I was trying to remember where I'd seen it before - the president's behavior of the past week, in which he changed his voice to alto, and began to shout like a gym teacher, and to pound his fist on desks, saying things like "I want you to listen to this," before demanding the resignations of his Cabinet and White House staff, and handing out nutty report cards. And then the scene came back to me, in a puff of blue smoke, like the smoke of the original:

It was near the end of "The Wizard of Oz," when Dorothy and her three companions had brought the wizard the broomstick of the Wicked Witch of the West, as the wizard had commanded, only to find him as bombastic and evasive as ever - his oversized, disembodied head bobbing like a giant egg yolk in the smoke and flames. Behind a curtain in the corner, we soon learned, was a mere man, manipulating the wheels and cranks that produced the false head, until Dorothy's dog, Toto, pulled aside the curtain to reveal the fraud.

At that the wizard looked around, and frantically shouted, as if through the head: "Pay no attention to that man behind the curtain" - hoping to persuade his audience that he and the head were separable. At that point, too, he became noisier than ever, the way our own national head has been of late.

Of course, the Wizard of Oz maintained a false head throughout his wizardom, and only pulled out the stops at the brink of exposure. Mr. Carter, on the other hand, did not grow noisy until recently, and has generally kept his head, whatever you thought of it. Now, however, he has cranked up a new head, a big shouting one, which is clearly not his own. Why?

Until last week, it seems to me, Mr. Carter was going along rather smoothly as an average, run-of-the-mill president. We have had many more average, run-of-the-mill presidents than great ones, although every president is paid the courtesy of being forecast as "one of the great presidents"; and there is no disgrace in averageness among presidents, any more than in averageness among all-stars. If it is true that one has to search carefully for Mr. Carter's strengths, an equally avid search is due his weaknesses. And if the polls show him even less popular than Richard Nixon, that too is meaningless, for Mr. Nixon wasn't unpopular, he was despised. And Mr. Carter is no more despised than he is loved.

That, I believe, is the historic truth of his presidency - a truth not available to change, by him or anyone else. Mr. Carter is an undersized president in an oversized job. Unlike other presidents, worse and better, he has not grown one inch in office, nor did he appear to be expecially troubled by that fact, until the heat of the Washington summer.

But then came Camp David circus week, and the parade of worthies and the sensationally boring night visit to Ma and Pa Citizen of Pittsburgh, and suddenly: Va-voom - a total and immediate stylistic change in a U.S. president. And not a change of hair-part, but of head. We were implicitly told to pay no attention to the man behind the curtain, because the "new" president, the "real" fist-banging, Cabinet-bouncing, ripsnorter of a president, was floating before our very eyes.

The only trouble is that the curtain in question was pulled a long time ago. We have already paid attention to the man behind it, have paid rapt attention to him for 2 1/2 years, and we would know him anywhere. Was it not that man behind the curtain who spent all that time tinkering with federal bureaucracies - though now he shouts that Washington is the enemy; and who never issued a whimper of disapproval about the Cabinet he now terroizes? The new head bellows that the old does not exist. But, heck, Jimmy, isn't that you puffing up that head?

For the rest of us little heads, the irritating thing about this presidential transformation is that if Mr. Carter went to all the trouble of creating a brand new head, he must feel that a big, smoking wizard's head is what we prefer - which is as much an underestimation of our intelligence, as his "loss of confidence" speech was an underestimation of our buoyancy and practicality. All we ever sought from the man behind the curtain was a brain, a heart, some nerve.