Move to full democracy apt to be slow, gradual. Change seems just ahead for Mexico's authoritarian political system. But just what shape the change will take remains unclear. Much will depend on how President de la Madrid's successor - expected to be named any day now - will respond to mounting grass-roots political pressure to open up the system.

When Mexico sank into economic chaos in 1982, foreign analysts sounded the alarms: Get ready for a political collapse, they said. It never came.

Five years later, many of the worrisome symptoms persist, but Mexico muddles along in relative tranquillity. There have been protests - even within the ruling party - over the government's secretive and sometimes fraudulent electoral practices. But even as the nation moves into the uncertain succession period, Mexicans stoically endure unrelenting inflation, flagging wages, and the effects of a $103 billion debt. Mexico's closed political system survives.

But for how much longer?

Like a pounding waterfall, the five-year economic crisis has slowly eroded the bedrocks of the political system - the power of the President and the ruling Institutional Revolutionary Party (PRI). Analysts say with restricted resources, a restructured economy, and a resentful public, the old-style authoritarian institutions can no longer function as before.

Mexico stands on the brink of a new political era, according to a wide range of political experts - both Mexicans and foreigners - both in and out of government. Unlike in 1982, when alarmists warned of an imminent political collapse, analysts today are speculating about the potential for gradual democratic changes within the system.

What form these changes will take remains unclear. That depends on a kaleidoscope of variables that spin around the answer to one key question: How will President Miguel de la Madrid Hurtado's successor respond to political pressure? In fact, one focus of change will likely be the destape - or ``unveiling'' - of the PRI's presidential candidate.

``In this succession, the President still has the capacity of nominating the successor single-handedly, without consulting anybody,'' says Lorenzo Meyer of the Colegio de M'ejico. ``But the legitimacy of his decision will be diminished ... So perhaps this is the last president that will be able to control the transmission of power in a traditional way.''

Some changes are already evident. The process of choosing a successor, usu ally shrouded in absolute silence and secrecy, has been spiced this year by a greater degree of outspokenness.

The most startling change occurred in August, when the PRI unveiled the names of six potential candidates for the first time in its history. The timely move has alleviated internal pressure for reform, PRI officials and Mexican scholars say.

Even if the unveiling were a purely pragmatic step, it could end up generating its own momentum for change.

``It's destined to spark a more thorough transformation next time,'' says one Western European diplomat, noting that any backward steps in the 1994 succession would fatally damage the PRI's credibility. ``The risk is that expectations have been raised for the next succession.''

At this point, it's about the only risk they wish to take. When the six would-be presidents were trotted out in front of PRI leaders, they were models of loyalty, tact, and caution. Their speeches were both uncritical and uncriticized. No questions were asked. No campaign promises offered. As one diplomat says: ``They're playing it safe.''

When it comes to Mexican politics, ``playing it safe'' is a cardinal rule.

In books, speeches, monuments, even street names, Mexicans are constantly reminded of the price of instability: 10 years of revolutionary chaos. As a coalition that emerged from the political debris of the 1910-1920 revolution, today's PRI isn't about to let political change happen without control.

``The system is predicated on not allowing uncontrolled mobilization,'' says one political expert with close ties to the PRI. ``That's the lesson they learned from the revolution.'' Adds a prominent political economist: ``Playing the game of democracy is dangerous.''

Indeed, many political experts here say that moves to democratize the party or the country could fracture the pillars of the political system. Even seemingly simple reforms - like establishing open primaries to pick PRI candidates - could shift the party's traditional values from cooperation to competition, from discipline to dissension.

More importantly, such changes could disrupt the intricate network of patronage and privilege: For example, elected political posts could not continue to be controlled by the President or evenly distributed to the various political sectors - at least not with the guarantees that have helped hold the system together. ``If you open up the selection process, the PRI comes unglued,'' says Fernando Estrada S'amano, a former PAN deputy. ``There's a tremendous worry that the PRI is structurally impeded or incapable of change.''

Others feel that the PRI has the power to change - but neither the pressure nor the predisposition. Since no political force threatens the PRI from the outside, the party just has to make a cost-benefit analysis, says political scientist Juan Molinar. For him, the answer is easy: ``It's much riskier to open up the party than take the slap of abstentionism on election day.''

For party officials, it's a bit more complicated. ``If the party has one merit, it's that it has the ability, the virtue of adapting to changing circumstances of this country,'' says Alejandro Sob'arzo, director of international affairs for the PRI. He agrees that opening up the selection process - even gradually, as with the recent unveiling of the six pre-candidates - is a risk. ``But I think the party takes more risks of this type every time,'' he says. ``It can do it because of the strength it has throughout the country.''

The old-style PRI may be ill-suited for a restless and rapidly modernizing population. But it still seems to have the power to regulate the pace and direction of reform. Indeed, political analysts agree on the basic channels for changes:

They will probably not be forced from the outside, unless the opposition parties can suddenly mobilize the large pool of apathetic voters.

They will likely come from the President, possibly from the highest echelons of PRI, but probably not from the party's ``bases,'' or grass roots.

There will be gradual changes, since the PRI can and will pick its spots.

But beyond those conditions no one can predict what shape Mexican politics will take. One factor will be the economy. Will the recovery in Mexico's external sector continue? And will it eventually translate into improved domestic performance and social conditions? Even if the answers are clear, they could be read in different ways: Some analysts argue that the country needs a period of economic growth to cushion the cost of political change; others say that only an economic crisis could create enough pressure for change. In the end, only one opinion counts: that of de la Madrid's successor.

Yet even when the PRI candidate is revealed within the next few days, his ideas about reforming the system will likely remain unknown until he steps into office Dec. 1, 1988. It's a tradition: Would-be presidents show absolute allegiance to the current president; but once in power, they stake out an independent agenda.

Says one Western diplomat: ``They've been so closely associated with the policies of this regime that one can't predict where they may diverge.''

But one thing is sure: Mexico's next president will face a political pressure-cooker, one that will make change nearly unavoidable.

Last in a series. Previous articles ran Sept. 28 and 29.