ONE OF THE GREAT, crashing disappointments for a tourist in Washington has to be a visit to the United States Senate where, usually, no one in the World's Greatest Deliberative Body is deliberating. Instead, the Senate usually appears to be the world's most somnolent body, a place where true debate, like truth itself, is a rare commodity.

The poor tourist should be excused for expecting otherwise, for expecting, in fact, to see a rip-roaring debate -- for even associating debate with democracy. He, like all of us, has been schooled in the myth of the debate -- Lincoln-Douglas and all of that. He has learned of Cicero, the Forum at Rome and of Demosthenes who trained for debate by talking with pebbles in his mouth -- an innovation that in this political year seems to have become something of a fad.

Now we have the natural extension of the Great Debate Myth -- the Great Debate itself. It is coming, we are told, like some great prizefight: Carter vs. Reagan. It is supposed to go the political equivalent of 15 rounds (90 minutes) and God knows what to expect. There could be a knockout. There could be a TKO. There could be any of these things, but the chances are there will be nothing like it. It will be, like all debates, more myth than reality.

The myth, of course, is that America has something even approaching a parliamentary system in which debates play a significant role. The president, unlike a prime minister, does not have to journey to Congress to defend his administration with the skills of a debater. He need not parry and thrust, have a wicked and sharp tongue, take the petards of the opposition and hoist them upon them. America has never had a Winston Churchill because it has never needed one. He probably would have wound up talking to himself.

At the moment, though, we are led to believe that there is some sort of connection between debate and the presidency -- that the ability to debate has something to do with the ability to govern. It does not. In fact, it has very little to do with anything.

This is particularly true of the upcoming debate between Reagan and Carter. Presidential debates have taken on a mythology all their own, but there have only been two major ones -- hardly enough to make a myth. In the first, John Kennedy is popularly believed to have bested Richard Nixon and in the second Jimmy Carter is supposed to have beaten Jerry Ford.

Probably the poplular perceptions are accurate. In Kennedy's case, he was able to show the American public that young did not mean green, that youth did not mean inexperience. He robbed Nixon of the experience factor -- the edge he had by virtue of being Ike's vice president for two terms. Kennedy "won," but not because his ideas were better than Nixon's or because he was a better debater, but because he simply was not the person some people thought he was.

Somewhat the same thing is true of the Carter-Ford debates. Carter won when Ford impaled himself on a remark that Poland was not in the Soviet orbit. This played directly into what you might call the "dummy factor" -- the nagging suspicion that Jerry Ford wasn't quite up to the presidency -- and it played off what is one of Carter's strongest suits -- his evident intelligence.

But this year, we are about to have a debate between a sitting president whom we all know to the point of either exasperation or boredom (excitement is out of the question) and a challenger we all have known in one role or another since the late 1930s. Ronald Reagan has not been an unknown since he made his first film. He has been a longtime GOP spokesman, a two-time governor of California and a presidential candidate since, it seems, time immemorial.

It's possible, of course, that Reagan will play into doubts about his intellect. And it is possible Carter will lose his cool and attack Reagan. But those are the sorts of things that could happen any day of the week on the campaign trail -- and, in fact, already have. If Reagan lapses into Latin he could not convince me he is smart and if Carter sprouts wings he could not convince me he's an angel -- or, for that matter, a terrific president.

In fact, the danger of the debate is that it obscures what we already know. It comes so highly trumpeted, so charged with high school images of ancient Greece, that it is hard to realize that it is nothing more than yet another performance having next to nothing to do with the presidency. It has become, like photogenic families and the boundless energy to campaign for, if necessary, years, an essential part of presidential politics having almost nothing at all to do with the presidency. That said, bring on the debaters -- and please try to stay awake.