MANILA, AUG. 15 -- There was a time, not too many years ago, when Agapito (Butz) Aquino, brother of slain opposition leader Benigno Aquino, was at the forefront of various leftist protest marches against the presence of American military bases on Philippine soil.

Now Aquino is a member of the newly formed Philippine Senate, which must ratify any new agreement to retain the bases. And on the Senate's opening day last month, Aquino explained to reporters his current thinking on the continuation of the American military presence here.

"What I would like to see is an orderly dismantling arrangement," he said. "I know they're going eventually. The question is, how soon is eventually? If we are made an offer we cannot refuse, then we will accept it." He added, "You can be nationalistic and still vote for the bases."

Aquino called this new stance "pragmatism," and it is a sentiment that appears to be gaining ground even among those once considered staunchly against any new agreement on U.S. retention of Clark Air Force Base and Subic Bay Naval Station, the two largest overseas American military installations.

It is a sentiment that appears to have raised the likelihood that even with a Senate composed of many nationalists ardently opposed to the bases, such as Aquino, a new agreement will be reached and the bases will stay -- albeit under different conditions -- after the present accord expires in 1991.

"I am against the indefinite stay of the American bases in this country -- with the {word} indefinite being italicized. Many people do not catch that nuance," said Senate president Jovito R. Salonga, who in his past role as opposition Liberal Party leader had been an outspoken critic of the bases.

"Before, a lot of people were anti-bases because that was {former president Ferdinand E.} Marcos' trump card with the United States, and they wanted to take that away from him," said Benedicto David, a presidential spokesman. "Now, they are the ones holding the trump card. They realize the economic benefit of the bases."

Still, this spokesman and others noted that since it takes a two-thirds vote to ratify a treaty under the new constitution, it takes only nine senators to derail any new bases agreement.

Philippine and U.S. government officials mentioned several reasons for the evident shift by longtime nationalists away from a strict posture opposing the bases. While next year's negotiations will be between Washington and Manila, the Philippines' Southeast Asian noncommunist neighbors as well as Japan have made it clear that they see the American presence in the Pacific as vital for regional security, in the face of a growing Soviet presence at Cam Ranh Bay in Vietnam.

One top-ranking Philippine diplomat said in an interview last week that the presence of the bases here is a positive factor for this country's investment climate.

Philippine military officials say the continued American presence here frees the armed forces from the financial constraint of maintaining an external defense, allowing the Army to use all of its resources to battle the internal problems of insurgencies by two rebel groups and a wave of lawlessness.

The bases also provide about 40,000 jobs in a country hard-pressed to combat unemployment and underemployment.

But the primary reason for the shift is the benefit of huge American infusions of economic and military aid.

Salonga, like other senators and Philippine government officials, said that with the country facing a crippling $28 billion foreign debt that eats up almost half its export earnings, the dominant question has shifted away from whether the bases should stay and toward how much the Philippines can persuade the American government to pay.

With most of the country's debt owed to American banks -- and a mood for repudiating at least part of it gaining momentum -- several senators have suggested linking the debt question to the bases agreement.

During the last review of the bases accord four years ago, the Reagan administration promised about $900 million to the Philippines over a five-year period, including military grants and economic assistance. Secretary of State George P. Shultz, when he visited here in June, said the U.S. aid will surpass the promised amount.

Several Philippine and American officials noted, however, that the atmosphere for the bases review has changed markedly. The "People Power" revolution of February 1986 that toppled Marcos and swept Corazon Aquino into power as president has also unleashed a new feeling of nationalistic pride and independence. The feeling is widespread that the Philippines should make the United States commit itself to specific amounts, like a tenant signing a long-term lease.

Shultz and other American officials have repeatedly said that as a matter of principle, the United States will not agree to pay "rent" for continued use of the bases. They say the bases are not here just to protect American interests but to contribute to the protection of the Philippines and all of Southeast Asia and the Pacific against external threats.

Most Philippine officials, who are insisting that the next round of talks center on a specific amount of "rent" to be paid, play down the external threat. One Philippine diplomat who plays a key role advising the government on the base agreement said the American insistence on an external threat is simply "propaganda."

"The feeling here is that the bases are primarily here because of American global and regional strategy," this diplomat said.

A lingering question is whether the die-hard nationalist bloc in the Senate might insist on a clause in any new agreement stipulating that the bases must go at the end of the new contract period.

The unknown factor is President Aquino, whose position remains a closely held secret here. She has steadfastly maintained that she will "keep her options open" until the current accord expires in 1991.

Many Philippine and foreign political analysts suspect that the Senate will submit the next bases agreement to the public in the form of a referendum, mainly to insulate the politicians from having to make such a crucial decision. And observers here said that with Aquino's extremely high popularity, her voice will no doubt be the most powerful one in swaying the public for or against the bases.