There can no longer be any doubt about it: the charge of "red-baiting" or, alternatively, "McCarthyism" has definitely replaced patriotism as the last refuge of the American political scoundrel.

Consider as the latest instance the debate raging over Nicaragua.

For some days now, one opponent after another of aid to the contras has been hurling one or another of these accusations at President Reagan, several of his supporters in Congress, and especially his communications director, Patrick J. Buchanan. Even a few congressmen favoring aid to the contras have joined in with the warning that the administration's rhetoric is alienating the centrist Democrats whose votes are needed if the aid bill is to pass.

It remains to be seen whether the administration is committing a tactical blunder by defining the situation, in the way Buchanan did, as a choice between "Ronald Reagan and the resistance -- or Daniel Ortega and the communists." But the more important question is why so self-evidently accurate a formulation of the alternatives before Congress should have provoked so hysterical a response.

One reason is itself tactical: the Democrats, along with their Republican allies, are trying to ensure that if the aid bill is defeated, the responsibility can be shifted away from them and onto the administration. They can thus simultaneously vote against the president and blame him for making them do it.

But there is a deeper purpose driving this maneuver, and to understand it we have to step back and review a little contemporary American history.

Once upon a time, then, there were people in this country who indiscriminately described their political opponents as "reds" or "communists." These people were in turn stigmatized as "red-baiters" and "McCarthyites" -- rightly so in the sense that they sometimes slapped the communist label onto persons who were nothing of the kind.

Obviously, however, the fact that some noncommunists were wrongly called communists did not mean that there was no such thing as a communist. Yet that in effect is exactly what it came to mean in the minds of many liberals.

So far as these liberals were concerned, to speak of, let alone to attack, anyone as a communist was to engage in "witch-hunting." Indeed, at the height of Joe McCarthy's power, Arthur Miller wrote a play entitled "The Crucible" based on that very analogy.

Applause for "The Crucible" was deafening. It was so loud, in fact, that it drowned out the voice of a lonely critic, Robert Warshow, who observed that in implicitly comparing the cases of Alger Hiss and the Rosenbergs with the Salem witch trials, Miller was suggesting that communism, like witchcraft, was a delusion of its persecutors' paranoiimagination.

As a measure of how powerful this compulsion to deny the reality of communism became, when Angela Davis ran for vice president on the Communist Party ticket in 1980, she was rarely referred to in the press as anything but a "militant" or a "radical." Evidently many liberals had reached a point where they felt calling even a high official of the Communist Party by that name was to commit the sin of McCarthyism.

Nor was this ban on the word communist limited to American citizens. Even in talking about countries ruled by communists, liberals tended to employ euphemisms and circumlocutions like "Marxist" or "Marxist-Leninist" or "socialist" -- anything, in short, but the supposedly McCarthyite epithet communist.

By thus indiscriminately extending the charge of McCarthyism from the false to the truthful use of the term communist, these liberals have become a mirror image of McCarthy himself.

Worse yet, they now stigmatize as McCarthyite the argument that in a conflict where the only choice is between communists and anticommunists, anyone who refuses to help the anticommunists is agreeing to help the communists, whether or not he wishes to do so and even if he is unsympathetic to communism in general.

The current debate on Nicaragua is an excellent example of how this stigmatizing process works. When the president and Buchanan point out that opposing military aid to the contras is inevitably and inescapably to side with the communist regime in Nicaragua, where no alternative "third force" exists, they are neither misrepresenting nor distorting nor oversimplifying the facts of the case. On the contrary, they are telling the plain truth. For this they are smeared as red-baiters and McCarthyites.

Those who are doing the smearing would surely have been recognized by Dr. Johnson (who was, of course, the author of the famous epigram about patriotism) as the true political scoundrels of today.

They are scoundrels because, bereft of any solid argument, they retreat to the last refuge of character assassination.

They are scoundrels because they cry foul when they are held to account for the logical implications of the things they say and the inexorable consequences of the things they do.

Most of all they are scoundrels because, in the name of liberal values, they are trying their utmost to interdict the open and honest and free discussion of the main danger facing this nation, not only in Nicaragua but in the world at large.