Whether President Ferdinand Marcos succeeds or fails in his bid for reelection next Friday, the post-Marcos era has already begun in the Philippines.

That is the judgment of many of the major actors here, including a substantial part of the business community, the powerful Roman Catholic clergy, the Communist guerrilla movement fighting in the countryside and U.S. policy makers.

These groups, and others, are busily positioning themselves in a transition that they believe already has begun. Even some of Marcos' Cabinet ministers now publicly stress their differences with the boss as they eye the uncertain future.

There is one significant doubter of this fast forming consensus, however. It is Marcos himself.

A skillful survivor who has defied debilitating illness and predictions of his political downfall before, Marcos makes it clear to visitors to his office at Malacanang Palace that he has no intention of yielding or significantly diluting the power he has held and hoarded for two decades.

Marcos does not appear to entertain the idea that he will be beaten by opposition candidate Corazon Aquino, despite a surge of popular enthusiasm for Aquino that is turning out huge crowds at many of her rallies.

Observers here and in Washington feel that Marcos controls the levers of power so tightly that he can ensure being reelected no matter what the popular mood is. He can probably even tailor the results so that he wins by a sufficiently small margin to defuse criticism from Washington that he won through fraud, in this view.

To Marcos, these elections are not about who will hold power. They are about his future relations with the United States.

Officials in the Reagan administration and Congress have concluded that Marcos heads a regime so corrupt and enfeebled that he will not be able to control an increasingly explosive situation if he wins.

"He is a master tactician," said one diplomat, who believes that Marcos called the elections to deflect American pressure on him for wide-ranging reforms. "But the Philippines needs a strategy now, not clever tactics, and Marcos cannot provide it."

But Marcos appears to believe that his long friendship with Reagan will help him weather the political and economic storms now sweeping the Philippines once he has demonstrated next Friday that he still runs the show here.

Marcos does not wait until he greets an American visitor to remind one that he and his wife, Imelda, have considered themselves good personal friends of the Reagans for most of the 20 years that they have ruled the Philippines. In the waiting room just outside the president's office, among the autographed photographs of the powerful and titled personalities that the Marcoses have met over the years, only one rates a separate display stand at the front of the room.

Enclosed in a mother-of-pearl frame is a photograph of Mrs. Reagan, inscribed, "To Imelda, With my love, Nancy."

And sprinkled among the recent full color photos of Soviet leader Mikhail Gorbachev, Princess Margaret of Britain, Prime Minister Yasuhiro Nakasone of Japan and others, are black-and-white snapshots of the Reagans and the Marcoses dancing together during the ceremonial visit that the then-governor of California paid here in 1969.

These snapshots have a vintage air about them compared to the others, as if they had been carefully tucked away in another era, to wait for subsequent events. They provide a study in patience and in power politics as practiced by Marcos.

"I don't think there is any problem with the Reagan administration," Marcos said in an interview in his office, noting that he would have to be "formally notified that there are problems" by Reagan himself before he would believe it.

"The problem is with some bureaucrats and with some members of Congress" whom Marcos characterized as "sympathizers with the opposition or communist-inclined."

Seated behind an imposing desk, Marcos appeared alert and composed during the 45-minute discussion. Reportedly seriously ill with a disease that has severely affected his kidneys and periodically lays him low, then recedes, the 68-year-old Marcos was in a relaxed mood after what had been a good week on the campaign trail.

Coughing several times into a handkerchief, Marcos did abruptly break off the interview at one point and walked somewhat unsteadily from the room. But he returned five minutes later and resumed the discussion.

Marcos made his decision to hold the snap elections and to seek a new mandate for his rule after a visit here last October by Sen. Paul Laxalt (R-Nev.), an official emissary as well as a close friend of President Reagan.

That visit was portrayed in Washington as an effort to impress on Marcos Reagan's personal commitment to the program of reform that the State Department and Pentagon agree is needed to restore economic health and to stem a growing insurgency.

But as he approaches the election, Marcos now dismisses the importance of the discussion with Laxalt and suggests that he and Reagan can shortly clear up what Marcos views as "the present haziness of our relations with the United States."

Laxalt "did not speak of problems; he spoke of concerns," Marcos said in the interview. A letter from Reagan that Laxalt handed to the Philippine leader was filled with information about the insurgency and the economy that came "from intelligence sources that were misinformed," Marcos added in detailing how he rebutted the American calls for reform.

In Washington, knowledgeable officials report that Reagan has not let his past association with the Marcoses lessen his support for a policy of distancing the United States from Marcos and his policies.

But Reagan is also known to feel strongly that his administration should never treat a friendly leader in a way that could be compared to the Carter administration's handling of the shah of Iran.

Thus, in the confused jockeying that is expected to follow the elections, Reagan's personal attitude is likely to be decisive in charting a U.S. response to what has to be a messy aftermath.

The campaign already has demonstrated a stinging rejection of Marcos by many segments of this society, including those that normally might be expected to side with an essentially conservative incumbent who is perceived to have more than enough money, party organization and, if needed, military muscle to stay in power.

Moreover, Marcos is the most likely figure to emerge with control of the purse strings in the future.

But at a gathering last week at the Manila Rotary Club, businessmen stood on chairs to whistle and show loud support for Aquino, widow of Benigno Aquino, who was taken into custody on an arriving airliner by the Army on Aug. 21, 1983, and immediately assassinated in disputed circumstances.

Marcos spoke to the same group this week, and this time the crowd sat on its hands. When Marcos asked executives in the audience if they were prepared to appoint the "inexperienced" Aquino president of their companies, he was surprised by a round of "yes!" answers shouted up at him.

"Marcos and his people have looted the country of its economic resources and confidence," said one industrialist. "No one will invest another peso here until he goes."

"Our planning cycle right now is looking at investments three years from now," said another businessman, who estimates that the country's gross national product has declined by 10 percent over the past 24 months. "We're making our decisions now on the basis that Marcos will be gone in three years, and we need to be positioned to take advantage of that."

An entirely different set of calculations is being made by the underground political organization, the National Democratic Front, which is controlled by the Communist Party of the Philippines and its military arm, the New People's Army.

"We do not see these elections as decisive," said a member of the front's secretariat. Predicting a significant escalation of military attacks by both government and guerrillas after the elections, he added: "The situation has developed to the point where armed struggle will be the decisive element."

The front decided to boycott the elections partly because Aquino would not bring significant change to the capitalist and pro-American orientation of the government, he indicated. Her victory would merely "hasten U.S. intervention" since Congress would be more likely to go along with increased military aid if Marcos were defeated at the polls, he added.

"Marcos may also try the military option," said the front member, who declined to give his identity. "But his era is over. He has been pushed against the wall."

Against such assessments, Marcos' loyalists, like Labor Minister Blas Ople, counter that the elections will produce "a new Marcos. . . . The elections make him look more critically at his own record. It is a cleansing and cathartic process."

But Ople, considered by many to possess ambitions to succeed Marcos, also dwelled during a two-hour interview on the need for "a future national consensus." Ople recounted that he had urged Marcos to hold the election by arguing that "things are coming to a standstill here. We must not be stuck in a sterile situation."