Seventeen years ago, Mexicans believed they had finally achieved a truly democratic government with the election of a president different from those of the Institutional Revolutionary Party, or P.R.I., who had governed for 71 years under a sophisticated simulation of democracy. For the first time, an independent citizen electoral institute took charge of the election and vote count and, to everybody’s surprise—since they doubted that the still-dominant P.R.I. would permit it—Vicente Fox, of the rightist…