MEXICO CITY, AUG. 30 -- For the past two weeks, six men who have been named "precandidates" for president, have given what amounted to campaign speeches before Mexico's ruling party, an extraordinary development in a political system in which candidates never used to acknowledge their ambition until the president picked the nominee.

None of the candidates seemed significantly helped or hurt by his "acceptance" speech. But, despite the near-universal assumption that this year's candidate, as in the past, ultimately will be selected by the president, many observers believe that public opinion and the preferences of party factions will weigh more heavily in the president's decision than previously.

Of the six "precandidates" announced Aug. 9 by the Institutional Revolutionary Party (PRI), the three leading contenders are considered to be Mexico City Mayor Ramon Aguirre Velazquez, 51; Interior Secretary Manuel Bartlett Diaz, 51; and Alfredo del Mazo, 43, the secretary of energy and mines.

Aguirre, a long-time close friend and ally of President Miguel de la Madrid, criticized the "social costs" of the government's economic policies -- a telling sign, analysts said, that any PRI candidate will feel obliged to promise a quick increase in employment and real wages.

Bartlett, by contrast, devoted most of his address to a vigorous defense of de la Madrid's programs. This was intended to compensate, some PRI veterans later contended, for the perceived liability that he is not as close to the president as are Budget and Planning Secretary Carlos Salinas de Gortari and del Mazo.

But Bartlett drew attention to the relatively few social confrontations in the nation's past five economically trying years, an achievement often attributed to his skills as a mediator. And in an acknowledgment that the PRI's popular support is eroding, he warned that the party should work "to recover the people's confidence."

Bartlett, the only experienced politician among the six, even though he has never run for elective office, received the warmest reception from the assembled PRI professionals, observers said.

Del Mazo, a former banker who joined the Cabinet 16 months ago after serving five years as a state governor, noted that he is the only contender who has ever run in an election. His 1981 gubernatorial campaign was "the greatest human and political experience of my life," he said.

The strongest of the second tier of candidates, Attorney General Sergio Garcia Ramirez, 49, protested to reporters that he was "neither dark nor a horse." Yet after his speech oddsmakers were still betting against him.

Despite his elegant prose, Garcia disappointed supporters, who hoped he would stress the broad practical experience he has acquired -- along with a reputation for integrity -- in decades in the top echelons of law enforcement.

Education Secretary Miguel Gonzalez Avelar was given little chance at the outset to win the PRI's backing. That judgment did not change after his speech, although he was widely applauded for a tough indictment of Mexico's failure to find jobs for its fast-growing work force.

Salinas, an economist with two Harvard postgraduate degrees, is viewed sympathetically in business circles and in the state financial bureaucracy but must overcome hostility from PRI unions and old-line politicians distressed by his budget cutting.

As a close presidential confidant and the government's main economic policy maker, Salinas is arguably the most powerful of the six candidates. He is also the staunchest advocate of trade liberalization and economic modernization.