárdenas has trailed far behind in all polls.

"The moment will come when we are ahead in the polls and above all, ahead in votes," Cárdenas said, calling for "a great patriotic, national and democratic alliance."

The PRD has not yet decided how it will chose its candidate. In earlier elections, Cárdenas was the overwhelming favorite of party members.

Asked about Cárdenas' intentions, López Obrador called the longtime leader "an important leader of the country, a precursor of the democratic movement of our times, so I see everything he does as good."

Both men started their careers in the PRI and participated in movements to democratize that party before leaving it.

Cárdenas had been the PRI governor of Michoacan state before making his first run for the presidency. In 1997, he became the first elected mayor of Mexico City in seven decades.

Cárdenas, whose father Lázaro was the president who nationalized Mexico's oil industry in 1938, called for using oil revenue as the engine of economic growth.

He promised "broad economic cooperation" with the United States and Canada, without abandoning hopes of greater integration with Latin America.

"This country has a historic debt to Cuauhtémoc Cárdenas and must return the presidency to him," said writer and political activist Paco Ignacio Taibo II.