FIVE YEARS AFTER Mexico established itself as an electoral democracy, its Congress faces a decision that could undermine that hard-won progress and invite political turmoil. As soon as tomorrow, the Chamber of Deputies will vote on whether to lift the legal immunity of Mexico City's mayor, Andres Manuel Lopez Obrador, a leftist who currently leads the polls for next year's presidential election. If the measure passes, prosecutors plan to criminally charge Mr. Lopez Obrador with contempt of court in a municipal land dispute, a step that could block his presidential candidacy. The short-term winners of this maneuver would be the presidential candidates from the party of the current president, Vicente Fox, and the largest opposition party, which between them control a majority in Congress. But Mr. Lopez Obrador's disqualification would be a disaster for Mexico's political system, and perhaps for its long-term stability.
Mexico's political establishment and its business community are deeply worried about Mr. Lopez Obrador, who promises to apply the leftist populism now gaining strength in Latin America in a country that has aggressively -- and mostly successfully -- pursued free-market capitalism for the past 15 years. Mr. Lopez Obrador has said he would "restructure" Mexico's foreign debt and renegotiate the North American Free Trade Agreement with the United States and Canada, even though NAFTA has produced an explosion of Mexican exports and, according to an exhaustive World Bank study, made Mexicans richer. Mr. Lopez Obrador might drive foreign investment from Mexico and destabilize the economy with massive government spending; at worst, he might imitate Venezuela's Hugo Chavez, who has wrecked his country's private sector and made most of its people poorer.
Those who wish to see Mexico continue to modernize and grow prosperous can hope that Mr. Lopez Obrador does not become its next president. But the way to stop this popular politician is not to force him off the ballot through a legal trick. The case against the mayor is trivial: Prosecutors would hold him culpable because officials in his administration allegedly did not comply with a court order halting the construction of a portion of a road to a hospital. Legal experts say government officials have ignored such orders many times without punishment.
If Mr. Lopez Obrador is unable to compete for the presidency, then the landmark achievement of 2000, when Mr. Fox became the first opposition candidate to win a presidential election, will be tainted. Mexico will return to the era when it was ruled by fraud and force; the next president will be discredited at home and abroad. Mexicans clearly value their new democracy: Recent polls show that 80 percent of citizens oppose Mr. Lopez Obrador's disqualification, even though only 37 percent say they would vote for him. Their representatives should listen, and refrain from perpetrating an injustice.