The North American Free Trade Agreement could start running into some flak from a new source: Mexico's small but feisty opposition parties. They are gearing up to bombard the U.S. Congress with biweekly fact sheets to bring it up to speed on Mexico before it takes up NAFTA in early 1993. Senator Porfirio Mu oz Ledo, a top official of the Party of the Democratic Revolution, says Mexican interests were slighted in President Carlos Salinas de Gortari's race to push through a trade agreement before his six-year term in office ends in 1994. The opposition's goal is to give U.S. legislators a "more balanced" source of information on Mexican wages, labor practices, and environmental problems. They believe prospective trade partners should know that things in Mexico are not quite as attractive as the official line put out by Salinas' ruling Institutional Revolutionary Party might indicate.