US looks to changed relations with Mexico. A more-democratic Mexico may be unpredictable for the US - but more stable in time. For Mexican opposition, it means opening up closed system, P. 9.

United States relations with Mexico will never be the same again. For 49 years, the US could count on Mexico's Institutional Revolutionary Party (PRI) to win every presidential election and every Senate seat. But with the surprisingly strong showing of the new center-left coalition in last week's elections, that is no longer the case.

This emerging pluralism will make Mexico less predictable, but it could make for greater stability in the long run, say State Department officials and independent analysts.

``Democracy is a risk; it does not automatically produce what the US wants,'' says M.Delal Baer, a Mexico specialist at the Center for Strategic and International Studies. ``But the risks of a closed system are even greater.''

The longer the system remained closed, the greater the people's frustration and the greater the potential for serious popular unrest that would cause the US major concern. But by yesterday, it appeared that the uproar over alleged electoral fraud was dying down and that President-elect Carlos Salinas de Gortari could get on with the business of building a consensus on his program with both the opposition parties and within his own party, the PRI.

It remains unclear how much impact these forces will have on Mr. Salinas's plans for US-Mexican relations. Some analysts suggest the new left-leaning National Democratic Front (FDN) will pressure Salinas to be more nationalistic than he might have been. They suggest the possibility of a tougher stand toward foreign creditors on Mexico's $107 billion foreign debt, but do not expect radical moves such as capping interest payments. Cuauht'emoc C'ardenas, who ran a surprising second in voting as the NDF candidate, had said Mexico should consider freezing debt repayment altogether.

Salinas may also feel the need to establish his independence from the US by turning up the rhetoric, for a while at least. The Reagan administration has kept its own distance by keeping quiet about the elections so far, an approach considered wise, given Mexico's great sensitivity about its powerful neighbor to the north - including lingering resentment over losing almost half its territory to the US in the Mexican-American War in the 1840s.

C'ardenas's impact may be muted, however, by infighting within FDN and the party's inability so far to turn its 30 percent vote share into a proportional number of congressional seats. Some analysts also suggest that many of his votes were more anti-PRI than pro-C'ardenas.

Privately, administration officials say they feel they can work with Salinas and are encouraged by his push to reform the nation's electoral system with its tradition of fraud. Salinas, currently budget and planning minister, has long experience with the US. He spent years at Harvard University's Kennedy School of Government and speaks excellent English.

Had Salinas won an undisputed victory, he would have been expected to continue President Miguel de la Madrid Hurtado's opening up of the economy, which included privatization of industry and more opportunities for foreign investment in Mexico. But his need to build a national consensus, a tradition in Mexican politics, throws doubt on how much this program can remain on track. Because of the magnitude of US-Mexican relations - encompassing the areas of most US government agencies (trade, debt, drugs, immigration, environment, education, etc.) - they are handled differently in the US from other bilateral relations. Issues are often handled on the Cabinet level, with the State Department as a ``pass-through,'' explains a department official.

Over the past several years, US-Mexican ties have operated on two levels - the high-visibility crisis level and the low-visibility cooperative level. If one judged the warmth of relations only by the headlines, one might think the US and Mexico are like the Hatfields and McCoys. The recent flare-up over Mexico's release from prison of William Morales, an alleged Puerto Rican terrorist wanted in the US, is one example. But behind the scenes, contacts on issues such as trade and environment are steady.

``Things work best when no one talks about them,'' quipped a former US diplomat who served in Mexico.

Successive administrations have grappled with how to manage day-to-day relations. The Carter administration set up special working groups, which failed for lack of well-defined goals. Under President Reagan, the US and Mexico set up bilateral groups on general issues and commerce. But poor interagency coordination has hampered their effectiveness.

With the US facing elections of its own, both sides of the US-Mexican equation are up in the air. But one thing is certain: Mexico's political upheaval has made a complex relationship even more so.