With preparations now under way for a scheduled May 1987 presidential election in the Philippines, a movement appears to be gathering steam to draft the widow of assassinated opposition leader Benigno Aquino to head a united opposition ticket against President Ferdinand Marcos.

According to Philippine opposition figures, Corazon Aquino, 52, is emerging as the most likely prospect for unifying the deeply divided foes of Marcos, 68, who has ruled the country for 20 years.

The search for a single challenger to Marcos comes at one of the most turbulent periods in the history of the former U.S. colony. Mired in its most severe economic crisis since World War II, the country faces a growing Communist insurgency in the provinces and rising leftist agitation in the cities. As the country is home to the two largest U.S. military bases abroad, the situation has stirred concern in the Reagan administration.

In this atmosphere, efforts to unify the opposition for elections have been taking on increasing urgency.

"In another 12 months the situation will be irreversible," opposition legislator Homobono Adaza predicted grimly.

At the center of U.S. policy toward the country is a push for free and fair elections in May 1986, when governors, mayors and other local officials are to be chosen, and in May 1987, when Marcos' current six-year term expires. Washington views the 1986 and 1987 elections as possibly the best hope for undercutting the spreading influence of the Communist Party of the Philippines and its armed wing, the New People's Army.

While Washington has not said so publicly, there are signs that some Reagan administration officials have given up on Marcos and are hoping that the elections will usher in an opposition government capable of dealing with the country's problems, especially the insurgency. In fact, some political analysts see hints from Washington that Marcos should not run again in 1987.

"If Marcos gets out, there's no reason the insurgency can't be reversed," said one foreign analyst. "If the government is credible, with a modicum of efficiency, the situation can be turned around."

For the United States, the stake in the Philippines' stability and the future of the two large air and naval bases on the northern island of Luzon are enormous. They are seen as vital to U.S. strategic interests and an important counterweight to growing Soviet naval power in the region. The presence of the bases is the main policy issue dividing the Philippine opposition. Aquino backers have said that they favor renegotiating the bases agreement in 1989 and possibly letting it expire when the current agreement lapses in 1991.

Of the other two main contenders from the moderate opposition, Salvador Laurel, 56, has advocated a referendum on the issue, while Jovito Salonga, 65, has called for their removal, a position that appears increasingly popular among Marcos opponents.

All three contenders favor a "restoration of democracy" in the Philippines and are prowestern in their outlook, but specific programs at this point are, at best, vague.

Corazon Aquino, popularly known as "Cory," has insisted that she does not seek the presidency and would rather yield to another opposition leader.

Yet, as a reluctant newcomer to politics who exudes sincerity, she is refeshingly attractive to a sizable part of the electorate that is fed up with ambitious traditional politicians, including opponents of Marcos who have failed to resolve their differences and unite behind a single leader. She also retains great public sympathy as the widow of Aquino, who had returned from three years of self-imposed exile in the United States to unify the political opposition here when he was gunned down at Manila International Airport on Aug. 21, 1983.

Long accustomed to living in her dynamic husband's shadow, Corazon Aquino, a member of the wealthy, landowning Cojuangco family, for years was content to be a wife and mother. She has emerged as a political figure only since Aquino's death, which she has publicly and repeatedly accused Marcos of having ordered.

Now, a consensus seems to be growing that all but one or two opposition presidential aspirants would give way if Corazon Aquino accepted a draft.

"Once she allows herself to be nominated, I think almost all of us will withdraw," said opposition legislator Ramon Mitra. "Nobody will want to oppose her. It's like opposing motherhood."

Often mentioned as a possible presidential contender, Mitra is among the major backers of a "Draft Cory Aquino for President Movement" formed last month.

Last week, after categorically rejecting the idea of running for president, Aquino opened the door to the possibility by setting conditions for her candidacy. She said that she would consider running if Marcos calls an early presidential election and if the draft movement gathers 1 million signatures endorsing her.

Marcos has raised -- and for the time being ruled out -- the possibility of calling an early presidential election. Nevertheless, the prospect has spurred opposition groups to seek a common candidate for fear of splitting the anti-Marcos vote in a snap election.

Salonga, leader of a wing of the opposition Liberal Party who returned in January from the United States after nearly four years of self-imposed exile, has said publicly that he would withdraw if Aquino runs. Laurel, the head of the United Nationalist Democratic Organization, an opposition grouping known as Unido, already has been nominated as the group's presidential contender.

Laurel, a former Marcos supporter who broke with the president in 1980, is seen as the most conservative of the contenders and the closest to the United States. He has the biggest and best organized political group, comes from a prominent, wealthy family and has experience as a former senator and assemblyman. But his opposition credentials are widely suspect, and he is opposed by the Philippine left.

Salonga, also a former senator, represents the center-left and long has been a staunch Marcos opponent. He was injured in a still unsolved bombing of an opposition rally in Manila in 1971 and was charged with subversion in connection with a series of bombings in 1979 and 1980. However, he was allowed to seek medical treatment in the United States in 1981, and the charges were dropped before he returned home this year.

Some opposition leaders who support Aquino for president see her essentially as a transition leader in a figurehead role. They say her main function would be to preside over a collective leadership that would usher in a new constitution and a series of political, economic and military reforms.

However, some who know her well warn against a tendency to underestimate her as a political lightweight without the intellectual credentials to be president.

"It's all steel inside that person who seems so frail," Mitra said.

"What we need now more than brains -- because we've had enough brains with Marcos -- is sincerity and honesty," said Teofisto Guingona, another leading opposition figure.

Certainly, the Communists tend to view her as a potential threat because she could undercut their efforts to "change the system" rather than simply replace Marcos.

"It's a problem for us," admitted a leader of a Communist front organization here. An Aquino victory "may distract people from what they have to do," he said.

Although she has been sparing with policy statements, Aquino has said that she would have nothing to do with Communists and opposes violent change.

Marcos has said that he intends to run again in 1987, when a vice president also will be elected for the first time since 1969. No decision has been announced on who his running mate will be, although speculation centers on his wife, Imelda.