The White House is said to be on the verge of unveiling what Washington describes as a mini-Marshall Plan for the Philippines.

Unlike the Marshall Plan of the 1940's that helped rebuild war-torn Europe, the new package of up to $10 billion in aid over the next five years would involve major assistance from other countries, with the U.S. playing a more limited role.The decision to spread the financial burden among Japan, West Germany, the World Bank, commercial American banks, and other countries reflects U.S. budgetary constraints and a growing international concern for a stable democracy in the Philippines.

So far, so good.

But one aspect of the plan is hard to understand and justify: The failure to tie the new aid package to Manila's renewal of leases on the six key military bases that the U.S. maintains in the Philippines.

With the leases due to expire in 1991, many Filipinos have been calling for an end to the U.S. bases, which they profess to see as an embarrassing vestige of colonialism.

What folly! Without those bases, the Philippines would be more vulnerable to communist insurgents and the Philippine economy more vulnerable to its financial woes, including a $30 billion foreign debt and scarce resources.

Of the six bases, two are especially critical - Subic Naval Base and Clark Air Base, both on the large island of Luzon. The two are America's largest and oldest overseas facilities and the key to U.S. operations in much of Asia.

Subic is the only facility outside the U.S. where planes can be picked off aircraft carrier decks and carried to hangars or repair centers. In addition, the Subic facilities allow training in almost every kind of naval and amphibious warfare. Clark includes America's only fully-instrumented bombing range overseas.

The bases are critical to security in Southeast Asia where, as U.S. ambassador Nicholas Platt notes, the Soviet military presence has grown from "essentially zero to one of substantial proportions."

Filipinos need the U.S. bases, too. The presence of these facilities enables the Philippines to limit its own spending on defense and concentrate on economic development while waging a 19-year war against communist guerrillas. At the same time, the bases account for 6 percent of the country's gross national product. Indirectly, the bases also are reassuring to foreign investors, who might understandably do business elsewhere if the U.S. presence is eliminated.

Clearly, Manila's decision on the leases could easily be the biggest foreign policy moves that Philippine President Corazon Aquino will have to make.

If the bases must be relocated, it would be a costly blow to the U.S. So costly that it's hard to see why Washington should have any qualms about attaching strings to the new mini-Marshall Plan of aid to the Philippines.