By Molly Moore
Washington Post Foreign Service
Wednesday, July 31 1996; Page A22

Mexico's Congress opened a special session today to debate the most significant revision of the national political system in more than six decades. The changes would dilute the power of the presidency and the ruling party, set limits on runaway campaign spending and allow one of the world's most populous cities to elect a mayor.

The agreement by the four main political parties to begin discussions this week in a special congressional session in Mexico City was a significant step after nearly 19 months of political warfare. But administration officials cautioned that "this is still part of a negotiation" and that debate is likely to be long and rancorous.

Revision of Mexico's authoritarian and corrupt political system, which has been controlled by a single party for 67 years, has been the cornerstone of President Ernesto Zedillo's troubled administration, and he has invested much of his political credibility in obtaining congressional approval.

In sending his package to Congress, Zedillo said, "We are taking a decisive and irreversible step in the construction of the full democratic development with which Mexico will enter the next century."

Pressures for loosening the grip of the ruling Institutional Revolutionary Party (PRI) are reflected in recent electoral gains by opposition parties. Further, the United States and European trading partners have called for political reform, with some making it a requirement for future favorable commercial agreements.

The proposed changes have been attacked by some PRI members as going too far and by the opposition as not going far enough. They would remove control of elections from the ruling party and reduce its voting power in Congress. Campaign spending limits are described as the means to remove disparities between the ruling and opposition parties as well as to curtail the influence of drug traffickers' money in elections.

In a demonstration of how tightly the PRI has controlled political power, the agenda includes passage of a law that would allow any party to use the green, orange and white colors of the Mexican flag in campaign banners and paraphernalia. Until now, the PRI has claimed a monopoly on use of the national colors.

The PRI holds about 60 percent of the seats in the two congressional chambers, but the changes would be derided as meaningless if they were passed without the participation of the opposition parties that are the principal intended beneficiaries.

As part of the package, Mexico's complicated system of proportional representation would be revised, modestly increasing the number of seats that at present virtually are guaranteed minority parties.

No part of Mexico would feel a greater direct impact of the changes than Mexico City, with an estimated 22 million population that makes it one of the world's largest cities. The capital's residents would win the right to elect their own mayor and city council. Recent polls have indicated that the PRI has fallen into such disfavor in the capital that it would be unlikely to win the campaign for mayor, which would be held next year.

In addition, Mexican citizens who live outside Mexico as legal residents of other countries would be given the right to cast absentee ballots, as can expatriate U.S. citizens. The government estimates that change would involve 4 million people, and some surveys have shown they are likely to vote against the PRI.

Some officials have said the change could bring Mexican electioneering into the streets of Los Angeles, border cities, Chicago and other places that have attracted large numbers of Mexican immigrants.

For more than a year and a half, opposition party leaders have been using the prospect of their support for the changes as a political club over Zedillo to obtain presidential concessions on unrelated issues. On several occasions, key opposition leaders have boycotted critical discussions in an effort to get their way.

Administration officials said today that they expect much of the same political maneuvering in the coming weeks.

"There will be a lot of noise," one administration official said. "That's part of the brinkmanship. You're going to have people saying, `I'm backing out of this.' They will push [the administration] to the limit, but at the end we will have the reforms."

While the warring parties have agreed in principle to most of the general concepts, forging the ideas into law could take months, officials say.

For example, one of the changes would attempt to give parties equitable access to television, radio and newspaper advertising. But the administration has yet to figure out how to divide the time or how to persuade the independent print media and some television outlets to agree to the proposals.

Limits on campaign spending will prove equally tricky, according to administration officials. The ceilings will require changes with each election and the proposals do not yet specify how to divide contributions between the public and private sectors. As in the United States and other countries, the laws also are unlikely to prevent all illicit campaign contributions, officials said.

But one of the most critical changes will involve taking the presidency and the ruling party out of the voting process, turning over the running of elections to an independent federal commission and giving the Supreme Court authority to adjudicate challenges and complaints -- a power now wielded by the executive branch.