Mario de la Torre opened his government-sponsored campaign to become mayor of Chihuahua recently by meeting with local garbagemen. Since then, to hear him tell it, he has met with every workers' group in town, "right down to the mariachi bands."

In his back-to-the-people race to win municipal elections July 6, de la Torre has become a symbol of sorts for Mexico's official Institutional Revolutionary Party, PRI by its Spanish initials. After more than 50 years as the country's invincible electoral machine, the party has suffered several recent setbacks here in northern Mexico. Smarting from the experience, it has set out to reinvigorate its ability to gather popular support behind a government rooted in a 1910 revolution and faced with a 1980s economic crisis.

The PRI's efforts as government vote-raiser will help determine how Mexico's unusual political system responds to the social and economic pressures accumulating as a result of slumping oil revenues and burdensome payments on a $98 billion foreign debt.

In the eyes of some knowledgeable observers, Mexico's struggle to deal with these pressures, although less dramatic, is as important to regional U.S. interests in the long term as the Central American conflicts that have occupied Washington's immediate attention.

On Monday, the Western Hemisphere subcommittee of the Senate Foreign Relations committee is to hold a closed hearing on the situation in Mexico. The subcommittee, chaired by Sen. Jesse Helms (R-N.C.), is to continue with open hearings on Tuesday.

The PRI named de la Torre as the most likely candidate to recover this northern town's city hall from an opposition mayor who has been in office since 1983. The opposition National Action Party, or PAN, to the right of the ruling party, won seven of the state's 67 city halls in the 1983 vote, including Chihuahua, the state capital, and Ciudad Juarez, the major urban area just across the border from El Paso, Tex.

Although minor on a national scale, the losses dramatized growing discontent with Mexico's traditional political system, particularly in the urbanized north, as a way to cope with the crash in oil prices and consequent blows to the economy known across Mexico as "the crisis." They added to earlier PAN wins in the cities of Hermosillo, Durango and San Luis Potosi.

"It was a vote of condemnation of our administration of this city," de la Torre acknowledged. "It was not a vote in favor of the PAN. It was a vote against us."

Equally important, the antigovernment vote here encouraged hopes among PAN officials that they could win a state governorship and solidify rule in Juarez and Chihuahua, along with city halls of other Chihuahua towns.

Although routine in the United States, and far from a threat to the PRI government's domination of the country, such changes would be significant developments in Mexico. They would be read here and in the capital, as well as abroad, as more signs of slippage from the system's perennial grip on politics.

In more than 50 years of stewardship, the PRI has never lost a state governorship and never allowed a major city to be governed by opposition leaders for more than a single term. Although the system is denounced as "party dictatorship" by some, officials point out that it has given Mexico a half century of stability in a region known for coups d'etat and turmoil.

The ruling party showed determination to maintain its record during elections for governor last summer in the northern states of Sonora and Nuevo Leon. Impartial analysts said before the vote that the PAN had a slim chance of winning in either state. But the PRI turned out a 3-to-1 victory in both. The opposition charged widespread fraud.

Now the test is here in Chihuahua, for the governorship as well as city halls. The PRI's gubernatorial candidate, Fernando Baeza, faces a tough challenge from the PAN candidate, Francisco Barrio.

Strongly backed by the national apparatus, Baeza has been going all-out to overcome this in the same manner as de la Torre. Pointing to the experiences of last summer in Sonora and Nuevo Leon, impartial observers here predicted both will win on July 6.

"We are not playing to lose," said Jose Diaz Redondo, spokesman for the PRI National Executive Committee in Mexico City.

One method has been to allow greater local participation in selection of the ruling party's candidates. In recent times, they have been picked by the national leadership in the capital. President Miguel de la Madrid, speaking in March, hailed the shift as "democratization" necessary to renew the party's strength and sensitivity to the national mood.

De la Torre, for example, said he was selected March 19 from four aspirants by delegates to a state-level party convention. But critics maintained that his selection, like that of Baeza, actually came from Mexico City, with the local conventions only giving the appearance of party democracy.

"That which has the PRI up against the wall in Chihuahua is its own discredit before a citizenry that demands the one thing the official party cannot give: respect for its democratic vocation," wrote Francisco Ortiz Pinchetti in the leftist magazine Proceso.

Frederico Osorio Altuzar, writing in the capital's Novedades newspaper, cited similar complaints on a national scale. These candidates, "far from being created by a collective will, more and more are made in the image of . . . the governing group," he said.

Neither de la Madrid nor his two immediate predecessors had ever held elective office before running for president, such critics point out. All three ascended through the bureaucracy. This marked a break with traditional PRI machinery politics and, according to the critics, was symptomatic of estrangement from grass-roots membership in favor of technocrats.

Disillusionment also has grown from a widespread belief here and elsewhere that the government has resorted to electoral fraud whenever its dominance was threatened. Archbishop Adalberto Almeida y Merino of Chihuahua, in a March 19 pastoral letter, charged:

"At the base of the corruption that affects the country is a greater corruption, which is electoral fraud . . . . it emerges not only in falsification of votes, but also in all those reproachable methods that lead to weighting the results in favor of one party, going over the head of free election by citizens."

Fermin Fernandez, who heads a local PRI-affiliated truck drivers' union, mentioned corruption in a conversation about things that trouble him. But he expressed confusion about how pervasive it is or what can be done about it.

"I just wish they would point out the corrupt ones, just say, 'You, you and you,' " he said, shifting his boots in the dust of a Chihuahua suburb. "I wish I knew where the problem is."