Mr. Calderon is barred by the Mexican Constitution from seeking a second term. Should a standoff be looming this time around, it would most likely result be between Mr. Lopez Obrador and centrist front-runner Enrique Pena Nieto.

Analysts say a stalemate is unlikely because Mr. Pena Nieto, 45, still enjoys a large lead over Mr. Lopez Obrador, 59, and Mexican leftists are unlikely to enter the streets for a candidate who’s efforts fell short in 2006.

“It’s unlikely that Lopez Obrador is going to find the same support that he did the last time, and there are two reasons why,” said Andrew Selee, director of the Mexico Institute at the Woodrow Wilson International Center for Scholars in Washington.

“One is that the electoral institute overseeing the voting is more credible this time around,” Mr. Selee said. “The other is that if he loses, many people on the left will see him as the past and not the future of their movement.”

The most recent polls posted by Mexican newspapers have Mr. Pena Nieto carrying 42 percent of the vote - a double-digit lead over Mr. Lopez Obrador, who, at 28 percent, is tied with Josefina Vazquez Mota, the first major female presidential candidate in Mexico.

Mr. Selee acknowledged Mr. Lopez Obrador “could conceivably have hidden supporters out there that we don’t know about,” a possibility some on the left are pinning their hopes on.

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