Once a man has become President of the United States, his next ambition is always to be a "human being." Usually they have some fiercely loyal supporter close at hand - some Julie Eisenhower - to keep insisting publicly that "He's really very human," in apparent contrast to other evidence.

President Carter has been hard at work for some time on this noble but quixotic goal, and has tried the usual methods, plus some innovations of his own. For instance, it is generally agreed in the society that the most humanizing of all human experiences is parenthood. That's gotten an awful lot of people not only to a humble state of humanity, but clear through to an inhumanity they never dreamed lay beyond. An independent-thinking small kid who tends not to disappear at company dinner time is practially guaranteed to humanize anyone, the way a wooden mallet will tenderize veal.

And of course, Carter showed himself aware that some clothes are more human than others, as for instance old cardigans, compared, say, to blazers with crowns or foxes on the gilt buttons.

But last week, he took two astounding, bold steps on that difficult road toward common humanity. He let it be known that he had 1) mice and 2) unexpected visitors.

You have to understand this is White House language. When you and I have furry creatures with long tails in our baseboards, they are not mice. Washington, D.C., outside of that one White House, is not noted for its mouse problem. At that, this is a franker administration than many. A previous one referred to these animals as "gerbils."

An unexpected visitor, in White House terms, is also not someone who arrives at an odd hour because he is interested in acquiring your television set, which you liked fine and have not described in any classified advertisement. It is an unknown and dangerous intruder who has been standing on the sidewalk for 20 minutes, missing an appointment he spent weeks in trying to set up, while a guard tries periodically to find out by telephone if the visitor's birth date coincides with today's lucky number in the White House Rolodex.

Nevertheless, President Carter has established himself as a man who hears funny noises inside the walls, and sees strange faces peering into his office from his backyard window.

It is what the rest of us think of as the human condition. One that could be avoided only if we rose to incredible heights, where we would have legions of people at our command to protect us from such things.