Mexico City — The subject: Mexican-United States relations under the four-month-old government of President Miguel de la Madrid Hurtado. The prickly issues: warfare in Central America, and (to a lesser extent) illegal immigration and bilateral trade.
The tone: optimism over an apparent warming of relations, tempered by concern that a resurgence of Mexican nationalism or US self-centeredness could scuttle progress.
This is the overall picture of US-Mexican relations drawn by foreign diplomats, Mexican government officials, and US and Mexican businessmen in interviews here. But it is the polarity between US and Mexican views of the US role in El Salvador that they describe as the most pressing factor in relations between the two countries.
A Mexican political scientist close to high-level policymaking at the Foreign Ministry says Washington is causing ''a great deal of distortion'' by explaining the Central American situation as ''an East-West problem,'' with Salvadorean guerrillas and Nicaraguan leaders as proxies of the Soviet Union.
The Mexican view, he explained, is that the Central American nations need to reform their social and political institutions from within - as Mexico did before them. It is a very difficult problem, he says, and cannot be solved simply by ''a matter of elections.'' A solution to strains tearing at Central American nations can be found only by ''a long conversation and negotiation'' among the various sides, he says.
This analyst says that Washington, by supplying arms, is ''pushing the next Salvadorean government into a very radical position, which will not be a healthy thing.''
Asked whether El Salvador's future might follow Cuba's, he said: ''If that happened, that would worry us; but we don't think that will happen.''
The broader implications of the Central American question - particularly the concern that the seven nations from Guatemala to Panama will all be caught up in revolution - have raised the issue of the ''domino theory.'' Some US politicians and analysts share the view of Sen. Henry M. Jackson (D) of Washington, who says Mexico may be the real target of ''all Castro-Soviet activity in Central America.'' Such views spark vehement reactions here.
Foreign Secretary Bernardo Sepulveda Amor does not refute the domino theory, but he observes that Mexico is an ''awfully big domino.''
Shari Rettig, managing editor of the Mexico City English-language daily, the News, pours water on the domino theory when she says, ''People here don't seem especially inclined to become communists.''
She adds, ''Everybody seems able to get into the middle class.''
Analysts who speculate on prospects of revolution here seem to see more of a danger in rightists than in leftists. Their analysis projects that the military, backed by a middle class growing disenchanted with the left-leaning tendencies of Mexico's ruling political party, could lead a right-wing takeover.
Such a possibility, however far-fetched, worries some observers. ''What is happening in Central America pales in significance before what is happening domestically in Mexico,'' says one diplomat. He adds, ''The No. 1 foreign-policy issue [of the United States] concerning Mexico is stability.'' The change in presidents.
Since he took office Dec. 1, Mr. de la Madrid has been watched closely by foreign-policy specialists. Gone, they say, is the ''virulence'' of anti-Americanism that marked the last six months of former President Jose Lopez Portillo's administration. One Western diplomat characterized the end of that term as ''very leftist, very nasty.''
Last week Foreign Secretary Sepulveda said that Mexican-US relations now have top priority, but, he asserted, in response to hints from Washington that Mexico should pay more attention to domestic matters and less to foreign affairs, that ''Mexico is in the international arena to stay.''
Western diplomats also note a desire on the part of the new administration to work more closely with the US government on day-to-day matters.
But the director of a group of US businessmen here says he will wait and see how the new administration functions. ''It's new like a snakeskin,'' he says, explaining that ''it's a little more sensitive and far more brilliant.''
This businessman worries, however, that the ''snake'' inside is unchanged - an entrenched bureaucracy accustomed to treating the US as something of a scapegoat. He adds that Mexico's one-party system tends to make each new administration simply a reincarnation of the prior one.
International University of Mexico director T. Noel Osborn says, ''The situation has changed a great deal in tone in the last few months, but we had the same expectations for Lopez Portillo when he came in.'' The economic crisis.
Mexico's $85 billion debt is another uncomfortable factor in US-Mexican relations.
Last month the US Department of Agriculture signed a $1.7 billion loan package, allowing Mexico to purchase at least 6.2 million tons of grain, oilseed , and dairy products from US farmers.
New lines of credit (worth up to $2 billion) have also been opened through such agencies as the US Export-Import Bank. Petroleos Mexicanos (Pemex), the state-owned oil company, has already taken advantage of the latter, signing a $ 100 million loan agreement with Crocker National Bank in San Francisco.
Pemex has opened a new office in Washington, which company spokesmen say is intended to ''establish improved relations with the United States.'' Mexico has already agreed to sell the US increased amounts of oil for its strategic petroleum reserves.
The flagging economy also has made the Mexican government change its attitude toward foreign investment. For years it had lambasted multinationals. Figures from the American Chamber of Commerce of Mexico show that new US investment in Mexico slipped from $1.1 billion in 1981 to $750 million in 1982, and estimates show even lower figures for 1983. But the administration is trying to change this, relaxing requirements that firms operating here must have majority Mexican ownership.
Mexico, which is not a member of the General Agreement on Tariffs and Trade, is also seeking bilateral trade agreements with the US. These could help soothe tensions over exports of Mexican beer and tomatoes, and even of Volkswagen beetles, which are manufactured here but do not conform to US exhaust emission and safety standards.
Another difficult issue in US-Mexican relations is illegal immigration. The numbers of Mexicans going to the US in search of work depends on the relative prosperity of the two nations. Many see the Mexico's complaints over the Simpson-Mazzoli immigration bill (which is designed to tighten the so-called ''tortilla curtain'' along the border) as rhetoric to appease Mexicans at home.