The peculiar preliminary rituals of Mexico's presidential campaign have entered their most public phase.

In grueling day-long sessions, cabinet officers deemed to be leading contenders for the 1988 nomination stand before Congress to present long policy speeches and suffer impertinent questioning from oppositionists. Televised nationally, these marathons make for less than compelling theater. But they are crucial opportunities for ruling party leaders to see if presidential aspirants can perform credibly in the spotlight.President Miguel de la Madrid, who as budget minister in Mexico's last administration was considered a second-tier contender, saw his own political stock boosted enormously by an assured, professional appearance before Congress six years ago. Late in 1987 Mr. De la Madrid will exercise his traditional right to designate his own successor. Yet this personal choice will be influenced strongly by the judgement of other senior members of the Institutional Revolutionary Party as well as by the perceptions of other critical power centers - the Mexican military, the international banks, the U.S. government and, perhaps most importantly, the private business community.

Ostensibly, the subjects of these ministerial dialogues with Congress are confined to those in each cabinet portfolio. Alfredo del Mazo, the minister of Energy and State Industry, detailed plans to modernize government enterprises in his Nov. 11 presentation. Interior Minister Manuel Bartlett Diaz followed two days later to outline a proposed overhaul of Mexico's congressional election system. Budget and Planning Minister Carlos Salinas de Gortari - still considered the third leading pre-candidate, though his political star is falling - is being called Tuesday to defend his 1987 spending program.

Yet the presentation's content is far less important than the ability to project an air of competence and a confident, presidential personality. By that measure, most analysts agree, Mr. Bartlett clearly bested Mr. Del Mazo. The interior minister appeared relaxed, speaking largely without notes and engaging oppositionists in spirited but respectful debate. Mr. Del Mazo, by contrast, seemed brusque and high-handed, crowding the congressional chambers with cheering political boosters and relying on a conspicuous 40-man computer team for a constant flow of prep sheets.

Mr. Del Mazo, Mr. Bartlett and Mr. Salinas are considered firm front- runners. But there are several darker horses in the race - among them Education Minister Miguel Gonzalez Avelar, anointed as the fourth leading contender by the PRI's Mexico City chairman; Manuel Camacho Solis, who as minister of Ecology and Urban Development has capably handled earthquake reconstruction efforts; Ramon Aguirre, Mexico City's appointed mayor, a close personal friend of the president; and Mario Ramon Beteta, the state oil monopoly chief and canny survivor of myriad bureaucratic wars. Any governor promoted to ministerial rank in the next few months would automatically join the favorites; anyone who leaves the cabinet - as former Finance Minister Jesus Silva-Herzog, once considered the strongest contender, was forced to do last June - is just as automatically excluded.

Mexican king-making is an arcane game, but it is prudent to pay heed. The personality, ideological inclination, and political commitments of the next president could be the biggest short-term determinants of Mexico's economic future. And deepening economic troubles in Mexico could have direct, profound consequences for U.S. farmers, manufacturers, senators, small-town mayors, big-city bankers, federal judges, corporate treasurers and a host of other political and economic actors not normally so affected by events beyond America's borders.

Mexico's new leader - whoever he is - will face hard choices from the moment he takes office on Dec. 1, 1988. The foreign debt, bankers and government economists concur, will have to be thoroughly renegotiated. The US$12 billion bail-out about to be delivered by creditors will at best get the country through 1988; in 1989, the debt servicing burden will once again be unsustainable. The president could feel forced, depending on the temperament of creditors, to negotiate a conciliatory default as an alternative to more borrowing and IMF-supervised austerity.

Other tough economic decisions await. The new president could opt to maintain the highest levels of protectionism permissible under GATT, mimicking the Brazilians. Or he could move toward more open trade, risking increased unemployment and the wrath of big business and labor. If triple-digit inflation persists, the president will be under pressure to either index the economy or subject it to the shock treatment of a wage-price freeze and currency reform on the Argentine model. Foreign capital must be courted, ruffling political sensibilities as non-Mexican investors demand to be cut in to agriculture, mining, oil refining and other businesses now restricted largely to nationals.

Mexico's intensifying political crisis could prove a more difficult challenge than the economic crunch. The president could choose to defuse anti- government hostility by opening the system's doors to the opposition. More probably, though, he will concentrate on shoring up the PRI, reforming the party internally but steadfastly refusing to cede power to electoral opponents.

Many experienced Mexican analysts expect the next regime to be marked by more dissent and more repression. Underlying this thesis is the widespread assumption that the next president will be either Mr. Bartlett, who as interior minister has strong-armed a series of fraud-tainted PRI election wins, or Mr. Del Mazo, who gained a similarly authoritarian reputation during his six-year governorship of the state of Mexico.

The two men are perceived by critics and admirers alike as hard-line PRI partisans who would work to expand the power of the presidency. Mr. Bartlett, 50, and Mr. Del Mazo, 44, both convey an image of vigor and decisiveness that many PRI professionals welcome as a contrast to the bland, seemingly vacillating Mr. De la Madrid. Most importantly, they are the only front-line aspirants with extensive practical political experience. Even foreign bankers have come around to the politicians' view that Mexico has been governed long enough by Ivy League economists.

As a potential candidate, Mr. Del Mazo enjoys many strengths: He is young, telegenic, an effective orator, and has close ties to top labor and military men - and, most critically, to President De la Madrid, a long-time mentor to Mr. Del Mazo and godfather to his child. As a former commercial banker, he is equipped to deal with the complexities of the foreign debt. Yet he is being tagged by business leaders as a champion of state industry and opponent of privatization - a perhaps inevitable consequence of his cabinet assignment, but an impression strengthened by his presentation to Congress. In laying out

plans for the modernization of big government-owned companies, he seemed to business critics to be portraying state enterprise as the industrial engine of Mexico's future. Bernardo Ardavin, president of the Mexican Employers Confederation and the private sector's point man, reacted by accusing the Energy and State Industry minister of plotting an economic dictatorship.

If businessmen are uncomfortable with Mr. Del Mazo, they may have nowhere to go but Mr. Bartlett. Carlos Salinas, the budget minister, a diehard central planner, worries the private sector even more.

Mr. Bartlett enjoys the political advantage of being one of the few cabinet members with no direct responsibility for the economic crisis. Yet in private conversations with U.S. officials and business leaders, diplomatic sources report, Mr. Bartlett has stressed a personal preference for freer trade, a freer domestic capital market, and less state intervention in industry. But a loosening of the government's hold on the economy could require tighter control of the political system, Mr. Bartlett's supporters argue.

Some foreign bankers, meanwhile, consider Mr. Bartlett the candidate least likely to embrace radical foreign debt proposals, such as a Peruvian-style limitation of interest payments.

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