The Reagan administration's anti-communist crusade in El Salvador has slowly moved to the inside pages and off the nightly television screen. That's a blessing when you think back on the earlier high-pitched shrieking about "drawing lines" and "striking at the source."

But it also invites a dangerously disarming misapprehension. Because the decibel level of the administration's preoccupation with El Salvador has been purposely reduced, you might be tempted to conclude that the pace and intensity of actual American involvement in El Salvador has been correspondingly tempered.

A little more restraint -- a heightened awareness, perhaps of the limits on effective intervention by the United States -- might have been in order after the outbursts of dismay and alarm at home and abroad that greeted the original, overwrought presentation of the Haig/Reagan line on El Salvador.

Instead, the administration is proceeding to develop and carry out an extraordinarily ambitious and comprehensive diplomatic, economic and military program for El Salvador. Designed for the long haul, it has -- never mind the weakness of the Vietnam analogy -- a familiar, not to say haunting, ring to it.

It is a carefully calculated plan to "end the war and win the peace" in El Salvador.

That, at any rate, is how the administration's plan struck me when it was laid out in some detail the other day in an interview with a high-ranking official. The basic ingredients have been well advertised: big increases in spending for military aid, a team of some 56 military advisers, a massive transfusion of economic assistance.

So have some of the program's essential and, for the most part, sensible elements. A high priority has been assigned to restraining the bloody repression practiced by government security forces; increased U.S. economic and military aid is to be used as leverage. Quiet diplomacy is being directed at broadening the civilian content of the largely military government. d

"If the United States can be seen to have some hold on the junta," this official explained, "this could encourage wider representation in the government on the part of labor unions, farm co-ops, private business people, peasant organizations and others alienated by government repression."

By reaching out to the "non-communist left," disaffected Christian Democrats in the opposition movement, and other "non-militants," the hope would be to set the stage for elections in 1983, or perhaps find some basis for mediating a settlement of the conflict.

All this, persuasively presented, sounds reasonable. It's when you get to the military effort, however, that you encounter an element that seems to be not only unrealistic buy at odds with all the rest.

The administration's military arm of the opposition. This would be done not just by the much-publicized efforts to choke off the arms flow to guerrillas via Nicaragua or other neighbors, although that's part of it.

The crucial, less-publicized part has to do with endowing the government with a military capability to conduct (and here, again, I hear that familiar Vietnam echo) "search and destroy" operations designed to crush the rebels.

"Right now, the government army is ineffectual," the official concedes. "It can sweep rebel-held areas, but it can't close on the enemy and destroy its forces." So what the U.S. advisers are doing is helping to create a "quick reaction" force of battalion size, equipped with helicopters and trucks.

In theory, this U.S.-supplied mobility will enable government forces to encircle rebel forces, and wipe them out.

If this tactic works, it will greatly simplify life for the ruling junta. So much so, in fact, that there would be precious little incentive for it to deal with the social and economic roots of the insurrection -- to broaden its political base or submit to popular elections, for which El Salvador does not exactly have an impressive record. The results of the last two elections, in 1972 and 1977, were simply voided by the ruling military/oligarchy.

Similarly, if the military strategy is as successful as the administration's policy-makers hope, there would be no incentive for the political opposition to engage in efforts to mediate a settlement. Stripped of a military capability, it would have no position from which to negotiate.

It can be argues that this is a relatively riskfree prescription for preserving the present government for a time -- always assuming that "search and destroy" will be more effective against El Salvadoran rebels than it was against the North Vietnamese. But that is a dubious assumption, given the nature (essentially homegrown) and history (protracted) of the uprising in El Salvador.