In the event of a faulty investigation of the killing of Philippine opposition leader Benigno Aquino, the Reagan administration might have to distance itself from Philippines President Ferdinand Marcos.
That distancing might even have to include cancellation or postponement of President Reagan's planned visit to the Philippines in November, officials say.
But because the administration values its strategic relationship with the Philippines, there are likely to be severe limits beyond which it will not go in loosening its ties with the Marcos regime.
The most important need for the United States is to maintain access to air and naval bases in the Philippines, officials say. The Reagan administration wants to avoid any move that would weaken President Marcos to the benefit of leftist-led insurgents or other opposition groups that advocate ousting the US Navy and Air Force from the bases.
The administration has been urging the Marcos government to conduct a full and fair investigation of the killing of Mr. Aquino, who was shot only moments after his return to Manila last Sunday, after three years in exile in the United States. The US Federal Bureau of Investigation has been asked by the Philippine government to assist in tracing the origin of the pistol that the authorities say was the murder weapon. FBI involvement could lend respectability to the investigation of the killing.
On Wednesday, President Marcos named Enrique Fernandez, chief justice of the Philippines Supreme Court, to head the investigation. But the Supreme Court justices are Marcos appointees, casting doubt on their independence.
In Los Angeles, meanwhile, White House spokesman Larry Speakes said that President Reagan's visit to Manila, planned as part of a five-nation trip to Asia this fall, is still on.
G.V. (Sonny) Montgomery, chairman of the House Veterans' Affairs Committee, said that in the interest of stability, the US should avoid overreacting to the assassination of Aquino.
But two other Democratic congressmen criticized the Marcos government and said that President Reagan's November trip ought to be canceled. Democratic Reps. Don Edwards and Fortney H. Stark, both from California, charged in a press conference on Wednesday that Philippine agents in the US illegally spy on and harass opponents of President Marcos and that the Reagan administration purposely ignores these activities.
A US State Department official, speaking on the condition that he not be quoted by name, said that it would be difficult to overestimate the importance of the access that the Philippines gives the US to two huge military bases. He described the bases as the southern anchor in a chain of American bases stretching from Japan and South Korea through Okinawa to the Philippines.
The Philippines bases, the official said, offer the US a gateway to the Indian Ocean, and sit astride key sea and air lanes leading from the Western Pacific to that ocean. Half of the noncommunist world's oil supplies pass through the Malacca, Sunda, and Lombok Straits that connect the Pacific and Indian Oceans.
The State Department official said that the Subic Naval Base and Clark Air Force Base in the Philippines, the two largest bases of their kind outside the US, could only be replaced at ''tremendous cost.'' The official said that the bases had grown in importance since 1975, when the North Vietnamese captured Saigon. Soviet ships and planes now use the former American bases at Cam Ranh Bay and Da Nang in Vietnam on a regular basis.
Some opposition politicians in the Philippines have opposed American use of the Philippine bases because they might make the Philippines a target for Soviet nuclear missiles. Some also attack the US-Philippine bases agreement because it allegedly violates Philippine sovereignty, bolsters the Marcos regime, and allows for misconduct by American servicemen in the vicinity of the bases.