HAVING CHOSEN to make support on El Salvador the first big test of its capacity to mobilize its non-Communist allies and friends, the Reagan administration must ask itself why it is getting such a dusty and disappointing response. Most of the countries solicited for support are ready enough to grant the president's point that foreign Communists have had a large hand in arming the El Salvador guerrllas. But he asks for more: he asks in effect that the various allies and friends license the United States to do what it feels is necessary militaryily to put down the guerrillas. This the others are unprepared to do.

Behind their reluctance lies, first, a difference of analysis. Where the administration says the main danger to the junta comes from the left, most European and Latin countries feel, more plausibly, that at this point the guerrillas are being held and that the main danger to the junta comes from its own right wing and from its outriders. A single-minded focus on the guerrillas, therefore, may simply have the effect of finishing off the civilian reformist component of the junta.

The second source of reluctance arises from the administration's very intensity. It is acting as though the fate of El Salvador is of global importance. In truth, this is the feeling the administration means to project. It is aiming for a display of boldness and resolution that will shock Communist countries and others into understanding that the United States is rough, tough and ready to do battle.

But the allies, whose security depends in large measure on the United States, want to see not only firmness but a sense of balance. They, and the important Latin countries. want to see a matching sensitivity to the political equation. None of them lacks a certain vexing propensity to hang back from even the most reasonable American step in whose benefits they stand to share. But the Reagan policy, taking a good point and carrying it to excess, gives them an easy out.

Though the Reagan administration's first big diplomatic campaign is dragging, it is not beyond revival. The administration will certainly want to keep on ensuring that the guerrillas are not resupplied, first of all by using the diplomatic means that it is employing with apparent success now. This should give it a full measure of international impact. At the same time, the civilian president of El Salvador, Napoleon Duarte, must ber bolstered in his last-chance effort to strengthen both his authority and his popular suport. In words anyway, Secretary of State Alexander Haig has shown he understands what must be done. This offers the best hope of generating the international respect that has eluded the administration so far.