Rosario Ibarra is not the sort of person you expect to be running for president of Mexico. Not long ago she was a middle-class mother of four with little political background. Today she is a symbol of important changes occurring in Mexico.

Mexicans vote today for a new president, and Rosario Ibarra's will be the first woman's name ever to appear on a Mexican presidential ballot. And although she has no expectation of winning, she has been traveling the country for weeks, carrying her minute body to the top of a van, and speaking out in a commanding voice to denounce conditions in Mexico.

A woman running for president in a country where machismo still thrives in its almost undiluted original form is an appropriately jarring image. Four years ago, there was only one name on the presidential ballot, Jose Lopez Portillo. That was the old Mexican politics dominated by the Institutional Revolutionary Party (PRI), which has long claimed to govern on behalf of the Mexican revolution but which has carefully preserved the privileges of the Mexican establishment.

The PRI's candidate this year will win again, but Rosario Ibarra and five other opposition candidates will take a significant portion of the votes. If that portion rises as high as a third, the PRI could be forced to share real power with the opposition groups.

That would shatter the traditional, authoritarian model of Mexican politics. In the long run, such a change just might put Mexico on the road toward the kind of unstable, multipolar politics that has been the bane of other Latin American countries for generations.

The immediate reason for this change in Mexican politics is a series of reforms that have permitted the formation and official registration of new parties on both the left and right. Their presence on the ballot makes today's the first Mexican election with significant participation of a broad opposition.

Equally important, the voting takes place under adverse circumstances for the PRI.

The oil boom has plunged Mexico into its worst economic crisis in four decades. Corruption appears to have kept pace with the boom; stories about officials who go abroad with vast fortunes abound. The peso has lost nearly half its value since February, and inflation, running at 60 percent this year, has deeply hurt the millions whose wages are not adjusted to compensate for the soaring cost of living.

As the credibility and authority of Mexico's highly centralized political system slowly erode, officials betray a nervousness that is unusual for this country's political establishment.

Miguel de la Madrid, the official candidate, has told reporters he will get at least three-fourths of the vote. But other political leaders do not seem so sure.

"We have never gone to elections like this," said a senator who belongs to the PRI. "We don't know how people will react to the inflation, which has never been so terrible."

The diminishing prestige of the official "revolutionary family," as the PRI is known, raises a question that in the long run will be far more serious than the outcome of the election. That is the extent to which the peculiar Mexican political system can retain its strength and the country its stability at a time when the magical powers of oil money are diminishing, both because there is less of that money, and because money by itself cannot satisfy growing public demands.

"The strength of the Mexican system has always been its capacity to conciliate and to adjust to popular demands," said a senior government politician. "What worries me is that our political and economic capacity to respond is becoming more and more limited. We are now 72 million people, the country is large and the problems have become large, but politics are still run like 30 years ago."

All the same, none of Mexico's eight opposition parties has so far provided an articulate or workable alternative. Three are essentially satellites of the PRI. Of the two rightist parties, the National Action Party, known as PAN, is the older and stronger. The Communist Party, recently legalized, is the best organized of the three parties on the left.

Why do these parties bother to campaign against a government party machine that controls the labor movement and the main peasant organizations and can draw on almost all the resources of the state?

All opposition leaders answer that they must capitalize on the country's rising discontent. In addition, they want their share of the 100 seats in Congress -- one-fourth of the total-that are reserved for minority parties. They admit, however, that in this country the Congress is virtually powerless.

Rosario Ibarra, the presidential candidate of the Trotskyite Revolutionary Workers Party (PRT), says, "The campaign gives us a chance to denounce everything that's wrong with the government, to raise people's consciousness, to get together the people who are fed up."

Ibarra, who insists she is not a Trotskyite herself ("just running on a the PRT ticket") says Mexicans are "politically astute" but tired of theories. "They've heard official political dogma all their lives, and they know that dogma has been used only to deceive them."

Ibarra's campaign style has been impressive. She's traveled this large country with a small group of volunteers, speaking on street corners or at popular events she happened upon. She's distributed leaflets in factories and harangued workers, urging them to speak up for their rights. She's visited the jail in every town she passed through. Her lively personality and straight talk have aroused the curiosity of bystanders. Being a woman in a man's country helps, she remarks:

"I find workers coming up to me saying, 'You're a woman and so small and thin and you dare stand up to the government and we don't.' They feel what I'm doing is almost a challenge to their machismo, an invitation to get active themselves."

Ibarra, 51, went into politics in 1975 after her 21-year-old son reportedly was picked up by police for alleged subversive activities and never seen again. During her search for him, she discovered other mothers knocking on doors, looking for their children. Her national committee for the defense of political prisoners and disappeared persons has not stopped pressuring the government since.

Like Ibarra, representatives of the rightist PAN and the leftist PSUM, the coalition led by the Communist Party, report finding deep discontent in the countryside.

"Mexico is full of little protest groups and angry communities," says Ibarra. "We only have to channel the discontent."

In the context of Latin America, Mexico has substantial tolerance for opposition groups, yet it still falls short of the standards of the democracy it purports to be. During their presidential campaigns, all the parties have reported varying degrees of harassment from local political bosses and police.

Campaign workers and local candidates land in jail and stay there until the Ministry of Interior gets involved and orders their release. The communist congressional candidates in Sonora and Michoacan have been fired from their jobs. In some Chihuahua villages electric power was cut off when PAN meetings began. Ibarra's home telephone gets cut off whenever she stays in Mexico City, and she receives frequent anonymous death threats by mail, she says.

The opposition believes its greatest challenge is to give credibility to the electoral system.

"People do not see elections as an instrument of change," says PAN spokesman Gonzalo Altamirano. Often his party has charged fraud in local, state and national elections. Voter apathy, PAN has argued, works in favor of the status quo.

Moreover, the setup of the electoral machinery, opposition members argue, suggests that elections cannot be altogether clean.

"The federal electoral commission is headed by the minister of the interior, and the polling stations are presided over by PRI representatives," remarks PSUM candidate Arnoldo Martinez. "There are 45,000 polling booths in the country. Only the PRI has the apparatus to man them all."

Ibarra brushes aside such points. She is determined that her party will get eight to 11 seats in Congress.

"We know Congress cannot do much, but we'll wake them up," she says. "We'll make sure their hibernation will end."