Sometimes these days when Walter Mondale's defenders start explaining him I am reminded of that now famous witticism about how Wagner's music is better than it sounds. Other times I am reminded of the feeble tag we all append to humorous stories that fail -- "Maybe you had to be there." Apparently we -- all 230 million of us -- have to be there. "If only he could talk to people head on head," Speaker O'Neill said the other day, "he's so talented . . . the election would be a landslide." Mondale's aide Robert Beckel, quoted in The Wall Street Journal, says the same thing: "If we could put Walter Mondale in everyone's living room, this race would be over."

But of course they can't. And, speaking as one who fell into the trap of making similar observations about Mondale a while ago, I feel compelled to say something different now. Gentlemen, forget it. It is not only silly to hypothesize about how Mondale's true, but unrevealed, self would prevail in a contest if only he could become personal friends with each of American's many million voters, it is also off the point. We don't "know" our presidents. We imagine them. We watch them intermittently and from afar, inferring from only a relatively few gestures and reactions what kind of people they are and whether they should be in charge. Much depends on our intuition and their ability at a handful of opportune moments to project qualities we admire and respect.

It's no good deriding all this as a cheap skill of actors or a political trick unrelated to the governing capacity of a serious man. For candidates need this gift to be elected and to lead -- to mobilize opinion and influence here and abroad. It is, in short, the essence of becoming and being president, and Mondale's failure at it so far is not something to be patched over with expressions of regret that everyone doesn't know him better. What too many people know is that he does not give them the wherewithal to imagine him as president.

Reagan had a few such critical, opinion-molding moments in the last campaign. One was surely during the New Hampshire primary debate when he seemed to override other people's dithering and bullying, making the acid remark about having paid for the microphone and being determined that it would be used in a way he found fair. Another was the debate with Jimmy Carter in which he conveyed a far more composed and masterful persona than that of the sitting president, never mind who won on points in the contest over policy and fact. In his own presidency Reagan has reinforced this impression of the leader -- the man who, when shot, makes graceful little jokes throughis pain to reassure and calm the rest of us.

I can hear the protests about how much of this is simulated, feigned, conceivably even memorized and how some of it comes from old movie scripts. But whatever goes into it, it is Reagan's authentic style and it comes out looking plausible to people as a mode of leadership. The same may be said of Geraldine Ferraro's virtuoso press- conference performance in answering the questions about her own and her husband's finances. True, some questions were not answered satisfactorily. Still, she also appeared clearly a person in charge of herself and ultimately of the others in the room; she seemed cool and strong. She, too, was plausible as a leader. This -- the plausibility element -- is the crucial one, and it is where Mondale falls down, where he has yet to make it. He does not seem the leader.

I have no doubt that a lot of unfortunate circumstances -- e.g., his having to battle for more than a year now with all manner of others to get the nomination -- and the diligent propaganda of his opposition have contributed to this impression. But it is there and it is not somebody else's fault, and if anybody is going to do something about it, it will be Mondale or no one. This is worth stressing, as he is getting some perfectly terrible advice right now, mostly having to do with image transplants.

Be "Fighting Fritz," he is admonished -- rough up the other guy, show you are mean and tough, etc. But Mondale, though he may have prevailed over Gary Hart by kicking him around and badly damaging his knight-in-shining-armor image, is at his least attractive in strenuous, angry one-on-one combat. It doesn't suit him. The other principal urging, all that lamentation about the absence of a "vision" and so forth, accompanied by calls for him to delineate the issues, only invites the candidate to an equally unattractive presentation: Mondale the goody-goody, the guy you love to tune out.

Mondale-Ferraro campaigners, seeking to rally their troops, often hark back to the Harry Truman upset. I think a more useful model would be the Kennedy-Nixon campaign of 1960. It has, for Mondale, both a "do" and "don't" lesson. The "don't" is what Nixon did. I well remember reading at the time how Nixon had acquired different style suits for campaigning in different regions of the country. He tried to calibrate his identity to the audience. Kennedy (it is now forgotten) seemed to many, many people -- I was one -- marginal, tinny of voice and short on either content or leadership potential in the beginning. He was the utterly implausible presidential candidate. He appeared to be going nowhere and deservedly so. He did two things. He managed to retain and impose his stylistic eccentricities on the public consciousness -- he wore what he wore and spoke the funny "Cub-er" way he spoke. So he established authenticity and then, in the first debate, he established that this personality was that of a leader, a plausible president. I remember that it was not until the middle of that debate that I began to think of Jack Kennedy as "my" candidate.

All I am saying is that there is a great opportunity here to recoup, to stage a comeback showing what you can do under pressure. Kennedy won that way. Humphrey almost did. But you have to do it on your own. In other words, if Mondale is such a terrific guy, it is up to him and no one else to prove it. America is not going to be bused into his living room. Advisers with ideas about how he should change his personality and image are not going to do him any good (but probably much harm). If he is not, as the current criticism has it, a whiner and a yielder and a guy who can bore you to death, then let him show it. I for one think he can.