MEXICO CITY, AUG. 30 -- Mexico's long ruling Institutional Revolutionary Party, under pressure to change a mysterious system of choosing presidents, has opened its internal political process a bit by officially naming six presidential "precandidates" and putting them on display.

Senior government officials and leaders of the party, known by its initials in Spanish as the PRI, are hailing the opening as the end of tapadismo, a system in which the incumbent president essentially handpicks his successor. But party dissidents dismiss the change as a "tricky maneuver" to relieve pressure for greater democracy in the presidential succession system.

Although the opening falls well short of dissidents' demands, political analysts said, it does carry some risks for party leaders and could herald major changes in the future.

"The next administration will have to make more than a cosmetic effort to change the political system," said one diplomatic analyst. "If the PRI tries to put the cork back in the bottle, they might find it blowing up on them."

The presidential successor, called the tapado, or "covered one," traditionally has been chosen by the serving president from a short list of "precandidates" and then, in the year before the elecion, suddenly "uncovered" as the party's standard-bearer as if by popular acclaim.

Since the party has never lost a race for president, governor or senator in the 58 years since its founding, the PRI's candidate is virtually assured of becoming the next Mexican head of state after the July 3, 1988, presidential election.

An indication of the opposition to the system came during a demonstration Friday in front of the Mexican legislature by leftist protesters, who burned a huge papier-mache finger in effigy. That bit of symbolism referred to another term for tapadismo: the dedazo, or fingering, of the presidential successor.

In an effort to undercut that sort of criticism, the PRI on Aug. 9 announced the names of six presidential contenders: Mexico City Mayor Ramon Aguirre Velazquez; Interior Secretary Manuel Bartlett Diaz; Energy, Mines and Parastate Industry Secretary Alfredo del Mazo Gonzalez; Attorney General Sergio Garcia Ramirez; Education Secretary Miguel Gonzalez Avelar and Budget and Planning Secretary Carlos Salinas de Gortari. In a process completed last week, the six then appeared individually before the party leadership, in alphabetical order, and gave nationally televised speeches.

"All the party has done is institutionalize a process that existed before," said Jorge Castaneda, a professor at the National Autonomous University of Mexico. Although the PRI was loath to admit it, a secret list of candidates existed during previous successions, and those on it engaged in an unofficial "show-and-tell process," he said. "What's important is that the effects this can produce may be long-lasting and very significant," Castaneda said.

"The opening can only get larger," said a diplomat. He added that Mexicans "have been turned off politically" by the system, upon which many feel they have no real impact. He cited what he said was growing abstentionism in elections, notably a July 5 election for governor in Mexico State, where an opposition party estimated that 75 to 80 percent of the electorate did not vote. The government said it was about 50 percent.

The risks in opening the system, this diplomat said, were that eventually the president and the PRI leadership might lose control over the presidential succession and other candidacies for high office. But, he added, "I don't see that happening anytime soon."

Although senior government and party leaders deny it, Mexican and foreign political analysts agree that pressure from a dissident movement within the PRI called the Democratic Current was instrumental in the decision to present the six presidential "precandidates." The movement is led by Porfirio Munoz Ledo, a former president of the PRI, and Cuauhtemoc Cardenas, a former governor of Michoacan State and son of the late president Lazaro Cardenas, one of the most revered in Mexican history.

In addition to more populist economic and social policies, the two are demanding an open registry of PRI presidential contenders, which would allow Cardenas to run, and a genuine party convention to choose the PRI's candidate.

"They are just two guys without a job demanding things that they did not do when they were in power," said a senior government official in denying that the Democratic Current wields any influence. "They don't mean a thing."

However, Castaneda disagreed. "There is no question that the Democratic Current played a major role" in pressing the party to open up, he said. In addition, he cited "an intellectual clamor" against tapadismo and bad publicity about the system internationally.

According to another diplomat, the announcement of the candidates was a move to "take the wind out of the sails of the Democratic Current," but one that amounted to "window dressing." A leading Mexican news weekly, Proceso, called the move "a farce."

"There is still no open competition, and the PRI still takes it upon itself to list the candidates," the diplomat said. "Tradition still doesn't allow them to campaign on an independent platform, and they have launched into a period of self-congratulations." She said she found the contenders' speeches "almost nauseating in their praise" of President Miguel de la Madrid, who by all appearances will still be the one to choose the next president.

Indeed, those who had hoped for a real debate among the contenders were disappointed by the similarity of views expressed during their appearances. Newspapers have been reduced to the journalistic equivalent of tea-leaf reading to discern how each might differ from de la Madrid. But the appearances have captured the headlines, with some papers going so far as to report in excruciating detail how the contender said he slept the night before his speech and what he had for breakfast that morning.

According to some analysts, the real short-term risks in the process will come in the tense period now beginning -- the several-week interval between the just-completed appearances of the precandidates and the unveiling of the nominee. This could be a time in which followers of the contenders try to undermine their main rivals, the analysts said.

Suspicions of such maneuvering already have cropped up with the recent publication of two books. One, a political thriller that was seen as disparaging the country's powerful oil workers' union, reportedly prompted the union to blame Budget and Planning Secretary Salinas and commission a book in retaliation, a diplomat said. The second book, entitled, "A Killer Can Become President," recounts an incident in which Salinas, as a child, accidentally shot to death a family maid.

Salinas, who has been most emphatic about carrying on de la Madrid's policies of economic austerity, is said to be the least favorite candidate of Mexico's powerful 87-year-old labor boss, Fidel Velasquez, who reportedly has been influential in past choices of the PRI presidential candidate. Velasquez is said to favor del Mazo, who, although de la Madrid once called him "the younger brother I never had," is considered to be the least predictable of the contenders.