Among the voluminous Pentagon Papers there emerged a memorandum, dated April 6, 1965, from Lyndon Johnson's national security adviser, McGeorge Bundy, to the secretaries of state and defense and the director of the CIA. It listed three specific presidential decisions to increase the number and widen the role of U.S. forces in Vietnam. And it ended: "The President's desire is that these movements and changes should be understood as being gradual and wholly consistent with existing policy."

You do not have to summon up some all-encompassing, jot-and-tittle Vietnam analogy for El Salvador to find an echo in the response of Secretary of State George P. Shultz the other day when he was asked if the decision to send a hundred U.S. military advisers to a Honduran training base for Salvadoran troops might lead eventually to U.S. combat involvement.

"No, absolutely not; that's a foolish rumor; there is no change in policy," Shultz said. "We will not, repeat not, Americanize the conflict there."

I suspect he believes it, just as the Johnson administration believed that by small, incremental increases in the American effort in Vietnam it could somehow convince "the other side" that its cause was hopeless--without inciting a whole lot of domestic debate. There lay the crippling contradiction. The Johnson administration was unwilling to project, for fear of the political reaction, the threat of an open- ended effort.

To hear the Vietnam echo loud and clear, you have only to match the reassurances of Secretary Shultz against the reflections of Lt. Gen. Wallace Nutting, in an interview with The Post's Karen DeYoung, as he prepared to give up his post as commander of U.S. forces in Latin America after a four-year tour. He does not advocate "introducing U.S. combat forces in El Salvador. . . . For a whole lot of reasons, we should not want to intervene."

But he also makes a crucial distinction between actually intervening with combat forces and being prepared to state a readiness to do so: "As long as . . . limits on our willingness to engage in the ultimate resolution of the problem are evident to the guerrillas, they will persist--they have the example of Vietnam to refer back to." The general is unable to say whether the United States should have 150 or even 1,000 military advisers in El Salvador. He is "pretty sure" that the limit of 55 advisers imposed by Congress is not enough.

But his point is there should not be any fixed figure--"that's part of the complexity of this form of warfare." Until the United States makes an open-ended commitment, he's convinced that the cost of the U.S. effort, in whatever form, will rise. "But if we make the evident commitment without the limit," he adds, "then the cost will go down."

By this time, the Vietnam echo is almost deafening: the generals arguing that being willing to do more means having to do less; an administration making ringing demands for a bipartisan consensus accompanied by assurances, artfully qualified, of some limits on the cost; a congressional opposition increasingly inclined to impose limits of its own. No more so than in Vietnam is the Nutting approach to the psychological as well as the military nature of counterinsurgency likely to get a fair test in El Salvador.

The president has piled huge stakes on the table. He told the joint session of Congress: "If we cannot defend ourselves there (in Central America), we cannot expect to prevail elsewhere. Our credibility would collapse, our alliances would crumble and the safety of our homeland would be put at jeopardy."

But his appeal for a "bold, generous . . . bipartisan approach" is scarcely sweetened by the implicit threat that the loss of El Salvador will be charged politically to the Democrats if they don't fall in line. It loses something more with U.N. Ambassador Jeane Kirkpatrick's allegation that some members of Congress "would actually like to see the Marxist forces take power in that country." And it is in no way enhanced by the circling of the wagons we are now witnessing in Central American policy- making--the removal of the assistant secretary for the region and the American ambassador to El Salvador in favor of trusted "team players."

Both moves will remove from the inner councils officials who have expressed reservations, of one sort or another, about the administration's course. So the executive, presumably, will be assured of a consistent, tougher line, untempered by evidence of internal dissent. Only those with an ear for old echoes will recognize all this as a prescription for a deeper, more debilitating public debate.