MANILA — WITH the future of a key United States naval base in doubt, the US and the Philippines are scrambling to stretch out the remaining days of a century-old American military presence here.The lease for US military facilities will expire today as the deeply divided Philippine Senate appears poised to reject a 10-year extension for the massive Subic Bay Naval Station. In a preliminary vote last week, the 23 senators sitting in committee failed to muster the two-thirds majority needed to ratify the new treaty under which the US would pay annual rent of $203 million over a 10-year period. Amid debate on the Senate floor, some treaty opponents proposed a three-year phaseout, which would allow US negotiators to reopen talks and, senators hope, sweeten the deal. President Corazon Aquino, who led thousands of treaty proponents through the streets of Manila to the Senate last week, backs a referendum and hopes to tap popular support for the treaty to overturn a rejection and possibly challenge the Senate in court, aides say. The referendum could be held as early as December. Despite Washington's refusal to renegotiate, Western diplomats say, the US is ready to consider alternatives to prolong the US military presence. A shutdown of Subic, which some analysts say could be completed by early next year, would leave thousands of Filipinos jobless and reduce aid to the battered Philippine economy. The US would be thrust immediately into the costly dispersal of Subic's extensive ship repair and supply operations and the base's more than 7,000 workers. "Neither side is happy with this confrontation," says a Western diplomat. "If there was overwhelming support in a referendum, the US couldn't help but take notice." "Neither side is closing its eyes to face-saving options," says Amando Doronila, a respected Manila newspaper editor. Last week's Senate action capped months of often tumultuous negotiations and debate which has torn this sprawling island nation between nationalist pride and a longstanding economic reliance on its former colonial master. Treaty opponents also are angry over what they consider stingy US compensation, particularly in light of the volcanic eruption of Mount Pinatubo and a series of other natural disasters. Many Western and Asian diplomats concur. "We need to lessen this dependency on the American dream and have our own dream," says Eugenia Apostol, a treaty opponent and prominent publisher and businesswoman. "The common people are so much taken up with daily survival. It is the responsibility of the informed citizens to take away this large sympathy for America as the land of milk and honey." "The worst enemy of the Philippine people is internal colonialism. The real enemy is the elite," says Frankie Sionil Jose, a well-known author who at one time opposed the American bases but then shifted. "There is widespread disgust among the people against [the Senate]." Despite the US's frustration with the Philippines, Subic Bay remains a cornerstone of US defenses in Asia, Western and regional diplomats say. They estimate that shifting Subic operations for the US 7th Fleet to a number of existing facilities will cost Washington more than $2 billion. "Subic is more than a symbol," says a Western military analyst. "America has ruled the seas, and Subic as a major naval base has carried a lot of weight." The threatened Senate rejection also has aroused fears of another coup by a military deeply disgruntled over the Senate opposition, Philippine journalists say. Dissidents in the military, which has long relied on the US for weapons and assistance, have launched more than six unsuccessful coup attempts against the Aquino government. Speculation of a coup swirled last week, prompting Armed Forces Chief Lisandro Abadia to deny possible trouble.