The Reagan administration is under attack from Fidel Castro for allegedly hoking up a new fantasy of Cuban intervention in El Salvador-- this time floating it in the press in a form in which the charge does not have to be proved, rather than issuing it straight in an official White Paper--in order to lay the groundwork for "further actions" against Cuba.

Well, live by the pen, die by the pen. Since the vehicle for the latest American threat was a newspaper column, it was only fair for Castro also to use a letter to the newspaper as the vehicle for his reply. There is a certain reassurance to be had in seeing that the exchange is still on the level of psychological warfare, propaganda and public diplomacy. I would rather have the two countries shooting words than bullets.

But is President Reagan actually so frustrated that he may be turning to violence? He was not too helpful the other day at his news conference when he said merely that he had "no plans" for military action. Plans can be made at any time.

There are, however, a few other straws. I assume that what intelligence there is bearing on the dispatch of Cuban troops to El Salvador is mushy. That could be one reason why Castro demands that Washington furnish it, and why Washington does not. The administration would have to be not only very frustrated, which it doubtless is, but also very foolish, which I don't think it is, to take a military initiative without being able to make a strong, clear case for it.

But certainly the administration is in a box. It has not been able to decide whether El Salvador should or should not be a test case of drawing the line against communist expansion. It has achieved limited success, but only limited success, in attracting international backing for its policy. The condition of its domestic backing is perhaps conveyed by the recent 9 to 0 vote in a House Foreign Affairs subcommittee urging the president to "press for unconditional discussions" between the government and the guerrillas.

Meanwhile, the administration seems to have greatly exaggerated the ease of winning either a military or a political victory in El Salvador, so much so that Secretary of State Alexander M. Haig Jr. confessed the other day that "the situation has become essentially stalemated" and "needs reassessment."

Actually, I was cheered by this statement. It struck me as a tacit rebuttal of the heedless gung-ho spirit with which the Reagan people started out in El Salvador. That both the junta and the guerrillas contested Haig's verdict, each of them claiming it's doing fine, tended to confirm his accuracy.

There was a bright side, moreover, to the ensuing news reports suggesting that the administration had been canvassing military options in the El Salvador-Nicaragua-Cuba triangle. Responsible officials would want to canvass the options whether they intended to make good on their oft-repeated pledge to "go to the source" (Cuba) or whether they were of a mind to let that pledge wither on the rhetorical vine or whether they meant to blend those two courses, conducting some sort of "Christmas bombing" to demonstrate presidential resolve and then looking for a table.

In the classical diplomatic view, of course, a "stalemate" is a positive development if it makes possible the sort of negotiation that was out of the question as long as either side had reason to equate talking to the other with defeat. If the American purpose were to settle down El Salvador, and Central America as a whole, then the Reagan administration would now be moving cautiously toward the various middlemen and formulas available for testing that purpose in the region.

But does President Reagan see it this way? His general view has long been that communists are expansionists at heart and not principled interlocutors. Now, for the first time in his administration, he is in a specific context in which his views must be applied to policy.

That makes the present moment among the most revealing the president has faced so far. He did not tip his hand at his news conference on Tuesday. If he can bring himself to accept his secretary of state's verdict of stalemate, however, and to act on its implications, he could conceivably enjoy a dazzling success as a peacemaker. Does that appeal to Ronald Reagan?