In the aftermath of the Bay of Pigs debacle in 1961, President Kennedy wanted a postmortem from someone in the White House who had not been directly involved; so he asked Walter Rostow for a recapitulation of how it went so wrong.

The report was secret. But I recall indelibly what Rostow told me was his bottom line: leaving aside the execution of the operation, he thought it only fair to bear in mind that "the CIA only gets terminal cases" when conventional means have failed. The CIA was being asked to undo covertly what the policymakers on high had been unable to prevent overtly, for whatever reasons: lack of foresight; ideological hang-ups; domestic political inhibitions; clumsy diplomacy; a mistrust of the potential of timely foreign aid.

Rostow's rule of last resort goes to the heart of the current controversy over the Reagan administration's Central American policy as it is nicely laid out in a recent report by the House Permanent Select Committee on Intelligence.

Contrary to the complaints of some critics who would have preferred a mishmash of bipartisan compromise, the force of this report is in the refreshing party-line precision with which the issue is joined. The nine Democratic committee members tell why they voted to end covert CIA support to insurgent forces inside Nicaragua, while voting an extra $80 million in overt help to Honduras and El Salvador. The money would mostly be for building barriers and installing radar, sensors and other sophisticated equipment to choke off Nicaragua's supply lines to the leftist rebels in El Salvador. The five Republicans tell why they think this approach would gravely undercut the Reagan policy.

The argument was over means, however, not ends. The importance of El Salvador's salvation from communism is taken as a given. So is the Marxist- Leninist menace from Nicaragua. The El Salvador insurgency "depends for its lifeblood . . . upon outside assistance from Nicaragua and Cuba," the Democratic majority freely concedes.

At this point we confront Rostow's Rule: the committee Republicans would turn the patient over to the CIA for covert treatment. They are careful to argue a distinction between covertly aiding Nicaraguan insurgents for the purpose of "interdicting" supplies to El Salvador (which is all the current law permits) and actually contributing to the insurgents' proclaimed purpose of overthrowing the government in Managua.

But the minority members speak of "attempting to disarm or neutralize Nicaragua" in a way that unmasks their (and Ronald Reagan's) real design: to undo the past by replacing the Sandinista regime with one more congenial to U.S. interests. It is, with appropriate modification, the "Bay of Pigs" mentality at work.

The majority argument, quite simply, is that this won't work. The CIA will be "hurt" by being asked to "continue an action whose principal elements are known to all the world." And this common knowledge will fuel hostile propaganda in a hemisphere with long memories of heavy-handed and imperious Yanqui intervention.

More important, if the administration's own cries of alarm are to be taken seriously, the year-long "interdiction" mission has failed; when it fits the administration argument of the moment, we are told that El Salvador is endangered by an ever increasing flow of supplies from Nicaragua.

The administration (and the committee Republicans) would also have us believe that inciting insurgency in Nicaragua will turn the Sandinista government inward in its own defense, discourage external adventurism, encourage a willingness to negotiate. To which the committee Democrats respond that the results have been just the opposite:

"Inflicting a bloody nose on nations achieves a purpose no different with nations than with individuals. It tends to instill a deep desire to return the favor. The Sandinistas are no different. Their policies have not softened. They have hardened."

What's more, a U.S.-supported Nicaraguan insurgency actually "tends to bind the Nicaraguan population-- even those with little enthusiasm for the Sandinistas--together against the threat of attack."

The Democrats argue that their open approach is the best way to show U.S. determination and commitment. The Republican answer reads like a declaration of bankruptcy: "With the stakes so high and with the uncertainty of U.S. resolve, the governments in Central America would be unwilling to work with us in the overt program to reduce the flow of external support to the Salvadoran guerrillas."

If that's so--if our friends won't work openly with us in their own defense--then the U.S. position may be so weak that it cannot be salvaged by either overt or covert means. But overt-versus-covert is nonetheless what the current debate is all about. And Rostow's Rule is an apt reminder that when you are talking "covert," you are talking last resort.