THE eruption of Mt. Pinatubo drove the United States Air Force out of its Philippines air base in June; now a political volcano in Manila may push the US Navy out of that country as well. The committee vote in the Philippine Senate this week rejecting a new base treaty perhaps signals the end of America's longtime military presence in its former colony.The fate of the treaty, which would extend Washington's lease of the huge Subic Bay Naval Station for 10 more years, is not yet settled. President Corazon Aquino is leading a grass-roots drive to save the pact. Perhaps the current senators, or their successors to be elected next summer, will be persuaded to prolong the US naval presence. Some observers speculate that the committee vote was just a bargaining tactic. But Philippine politicians miscalculated if they think Washington can be induced to sweeten the deal it's offering Manila. President Bush evidently was serious when he said the US has made its best offer - $203 million a year to use the base. It would be inconvenient and costly for the US to transfer naval operations from Subic Bay to Guam, Japan, and possibly Singapore. Washington could do so, however, without jeopardizing American security interests in the Pacific. The greatest loss for the US would be the highly skilled civilian work force that efficiently maintains and outfits Navy vessels. Yet the loss would not be critical at a time when the Soviet Union, faced with severe budget problems, has abandoned dreams of a blue-water navy able to shadow US forces throughout the western Pacific. In the short run, Manila would pay a high price for ejecting the US from Subic Bay. The Navy is the Philippines' biggest employer besides the government, and with the US departure would go billions of dollars annually in local spending. Foreign investors may be more cautious about putting money into the country without the "Good Housekeeping seal" provided by the US presence. And Manila would displease many of its East Asian neighbors, who regard the American naval presence as a stabilizing force. Yet it may well be true, as many Filipinos insist, that their land can never become a mature self-governing nation so long as their former colonial masters maintain such a towering presence. Whether the colonial legacy actually lingers in the US's demeanor or simply in Filipino consciousness is beside the point. Philippine nation-building (and an end to the smoldering communist insurgency) may require the departure of the American fleet. In prickly US-Philippine relations, moreover, absence may make the heart grow fonder.