Marcos's power base eroding The Philippine President's grip on power is loosening. Once-supportive institutions are defecting, the economy is sliding deeper into recession, and the costs to US firms of doing business there are on the rise.

Even if Corazon Aquino fails to force him out of office, President Ferdinand Marcos will remain at best a lame duck. The institutions that once supported his regime have now been weakened or have defected. The process of social alienation that began with the August 1983 assassination of oppositionist Benigno Aquino Jr. has accelerated dramatically with the Feb. 7 election, in which Mr. Marcos is widely believed to have defeated Aquino's widow, Corazon, by fraud.

At the peak of his power in the mid-1970s, Marcos had a firm grip on the main institutions of Filipino society. Those he did not control were confused or divided.

The press, monopolized by Marcos relatives or confidants, faithfully projected his views. The military enforced them. The business elite was compliant. The Roman Catholic hierarchy was usually neutral and occasionally inclined toward the government.

Today, by contrast, businessmen are leading the middle-class revolt against Marcos. The official press is being outstripped by aggressively pro-Aquino dailies. The military's loyalty to the regime is eroding. Last weekend, Catholic bishops stopped just short of calling Marcos's rule illegitimate.

Businessmen, the press, and the church are working together closely to overthrow Marcos. And elements of the military no longer hide their desire for change.

The military has been the last to weaken, but the cracks have widened markedly since the election.

``The President's capacity to command obedience has weakened a lot since the elections,'' says one reform-minded officer.

The clearest expression of discontent within the military is the Reform the Armed Forces Movement, usually known as RAM. The movement, which emerged last year, initially portrayed itself as an apolitical body. It has now moved visibly closer to the opposition. RAM is certainly a minority group inside the military, but it is an important minority -- its leaders are among the most successful and senior of their generation.

Last week, RAM published a letter calling on fellow officers to use maximum tolerance in handling demonstrations. The letter is said to have infuriated Gen. Fabian Ver, the armed forces chief of staff who is scheduled to resign March 1.

Recent actions by RAM have brought it to a state of near-confrontation with Marcos and General Ver. RAM leaders say that they trained some of the security people who protected Mrs. Aquino during her campaign. They stress they did so in the interests of seeing the survival of ``a strong opposition.''

And when, two days after the elections, 30 computer technicians working for the Commission on Elections walked out saying the commission was falsifying election results to show Marcos in the lead, they did so with the protection of a RAM leader. The group's spokeswoman is in fact the wife of a RAM organizer.

RAM members say they later took steps to protect the computer group from unidentified military teams who were apparently tracking them. Two sources say that the teams had been ordered to kill one or more of the technicians.

But alienation goes beyond RAM. One respected Marine commander in the south makes little effort to hide his unhappiness with the regime. And two of the twelve Regional Unified Commanders -- Brig. Gen. Renato Devilla and Brig. Gen. Cesar Tapia -- have quietly distanced themselves from the Ver-dominated high command. Officers say that hostility to the Marcos government inside the military began after the Aquino assassination and deepened as frustrated officers watched the increase in communist insurgency.

This frustration intensified during the presidential campaign. One reformist officer interviewed before the polls expressed the hope that Marcos would not resort to fraud to win the election.

``If he does, it will only benefit the communists,'' the officer said. Asked how like-minded officers would react if there was major fraud, the officer laughed. ``You shouldn't ask that sort of question,'' he said, ``You can guess what the logical conclusion would be.''

Reformist officers routinely deny they are planning a coup, citing practical problems, such as lack of firepower. But some officers have been known to theorize out loud about the best way to assault the presidential palace. They add, however, that they are doing this ``purely as an intellectual exercise.''

Military sources report that one group of junior officers seemed to have run out of patience on election night. The officers, all combat veterans, were intercepted with arms and what appeared to be a ``hit list'' of government officials and military commanders.

The change in the press is more immediately visible. Paperboys now sell two types of papers: the sex-and-violence tabloids and the anti-Marcos press.

The Philippines' biggest-circulation paper is now probably the Inquirer, which claims 250,000 in daily sales. The Inquirer is followed by another mass circulation daily, Malaya, and the Manila Times -- started up again two days before the election, after a 15-year hiatus.

All these papers are strongly and sometimes stridently antigovernment. Malaya and the Inquirer have in the recent past vied with each other tp be the first to reprint such damaging expos'es as allegations that Marcos's war record was spurious and that he had a large unexplained fortune in the United States.

The decline of the pro-Marcos news media has been accelerated by Aquino's call last Sunday for a boycott of the progovernment press. Bulletin Today, the largest of the Marcos-controlled dailies has in the past ignored boycott calls. But it recently published a rare front-page editorial denying a Marcos bias. It has also reportedly increased incentives to retailers in an effort to stop its sliding sales.

Government spokesmen point to the opposition press's upsurge as a sign of Marcos's benign rule. The change is, in fact, a remarkable departure from conditions in the late 1970s, when, for example, the government closed down a weekly paper in a remote province because of its antigovernment stand. Oppositionists give a different interpretation to the new liberalism.

``I'm sure he'd love to move against the press and the rest of the opposition,'' said one professional. ``But I doubt that he will. In the '70s, we were cowed, and he had the muscle. Now we're not cowed, and he's no longer sure he has the muscle.''