AMERICA HAS GOT into the habit of building up its presidents too high in the beginning and then all too soon bringing them down much lower than they deserve. There is not one president elected in the past 20 years -- after John F. Kennedy in November 1960 -- who is not today reviled or at least disdained. Unless I am very much mistaken, it is about to happen all over again.

President Reagan was wafted to California at the beginning of August as an all-conquering hero. He is now being presented to us at the beginning of September as an embattled victim of his own promises and evasions. Neither of these pictures is true. For a man of notably modest talents, his performance has been equably mediocre. He is neither more nor less distinguished than he promised to be. Yet the notion of a "credibility gap" is again being bred.

But the credibility gap is now, as always, in us. It is born of our own absurd expectations, and then fed by our so virtuous disillusions. One could offer many explanations of this new destructiveness in Americans' attitudes to their presidents. I want to concentrate here only on one.

There is a lamentable failure to appreciate politicians on a level no higher than their usually very ordinary humanness. They eat, they drink, they sleep. How they do these things usually tells more than anything else about them. One can neither build them up too high nor pull them down too low if one attends to their ways of getting through a day.

The opulence of President Reagan's inauguration is likely to remain a symbol of his administration far into the future. Washington had the uncomfortable sense of an alien and occupying power. Conquerors show what they are like by the food they like and how they order it. The newly subject natives will be only too willing to meet their whims and needs. We may go back 800 years.

It is one of the conveniences of modern cities that, if one is feeling too lazy to whip up a meal, one can send out for fried chicken or spareribs, or just pick them up on the way home. How up to date! Yet one William FitzStephen, writing about 1173, described a restaurant near the Thames which offered "expensive dishes of a Lucullan luxury," and other hot packed meals, for citizens to take away.

The interest of the story is that FitzStephen was a conqueror. There was no classier name among the Normans who fastened themselves on the necks of the Anglo-Saxons in 1066. Within a hundred years of the Conquest, their French tastes were being supplied by a carryout restaurant. Far more than any of the multiple regulations by which the Normans controlled England, that one reference tells us exactly what politicians aexpect from those they govern. They, like the rest of us, must eat.

Virginia Woolf once observed that meals are not described often enough in novels. So much takes place at them. I remember one story of Gogol's which captures all the terrors which a child feels at the table of overbearing grownups. The hunger for food and the thirst for drink are born with us. They die only when we die. The condemned man on the way to his execution is allowed a hearty breakfast. He usually eats it.

So do politicians eat. As with the elephants of Lake Manyara, the ecology of politics is, first and foremost, also one of food and drink. If one tries to imagine how politicians decide to build a neutron bomb, one must remember that they always decide more quickly when it gets close to lunchtime.

This was one of the rules laid down by Clement Attlee, a most expeditious man, for getting a difficult decision through his cabinet. "Call the Cabinet for 11:30 a.m.," he once told a journalist. "Fill the first hour with the unimportant matters. Then raise the difficult question at 12:30." Every one of his ministers then glanced at his watch, and the issue was at once decided as they went to lunch.

Politicians are energetic. Only an energy which few of us can command would sustain them in activities which to the rest of us would seem so demanding and at the same time so banal. (But also, I do not deny it, so necessary.) The main function of all political systems must be to drain some of that energy away into activities that are relatively harmless. Sex seems to excite them to political activity only more. We have only one recourse. Let them eat and drink and be satiated.

Politics is an incoherent activity; it should be left incoherent, be recognized as incoherent and described as incoherent. Food and drink are part of it. One never knows what happens at those state banquets, for example, until a future generation can read the personal journals. More than we readily acknowledge, that is where powerful men are softened. Menachem Begin would not have made the speech which he made at the White House the other night if he had been in front of a mere press conference. This is the jolly side of politics: its irrepressible display of ordinary appetites.

There is something at fault when we sit down in front of our television sets during the weekend to listen to what we are invited to believe are discussions of politics, and all that we get is lugubriousness. Four lugubrious commentators on "Washington Week in Review" on public television; four who are equally lugubrious on "Agronsky & Company" the next evening; and an assorted number of the lugubrious commentators interviewing no less lugubrious politicians on all networks on Sunday morning. Not a mention of food or drink.

This is not what politics is or should be like: commentators pretending that they know what they are talking about, and politicians pretending that they know very clearly what they are doing. Politics moves among shifting sands to find momentarily some sure ground. All the time, it is risk; it is gamble, it is dare. Opportunism is barely discernible from principle; and supposed principle is rarely what it is advertised to be. Often the most we have to go on is the evidence of how politicians eat or drink.

Friedrich von Holstein, who was political counselor to Bismarck, wrote of Bismarck's son, who was then the secretary of state for foreign affairs: "Herbert is completely taken in by the Russians. He dines with Suvalov two or three times a week, is served (as he says himself) seven different kinds of wine and is grossly flattered. But on the diplomatic level Suvalov treats him as a callow youth." The same happens now in politics everywhere.

We are amazed when we read of the political feasts of the past. George Neville in 1466 gave a feast to celebrate his enthronement as archbishop of York. One of the guests was Richard, the young duke of Gloucester, whom he was seeking to detach from his father, Edward IV. The archbishop did not seat Richard at his own table; Richard was the only male to be seated with the ladies of rank. Such flattery! Yet little different from politicians' hospitality today.

"The banquet was one of the most sumptuous of the age," a historian has written, "its opulence far exceeding that of which the king himself was capable." Sixty-two cooks labored to prepare 104 oxen, six wild bulls, some 4,000 sheep, calves and pigs, 500 stags, 400 swans and ''a galaxy of other meats which were washed down with 300 tuns of ale, 100 tuns of wine, and a pipe of hippocras." There followed 13,000 sweet dishes.

That is politics, and the Nevilles, of course, last to this day, mainly by such hospitality. Some 500 years later, Disraeli described a dinner which the speaker of the House of Commons gave to Palmerston, then the prime minister:

"Dined with the prime minister, who was upwards of 80 years of age. He ate for dinner two plates of turtle soup; he was then served very amply to a plate of cod and oyster sauce; he then took a,e; afterwards he was helped to very greasy-looking entrees; he then despatched a plate of roast mutton; there then appeared before him the largeo st and, to my mind, the hardest slice of ham that ever figured on the table of a nobleman, yet it disappeared, just in time to answer the inquiry of his butler, 'Snipe, my Lord, or pheasant?' He instantly replied 'Pheasant,' thus completing his ninth dish of meat at the meal." That is politics. Feed the brute; then work on him.

War also is food, judging by the memoirs. One is not surprised to read Gen. Serrigny's description of Marshall Joffre: "I never saw a man eat so much," as he watched the great man put away eight chops and a bottle of Bordeaux by himself. The young Lt. Patton noticed how his hero, Gen. Pershing, dressed exquisitely in the field, and later followed his example. But Patton was no less impressed by Pershing's mess in the field in Mexico in 1916:

"Oyster stew, celeries and ripe olives, roast turkey and dressing, cranberry sauce, banana fritters with cream sauce, mashed potatoes, string beans, lettuce with mayonnaise, apple pie and cheese, assorted fruit, coffee, tea, milk, distilled water."

It may seem homely American fare, but one understands what so impressed Patton that he wrote home about it. There in the middle of Mexico, the general orders up an American meal. That is what impresses the natives and, by jingo, that is what impresses any statesman as well. I have never understood why the reports of state banquets are confined to the social pages. That is were much of the political work of softening up is done. A speech at any state banquet ought to be parsed.

But there is something even deeper behind this subject. I happened to read last week Joseph Alsop's book, "From the Silent Earth: A Report on the Greek Bronze Age." It is, as one would expect, impressive. It is, as one would expect, moving. He picks his way through the treacherous ground of archaeology, the minefields which scholars in such a field habitually lay for each other, with a learning and cultivation which few journalists now can emulate. He quivers with sensitivity to a high civilization now 3,000 years buried in the past.

And on what evidence -- as a political reporter, as he anounces himself -- does he build his picture? On the same evidence, of course, as the scholars themselves: on a few tablets of scripts -- Linear A and Linear B -- baked hard in the final catastrophe; and on things as passing, one might possibly think, as wine jars filled and wine cups ready, when the catastrophe struck. How much of our evidence of the past, in memoirs or dug-up artifacts, is food and drink.

The food and drink of the rulers, the food and drink of the common people; whether it is in the silent earth of ancient civilizations, which we try to make speak to us; or in Cicero's complaint that he was despised by the aristocrats if he did not serve peacocks at a meal; or in some otherwise mute roll of a medieval monarch which comes to life when it tells us what his court ate; or in the spendid descriptions of 19th-century repasts: Our best evidence often is from the stomach.

Joseph Alsop's book on preclassical Greece explains in large measure why he was a supreme reporter. He had the reporter's proper sense that only the past and the future are real, and that out of them he must create some possibly imaginable sense of what is going on in the present.

Journalism cannot long survive if it thinks that it can trade in hard-hitting certainties, of when a president is on the rise and when he is falling. Journalism is like life: hard-hitting uncertainties. To pretend that there is any coherence to politics is to diminish it. Politics moves like a mountain goat, precariously but skilfully balanced on the precipices of life. So should our observations of it. It is ridiculous to pretend that Carter or Reagan know what they are doing. They balance; they adjust. We scarcely know why, but at least we know that they feed.

When the champagne at last was served at a dinner at which Disraeli was present, he said in a whisper which could be generally heard, as it was meant to be: "Thank God for something warm." What else does one need to know? The interest that is apparently given to politics in America is overexerted. There is precious little interest to be found in politicians as such. They are neither saviors nor villains. All they want to do, like us, is fill their stomachs. President Reagan does it the same way in Washington now as in California last month. Nothing has changed.