THE CIA manual advising Nicaraguan guerrillas how to kidnap, assassinate, blackmail and dupe civilians is an appalling production, and its disclosure has produced a first-class storm. The Democratic ticket has seized on the manual as evidence of the darkest and least defensible tendencies of the Reagan administration's foreign policy; no doubt Walter Mondale will bring it up in his debate with President Reagan tonight. Mr. Reagan himself is desperately trying to flee responsibility for the document. He is doing just about everything, Sen. Daniel Moynihan suggests, except to blame it on Jimmy Carter.

The damage, however, has been done -- several sorts of damage. To the extent (unclear) to which the manual's advice was applied, people have died and a whole style of terror and counter-civilian violence and deception has been condoned. The democratic elements of the Nicaraguan insurgency will now be widely represented -- misrepresented, we believe -- as people who need and use terror to make their way. In this regard, the disclosure cannot fail to lengthen the already very long odds against revival of CIA funding for the contras when the current suspension of funds ends next March.

The Reagan CIA's earlier inspiration in Nicaragua, to mine the harbors, had brought the United States worldwide embarrassment. This incident aggravates those diplomatic costs. Further, it is taking the CIA back to the atmosphere of scandal that drenched it in the mid-1970s -- this in an administration quick to boast that it has restored the morale as well as the effectiveness of American intelligence.

The premise of covert action is that it can be properly conducted and effectively overseen. But in this case the CIA surrendered its professional judgment to a lame-brained idea launched on the political side of the government, and an uninformed Congress lagged months behind the deed. The manual tramples upon legislation specifically outlawing assassination as a tool of American policy. It mocks the American government's campaign against state-sponsored terrorism. It flouts the statutory requirement for the CIA to "report (to Congress) in a timely fashion . . . any illegal intelligence activity."

The president has ordered an investigation. This is not a terribly complex affair, so the investigation should take, perhaps, an hour and a half. The measure of Mr. Reagan's seriousness will be whether the findings are reported before Nov. 6.