IT'S DO OR DIE for the president's Caribbean Basin Initiative, whose central feature is about to go to the House floor. The CBI was the aid, trade and investment package in which Mr. Reagan sought to wrap his policies toward the small and poor nations of the Caribbean and Central America. The aid has already been voted -- $350 million chiefly for International Monetary Fund stabilization programs. Extension of the president's domestic investment credit to American investments in the Caribbean fell victim to philosophical and political differences in the House Ways and Means Committee. There remains the CBI's symbolic heart, the proposal for an increase in duty-free imports from Caribean countries.

In broad American terms, it's not much. Some 27 percent of Caribbean exports to the United States currently arrive duty-free. Oil is excluded from the reach of the CBI measure; the administration excluded textiles; Ways and Means excluded footware and leather goods. Other special interests -- steel, mushrooms, tuna, roses -- were fended off by Ways and Means Chairman Dan Rostenkowski. Some necessary protections for Puerto Rico and the Virgin Islands, the American territories that first saw the CBI chiefly as a threat to their existing privileges, were thrown in.

At that, only an extra 10.5 percent of Caribbean imports, worth less than $1 billion, is expected eventually to enter through the new opening. Even this will not be automatic, for most of the imports due to be so favored are in industries that the Caribbean countries haven't established yet. For the United States, the impact will be minuscule, but for the tiny economies of the region, it could make an important difference.

Foreign aid has its continuing role, but the domestic constituency for it clearly is not getting any larger. That leaves the private sector as the place from which most of the relief and development of the Third World must come. President Reagan, in the CBI, recognized the special American interest in the region on America's doorstep. The CBI itself is an imaginative approach and has stirred hopes throughout the region. The Caribbean countries cannot possibly count on getting all the aid they need. If they cannot earn more in the hemisphere's largest market, their distress will only grow, and with it their disillusionment with the United States.