Responding to those now complaining about the Reagan administration's policy on El Salvador, Sen. Jesse Helms observed the other day: "When tough decisions come up, there are always some nervous Nellies who wouldn't turn on the garden hose if their house was on fire."

Now, where did we first hear words to that effect? From President Lyndon B. Johnson, in May 1966, as I recall. He was deriding "some nervous Nellies and some who will become frustrated and bothered and break ranks under the strain" of -- you guessed it -- the Vietnam War.

The Vietnam parallel, President Reagan has twice reassured us in recent days, doesn't fit. El Salvador is 10,000 miles closer, and a far cry in a lot of other ways from Vietnam. Half a million American troops are not going to wind up fighting in El Salvador.

But having said that, you have by no means disposed of the Vietnam analogy. On the contrary, the more you see and hear of the Haig/Reagan approach to El Salvador, the more there is about it that is familiar to the Vietnam experience and the more there is for the re-emerging nervous Nellies to be nervous about.

And nothing better illustrates the point that an incident recently alluded to more than once in public, and with more elaboration in private, by the recently U.S. ambassador to El Salvador, Robert E. White.

Briefly, he reported that on Jan. 19, inauguration eve, the top military man in the embassy presented him with a six-page telegram for Washington. It laid out the argument for, among other things, the 75-member military advisory team (the Reagan administration is envisaging expansion to only 50 advisers).

This "bizarre" proposal, White claims, would have "totally" transformed U.S. policy. He asked where it originated, and was told by his military chief: "I am under instructions to do this from the Pentagon" and the U.S. military command in Panama. In short, elements in the Pentagon, as White sees it, were trying to force the hand of the new Reagan administration even before it had been officially installed.

Now this is significant, naturally, only if you are nervous enough to question whether the new increases in military aid and in the number of American military advisers will be enough to shut off the arms shipments to the Salvadoran rebels and successfully shore up the government of President Jose Napoleon Duarte. If everything works out as neatly as President Reagan assures us it will, then escalation isn't a worry.

But if it doesn't, two factors have to be taken directly into account. One is the traditional escalatory instinct of the U.S. military, with particular application to El Salvador.

"I think there is a driving need, which I do not pretend to understand, by the American military to involve themselves on the ground in Central America," White told a congressional hearing.

The second factor is harder to measure, but difficult to dismiss. That is the enormous weight the Reagan administration has attached to El Salvador as a symbol of its resolve to counter communist expansion worldwide. Put the two factos together and escalation, on however narrow and reduced a scale, in Vietnam terms, would become very nearly inescapable.

Reagan says it won't happen: "We do not foresee the need of American [combat] troops." Neither did Dwight D. Eisenhower, John F. Kennedy or Lyndon Johnson in repeated reassurances, from the time the first advisers arrived under Eisenhower until the first combat units landed in Vietnam in 1965.

The military advisers in El Salvador, Reagan insists, are not like real advisers -- they won't go out on combat missions. The first 500 military advisers in Vietnam were technically "civilians." When that wasn't thought to be enough, military groups arrived, up to 16,000 under Kennedy. When that didn't quite do it, the advisers were sent on combat missions, took casualties, were authorized to shoot back.

Our advisers in El Salvador will be made as safe as possible within their garrisons, the president promises. It was North Vietnamese attacks on American billets and garrisons, singled out as targets, that triggered the first American bombing of the North in Vietnam .

Too dark a drawing of an inappropriate analogy? Perhaps. But the answer is not going to turn alone on military developments. It will turn (as in Vietnam) on the unknowables of economics and local power struggles on the right and the degree of repression by the government. All of this is difficult to control -- and even harder to predict.

So, yes, I'm nervous -- when it comes to conflagrations of any kind. It is no trick to turn on a garden hose. The hard part is whether it can be counted on to save a house on fire.