WELL, IT WAS A great success in just about any terms you can think of, the President's Saturday afternoon radio cal-in show. There were earnest, friendly callers and earnest, friendly answers; the exchanges had a refreshing, authentic quality, not just because they were unplanned but also because the questioners were asking about things that interested them most, rather than things professionally designated as being "in the news." Life imitated art in that the combination of randomly selected callers presented a show balanced enough to have been the work of central casting. The President was snappy, intimate, and impressively knowledgeable. And, despite our own worst fears of what would happen, Richard Nixon did not in fact call in to see if he could get his tapes back.

We will confess that there are virtues to this particular political art form that we had not even guessed at. Somehow it is a benign and easy setting in which to enunciate policy: no leers and sneers and nasty follow-ups insisting that a President admit to a change of course or a conflict within his administration. The tone of his responses throughout - low key, reasonable, willing from time to time to say he didn't know what to do about something - was reassuring. People listening, we suspect, were able to identify with those whose calls got through and to be heartened by the knowledge that the President was in direct communication - as he himself so likes to say - with the people.

We have some reservations. They are not very strong or sharply formed, being more in the nature of a shadowy, down-the-road worry than an objection. It strikes us that there is a potential for demagoguery in this format and in Mr. Carter's populist pitch in general. And it seemed to us that in the occasional answer, though not in the tenor of the answers as a whole, we could hear a distant strain of it - a kind of us-against-them class warfare feeling and an almost ducal sense of the office, as if it were up to the President personally to dispense government benefits to the people.

Still, our reaction was mainly upbeat. We think this format, along with related innovations the President has in mind, may be instrumental in mobilizing for him the kind of public support that will be essential to getting hard jobs done. Mr. Carter has set about imaginatively to gain the confidence and goodwill of an electorate that chose him by a thin margin. He seems to be succeeding. That may not tell you much about the uses to which he intends to put his enlarged support. But it does tell you that he will be in a position to take tough stands and fight for them if he chooses.