FOR A LITTLE WHILE, things seemed hopeful, President Reagan, unveiling his much-touched plan for the Caribbean Basin to the Organization of American States, began his speech with an apparent apology to the nations of Central America for past U.S. belligerence. "Yes, at times we have behaved arrogantly and impatiently toward our neighbors. These experiences have left their scars." He then unveiled a program that in parts seemed too good to come from the New Right school of diplomacy. It included, for example, more favorable trade agreements for Central American countries, tax incentives to stimulate investment in the basin, increased training and technical assistance and cooperation with Mexico, Venezuela and Canada to further development in the economically troubled Caribbean.

But then, it was back to the usual anti-Communist rhetoric, and in a big way. "Very simply. "Reagan explained, "guerrillas armed and supported by and through Cuba are attempting to impose a Marxist-Leninist dictatorship on the people of El Salvador as part of a larger imperialistic plan." The United States, he continued, will do "Whatever is prudent and necessary to insure the peace and security" of the region. We can only presume that asking Congress for an additional $135 million in economic and military aid for EI Salvador represents such "prudent and necessary" action. And plainly, cutting the last drop of assistance to struggling but leftist Nicaragua is the best way to insure the "peace and security" of the region.

The tragedy is that the President's ideological rigidity seems certain toe choke off the promising long-term features of his new Caribbean policy. For the next 12 years, almost all basin exports to the United States will be duty-free; as a result, most experts agree, the President's initiative will likely stimulate the creation of many new products. And by calling for increased technology transfers and the teaching of marketing strategies. Reagan just might insure the region's future competitively in agriculture and industry. Additional U.S. financial assistance will probably bail out countries like Honduras and Costa Rica, enabling them to concentrate more on development programs.

BUT THE COSTS of Reagan's ideologically barbed measures will outweigh the benefits of his handful of good steps. For example, renewed financial aid to EL Salvador in reality means increase U.S. involvement in the fight against leftist guerrillas. The Administration is convinced that the upcoming election it supports constitutes the solution to EI Salvador's problems. Yet the guerrillas continue to win important victories in the field, suggesting that peace after the election is highly unlikely--especially if the extreme right emerges supreme. In addition, according to new U.S. intelligence reports, for every civilian killed by government forces-and even the most conservative of estimates attribute 6000 deaths to Duarte's regime last year alone--at least 25 guerrillas sympathizers spring up. It is painfuly clear now, as it should have been long ago, that reform, not repression, is the only way out of the Salvadoran nightmare.

Reagan's conspicuous denial of aid to leftist countries in the Caribbean will also prove counter-productive. Mexican President Lopez Portillo has declared he will not participate in any plan that excludes Nicaragua and Cuba--which is precisely what the Reagan program does. But without the support of Lopez Portillo--who is trusted by Reagan and Caribbean leftists alike, a rare combination--any initiative in the basin has only a dim chance of success. And cutting off Nicaragua will only increase the Sandinistas' reliance on the Soviet Union.

Last week, a Harvard Government professor argued forcefully that the Administration is wrongly straddling the fence in the Caribbean. "Some government officials." Jorge Dominguez said, "clearly care about Central America and its economic and social well-being. Others don't give a damn, they just want to prove an ideological point." The new Caribbean Basin plan reflects this ambivalence. But the peoples of the Caribbean will not trust such a two-faced approach and rightly not. If the United States is to have any positive influence in the basin, it must end its ideological posturing.