In trying to distance itself to just the right degree from authoritarian Philippine President Ferdinand Marcos, the United States is playing one of its most delicate diplomatic roles these days.
It gets high marks from some who say it has struck just the right balance. ''It's a remarkably subtle performance for the US - and usually we're not very adept at subtlety,'' says David Newsom, a former US ambassador to the Philippines.
But Washington policymakers also face pressures to retreat to a softer position and to push considerably harder for political and economic reform.
The two countries have enjoyed a close relationship. Although military abuse, corruption, and human rights violations in the Philippines had long bothered Washington - particularly Capitol Hill - the official US stance until a year ago was largely supportive of the staunchly anticommunist Mr. Marcos.
But since the murder of Philippine opposition leader Benigno Aquino in August 1983, an incident which undermined public confidence in the government and the armed forces and intensified Manila's economic difficulties, US diplomatic strategy has been shifting. It is, however, more a shift of tactics than of goals, notes Marjorie Niehaus, an Asian analyst with the Library of Congress.
Longtime quiet efforts to develop and strengthen democratic institutions there have been bolstered by more forceful public statements since Aquino's death. Among those speaking up much more often on substantive issues is Stephen Bosworth, US ambassador to the Philippines, who recently reminded Manila Rotarians that democracy has deep roots in their country and ''can be your national-security doctrine.''
The US aim is to keep just enough pressure on the Marcos regime to force democratic concessions without either contributing to a destabilizing shift of government or adding to the country's economic difficulties. Both changes could easily play into the hands of Marcos's most radical opponents.
The US does not want to provoke Marcos, a proven and determined survivor, to the point where he would whip up anti-American sentiment by complaining of US interference or take a tougher line on to US access to its bases. ''There's a great deal of resentment against the bases and worry that they could be a magnet (for attack),'' says George Kahin, a Cornell University Asian expert.
The US views the strengthening of democracy and reforms as its best insurance for when Marcos eventually leaves office. US officials insist they have regular contact with opposition groups. ''We try to make the Filipinos understand our interest is in the (government) system as opposed to individuals.''
From the day of the assassination - promptly labeled a ''brutal political murder'' by the State Department - Washington has taken a strong interest in the case. It has regularly reminded Marcos of his promise that those responsible, ''no matter who they may be,'' will be held accountable. Last month the US publicly prodded Marcos to turn over to proper authorities the two fact-finding reports on the killing - the minority opinion and the more controversial majority one - and later commended him for doing so. The reports suggest a military conspiracy was behind the Aquino killing. And the US has praised the civilian panel for conducting ''an impressively thorough investigation.''
The Philippine armed forces chief, Gen. Fabian Ver, who is a cousin and close associate of Marcos and was named in the majority report, is now on leave of absence. If, as expected, he is tried only for conspiracy to cover up rather than to commit the murder, or if the evidence is slim, he could conceivably regain his job. But the US considers reform of the now-tainted and divided armed forces more likely if he does not return.
Whatever happens, both Congress and the State Department expect to take their future cues from the Philippine public. ''What we're interested in is a civilian tribunal fairly . . . judging the case,'' explains a State Department official, who describes the US role as that of an ''interested spectator.'' He adds: ''What the US thinks about the trial will be determined by and large by what the Filipinos think about it.''
It is this kind of reactive riding of the waves, however, coupled with President Reagan's recent debate comment that a large communist movement is ''the alternative'' to Marcos, that has some Philippine experts urging the US to take bolder action. ''We have the power to influence events before things fall apart on us . . . but in the great sway of public opinion, I think we're going to (be punished) for having done too little too late,'' says David J. Steinberg, author of ''The Philippines: a Singular and Plural Place.'' He suggests a Marshall Plan of additional aid, making the money contingent on reforms made.
''Until there is substantial political reform there, it's not clear to me that sending more money will have a beneficial economic effect,'' adds Earl Martin, an East Asia expert with the Mennonite Central Committee.
Congress is showing an increased inclination to take such progress into account in giving US economic and military aid, which the Filipinos have long regarded as unconditional ''rent'' in exchange for the bases.
Widespread Filipino interest in American views gives US policy added leverage. Even those who argue it should be bolder concede that US pressures, including the cancelation of Reagan's visit to Manila last November, have played an important role in the limited liberalizing trend under way over the last year or so. Signs of such loosening include the establishment of a more orderly succession arrangement, the holding of parliamentary elections last May, a somewhat freer print press, and the scheduling of presidential elections for 1987. Marcos has announced his intention to run, but some experts here surmise that he may then step down, and that the announcement was intended to avoid his being labeled a lame-duck president.