MEXICO BOMB AT THE END OF A FUSE?

"Death to Reagan!" says one of the red, hand-painted slogans splashed on a wall in the marketplace in this lovely town in southernmost Mexico. "Yankees, get out of el Salvador," says another.

It is perhaps typical of the Mexican law students who are said to have painted these slogans that they have turned their wrath toward perceived injustices outside Mexico rather than those which persist within. The walls of San Cristobal are plastered with slogans. Only a few deal with this country's internal problems.

No doubt nearly El Salvador has worse problems than Mexico. Many Mexicans seem to feel that those two countries are ruled by corrupt, repressive regimes which will inevitably be overthrown.

But Mexico itself seems at first glance to have some of the same ingredients for trouble: a vast gap between rich and poor, with poverty in some areas actually increasing; social and economic injustices; malnutrition and high infant mortality rates; great unemployment and widespread underemployment.

In the United States, there is a tendency to look at Mexico's many problems -- and its oil wealth -- and to conclude that it is a huge domino waiting to be toppled at the end of a chain of smaller American dominoes. Most Mexicans don't seem to see it that way. The idea that Mexico could share the fate of El Salvador and Guatemala would strike many among the Mexican elite as absurd.

This reporter tried the domino theory out on a Roman Catholic priest who has worked for years with some of the most underprivileged Indians in the state of Chiapas, Mexico's oil-producing southernmost state. The priest is intimately familiar with the state's problems and is highly pessimistic about the outlook for its poorest inhabitants. He speaks of "real hunger" among some of them.

Couldn't the fall of neighboring Guatemala combine with revolutionary conditions in Chiapas to produce a domino effect?

The priest laughs, as if to say, you don't understand Mexico.

He explains that unlike the political parties and military rulers of Central America, Mexico's ruling Institutional revolutionary Party (PRI) has roots everywhere. It has come to power through elections. It has legitimacy. It has also adopted much of the left's revolutionary rhetoric. At the same time, the PRI's politicians have bought off, or co-opted, much of Mexico's leftist political opposition. While pursuing relatively conservative policies at home, the Mexican government has appeased its opposition to a degree by leaning to the left in the its foreign policy.

American diplomats tend to agree with the priest's analysis of the PRI's reach and control. The political section at the US Embassy in Mexico City makes five-year predictions as to Mexico's stability. The latest gives Mexico another five years without major disruption.

But some diplomats see possible peril at the end of that period. At that point, many of the youths among the nearly 50 percent of the population that is under 14 years of age will be looking for jobs. According to one estimate, Mexico's current rate of unemployment is 11 to 12 percent. Its "underemployment" reaches as high as 45 percent.

Where the Mexicans seem to have an advantage over most of their Central American neighbors is in the variety and sophistication of their response to protest. In Chiapas, for example, where much of the sizable Indian population lives in poverty, they have shown that they can be both tough and accommodating.

On the one hand, when peasants took over land at one at the big estates last year, the Army was called in and opened fire. Several persons were reported to have been killed.

On the other hand, when an opposition party pushed some people to assert themselves in a squatter community on the outskirts of Tuxtla gutierrez, the PRI responded with promises to install running water for the community.

By extension, the Mexicans seem to think they can buy off some of their leftist neighbors in Central America. Mexico maintains warm ties with the ruling Sandinista directorate in Nicaragua. It also has ties with the leftist-led opposition in El Salvador. It apparently sees itself both as a friend and competitor of Cuba.

One high-ranking PRI official told an American not long ago that now that Tomas Borge Martinez, the Nicaraguan interior minister, was wearing a Rolex watch --"Somoza's watch," was the way he put it --Mr. Borge's communist ideology mattered less than it once did.

Some US officials -- but certainly not all --think the Mexicans are deluding themselves.

Constantine Menges of the Hudson Institute, who acted as a foreign policy consultant to the Reagan administration transition team, thinks Mexico could be vulnerable to a new revolutio fed by a coalition of Marxists in Central America and communists and radical labor groups within Mexico itself. He calls Mexico the "Iran next door."

But many Mexicans are suspicious of any suggestion that their country may be on a Soviet, or Cuban, "hit list." Thanks to a history of US intervention in Mexico and elsewhere in the region, the Mexicans tend to keep more of a wary eye on the North Americans than on anyone else. Talk of Soviet intentions, in the Mexican view, could be camouflage. They think that US policy toward Mexico is dominated by a desire to gain access to Mexican oil. They do not rule out, under emergency conditions, a North American attempt to take over the oil fields.

What seems certain is that Mexico will continue to pursue independent policies abroad while maintaining the peculiar Mexican mixture of toughness and restraint at home.

"The Mexicans are a lot less democratic than they pretend to be," said a Guatemalan teacher who has been forced to flee his home country because of political repression. "But you have to admire them. They're a lot smarter about the way they repress people than our government is."