BY INVITING American reporters to see how

they are doing, the guerrillas fighting the regime in Nicaragua have done something very important: they have shredded the threadbare veil in which the Reagan administration had cloaked its policy. This has been one effect of Post correspondent Christopher Dickey's absorbing reports.

The revelations come at a moment when Congress is becoming increasingly agitated about the American role. Its concern is not simply that the United States is sponsoring an armed intervention, one fraught with heavy regional political and military implications, against the leftist regime in Managua. Legislators have different views on the policy. They appear united, however, in their broader concern that the administration is conducting it in evident violation of the law.

Congress, through its intelligence committees, had approved plans for CIA action to interdict the flow of weapons from Nicaragua to El Salvador. The hitch arose when the administration moved on to operations directed against the Sandinista regime. As Don Oberdorfer reported on Sunday, troubled intelligence committee conferees last August attached to the intelligence authorization bill a secret document, a statement limiting the purpose of the operation to interdiction. Congress found it necessary to reaffirm the limitation in December in the Boland amendment, which the House passed 411-0. But correspondent Dickey's stories from Nicaragua make it crystal clear that, notwithstanding that law, the guerrillas are fighting to bring the Sandinistas down.

The result is the sharpest pinch President Reagan has experienced since he undertook to fight communism in Central America. The guerrillas are demanding that he stay firm in support--they no doubt calculate that the new exposure of their American-helped effort will make that effort harder to terminate--and Congress may be gathering itself to enforce a law, or at the very least to stage a debate, whose thrust will be to trim back that support. A painful collision looms. If Mr. Reagan does not put his policy back within the confines of the law, however, Congress will have no responsible alternative but to do it for him.