He vows to increase pensions for older citizens, and educational grants for the young. He promises to reduce the top salaries in government, including his own, and lift the wages of the lowest-paid public workers instead. He says he will fight corruption and use the billions of dollars a year in savings to pay for social programs.

Many doubt he can eliminate graft or come up with the windfall he has promised. But after spending the past 18 years vacillating between Mexico’s two dominant parties, voters appear increasingly willing to try something else.

Mr. López Obrador’s positions are largely unchanged from his time as a young organizer for indigenous communities in his home state, Tabasco.

What has changed is the political climate of Mexico.

Stubborn poverty rates and vast inequality, coupled with corruption scandals and a rise in violence, have pushed voters toward Mr. López Obrador, who last held elected office in 2005 as mayor of Mexico City.

Beyond that, young people, who are expected to make up about 40 percent of the vote in this election, have widely embraced Mr. López Obrador, who, at 64, happens to be the oldest candidate in the race.

“It’s sort of like a pox on all their houses,” said Roberta S. Jacobson, the former American ambassador to Mexico. “He was the only one who could successfully paint himself as an outsider — and there are a lot of people in Mexico who feel that they are outside.”

Indeed, Mr. López Obrador’s electoral prospects owe as much to the current president, Enrique Peña Nieto, as to his populist language and promises to take on the powerful.