THE Philippine Senate's watershed defeat of a new American military treaty is churning new controversy among Filipinos and spurring security shifts in Southeast Asia.With treaty supporters and opponents massed outside in monsoon downpours, the senators formally refused to ratify a new 10-year lease for the giant Subic Bay Naval Base, one of the United States' major defense facilities in Asia. Stung by the Senate's recalcitrance, President Corazon Aquino pledged to tap public support for a US presence and challenge the rejection in a referendum during the next six months. In an about-face yesterday, the day the old treaty expired, US officials backed Mrs. Aquino's move and froze any withdrawal plans for the more than 7,000 military and civilian workers at the massive naval base, declaring, "The process is not over." Analysts say Aquino faces an uphill battle to overturn the Senate decision and buy time for this poor island nation. "There will be a huge upheaval as Filipinos watch the Americans steaming over the horizon," predicts a Western diplomat. "Economically, culturally, and security-wise, the Philippines is dependent on the United States." In Southeast Asia, the Philippine rejection comes as a US defense pullback already is under way, and the region searches for new security arrangements. Analysts say an eventual withdrawal from the Philippines, military spending cuts, and the unwillingness of neighboring countries to host a facility the size of Subic are reducing the US role in Southeast Asia. Still, in the near-term, diplomats say the US is needed to referee a region confused by the waning cold war, the chaotic decline of the Soviet Union, an approaching peace in Cambodia, and growing ties between entrenched Communists and fast-growing free-marketeers. Fears of Japan's pervasive economic power, China's aggressive ambitions, and India's emerging defense clout keep smaller Southeast Asian countries uneasy, analysts say. Singapore, Brunei, and Thailand are negotiating with the US to assume some of the functions Washington says it will disperse from the Philippine bases. Within the next decade, some military analysts predict the US could even look at resuming use of its former Cam Ranh Bay base in Vietnam, now being gradually evacuated by the Soviet Union. Noncommunist Southeast Asian countries also are looking more closely at proposed Asia-wide security ties and pursuing talks to resolve long-standing tensions over resource and land disputes in the region. "The Americans aren't going to leave the region. Ironically, some countries might see more Americans around," says an Asian diplomat. "However, [the rejection] will impact the security dialogue in Southeast Asia," he continues. "The rules of the game are new with Japan, China, and India. And we want to make sure there is no instability growing out of distrust for each other." The future of the US bases has exploded in a countrywide debate. Supporters point to thousands of jobs and other economic benefits. Opponents criticize the 10-year treaty as "monstrous," and urge an end to more than 400 years of foreign domination by Spain and the US. "The Senate is reacting to a long train of hurts and grievances. For them, it's a Pandora's Box of injustices and bad memories," says Teodoro Benigno, a political commentator and former Aquino spokesman. "But the president cannot take this defeat lying down," he continues. "There is a lot of time over the next few months for the issue to sweep over the provinces where Aquino has support." Aquino's most potent appeal will be to fears of economic upheaval. Treaty supporters say defeat will batter the already impoverished country, shave economic growth, endanger crucial foreign aid, discourage foreign investment, and add thousands to the Philippines' army of unemployed.