MEXICO CITY, OCT. 4 -- Mexico's ruling party today proclaimed as its presidential candidate Budget Director Carlos Salinas de Gortari, 39, the chief architect of economic rebuilding here.

Jorge de la Vega Dominguez, president of the Institutional Revolutionary Party (PRI), announced the nomination, tantamount to election in voting next year, at a morning press conference. "You will be the next president of Mexico," he said, presenting Salinas at a rally outside party headquarters two hours later.

"Filled with emotion, I accept the party's decision," Salinas said in a speech lauding the egalitarian ideals of the Mexican Revolution and the governance of President Miguel de la Madrid, his career-long political benefactor.

In an attempt to heal the wounds of a bitterly fought prenomination campaign, Salinas made a point of praising by name each of the five other Cabinet ministers who had sought to be the PRI standard-bearer. "Our world is changing with great rapidity," he said, pleading the cause of economic and political "modernization."

The PRI's decision appeared uncertain up until the moment of the announcement. From dawn, well-wishers had gathered at the house of another presidential aspirant, Attorney General Sergio Garcia Ramirez, to congratulate him for what had been erroneously announced in some local media as his designation.

One of Salinas' strongest rivals, Energy Secretary Alfredo del Mazo, in a radio transmission, was quoted as expressing support for the PRI's "magnificent choice" of Garcia Ramirez. Members of Salinas' political team saw the Garcia Ramirez boomlet as a last-minute attempt to derail the budget minister's candidacy.

A Harvard-educated economist respected for his intellect and skills as a political infighter, Salinas until recently was thought to be relatively unpopular among party regulars. But today, in an impressive demonstration of the PRI's fabled ability to rally rank-and-file support for the leadership's choice, about 50,000 rallying PRI union and peasant members enthusiastically chanted Salinas' praises.

Often somewhat somber in public, a contrast to the self-assurance and wit that he displays in private, Salinas today was beaming and waving from the moment he boarded the PRI bus that took him from his southern Mexico City home to party headquarters.

Salinas will step down from his Cabinet post as secretary of budget and planning. At a PRI convention called for Nov. 7, he is expected to be nominated by acclamation. The presidential election will be held July 6, 1988, for a single six-year term beginning in December. In its 58 years, the PRI has never lost a national election.

While even the PRI's opponents concede the inevitability of a massive ruling party victory next year, Salinas will still embark on a grueling, costly campaign tour to fortify local support. The campaign will be financed in part through mandatory paycheck deductions from managerial-level government officials, the PRI announced last month.

Few negative views or expressions of surprise about the nomination were aired today on Mexican radio and television. Yet the decision, though rumored strongly in recent weeks, disconcerted many analysts and politicians who had thought him outmatched by his two main rivals, Internal Affairs Secretary Manuel Bartlett Diaz and del Mazo.

A slight, balding man whose owlish countenance is beloved by Mexican political cartoonists, Salinas was also considered in some PRI circles to be insufficiently "presidential" in appearance.

Among six officially recognized PRI presidential hopefuls, only Salinas had provoked open opposition within ruling party ranks. Yet this resistance came from critics of administration economic programs and may have fortified de la Madrid's determination to see his policies maintained, sources close to the president said.

"The key thing is that de la Madrid was able to name him because Salinas has some real strengths, as well as the obvious drawbacks," said Jorge G. Castaneda, a prominent political analyst who has often been critical of Mexico's present government. "Even though his base is narrow, Salinas has managed to cultivate support in diverse sectors -- the private sector, of course, but also among some older politicians and the left. And the Mexican intelligentsia, to the extent that one can generalize about it, has a cultural and generational affinity for Salinas."

Moreover, Castaneda added, voicing a view shared even by the candidate's rivals, Salinas "is obviously extraordinarily intelligent. He has also put together a homogeneous team. People say the Salinas group is ready to run the country right now, and they are right."

But while exercising the incumbent's traditional right to designate his successor, de la Madrid took unusual care to secure party backing of his candidate beforehand. In one unprecedented move, the six Cabinet members were publicly identified as presidential aspirants in August and called to present their views publicly before party elders. Since then, the PRI has observed further candidate registration formalities that seemed designed to persuade voters that the nominee is chosen by the party, not the president.

"Nobody within the system ever doubted that the president would make the final choice himself," said a PRI congressman. "But it was thought that opening up the process might affect who that choice would be."

Born in Mexico City on April 3, 1948, Salinas would be 40 at the time of the December 1988 inauguration, making him Mexico's youngest president in four decades. "The Salinas candidacy represents a change of generational command," said Luis Javier Garrido, a historian who has written extensively about the PRI's internal affairs. "Youth may be his greatest political advantage."

Salinas' biggest political difficulty, Garrido said, will be to differentiate himself from the de la Madrid administration without seeming to repudiate its economic reforms.

"A secret of the Mexican system is its ability to shift politically from one administration to the next, but the central message of Salinas is the continuity of de la Madrid's economic program," Garrido said. "This could be a real problem for him."

Salinas is unlikely to reshape dramatically Mexico's independent foreign policy, analysts said, while cautioning that his true views on such issues as Central America may not yet be known. Many believe he may take a tougher line on the foreign debt, though, demanding eased repayment terms when negotiations resume in late 1988 or 1989.

As the Cabinet officer who directly managed Mexico's economic program, which is endorsed by the International Monetary Fund, Salinas and his advisers are esteemed highly in the international financial community.