I moved back to the United States from England a year ago. While I was away, something happened. The nation, it seems, was invaded by pigs.
Not that there haven't always been pigs. For years the rich farm states of the Midwestern ''swine belt'' have been going, as it were, hog wild. This, after all, is the nation that in 1979 produced 15.5 billion pounds of pork. And this is the nation that produced Porky Pig comic books and cartoons and Freddy the Pig novels.
In the past few years, however, the ordinary porker has been invested with a new mystique. I was reminded of this the other day, when the Boston Globe carried a front-page story with a Kansas dateline about a woman who works local circuses with a trained pig act. Called the Pork Chop Revue, it features 500 -pound stars who roll out a red carpet with their noses, jump over fences, and push one another around in baby carriages. It's nothing new: The act has been going since 1956. What struck me was that the paper should give it such play.
But there you are: America is going into its porcine phase. Consider the following:
* Miss Piggy, the TV character from ''The Muppet Show,'' has taken the nation by storm. A stout, formidable suitor with an unregenerate vanity, a petulant flounce, and a powerful left hook, she crops up on posters, in movies, on ads, and has ''authored'' the best seller ''Miss Piggy's Guide to Life.''
* The crew on the recent flight of the space shuttle Columbia, were awakened after a sleep period by a transmission from Houston of something else from ''The Muppet Show'': a taped segment of ''Pigs in Space,'' featuring Dr. Strangepork and a whole pigKery of fellow muppets dressed as astronauts.
* Jamie Wyeth, son of painter Andrew'Wyeth and a well-known artist in his own right, has kept and painted pigs for years. Asked to contribute a small piece of work last summer to an art auction sponsored by a group opposing nuclear power, he dashed off a pink ink drawing of his sow, Baby Jane, wearing a sash labeled ''NO NUKES'' around her midriff.
Even Boston's famous Quincy Market, full of sleek shoppers in alligator shirts and Calvin Klein designer jeans, has a tiny boutique devoted solely to things porcine. Called Hog Wild!, it is a veritable trove of piggy banks, lamps, dishes, glasses, pencils, carrying bags, and T-shirts. It has calendars featuring pictures of a pig in a toga titled ''Caesar Hogustus'' and another of a large and a small pig iN Elizabethan costume labeled, respectively, ''Ham'' and ''Hamlet.'' It has a brand of shampoo labeled Hogwash. It even has a line of turtlenecks with pigs embroidered where the alligators should be, bearing labels ''Calvin Swine.''
Why all this interest in the family Suidae? Part of it, I suppose, is in the name. Pig is a funny-sounding word. But then, so is aardvark - which means earth-pSg in Afrikcans, but which has yet to show up on many bumper stickers. Part of it, too, may be in the look of the beast itself.
But I suspect the answer lies deeper, in our present sense of ourselves. It is a divided sense. These days, one part of America is slimming itself down in what Time magazine describes as a $30-billion-a-year diet-and-exercise business. But another part is clearly on the plump. And both sides, I suspect, harbor a suspicion that the nation as a whole has got its priorities slightly askew. There is a nagging concern that America is sinking a bit too far into hedonism and self-indulgence - that in a world where 500 million people go to bed hungry, we hadn't ought to busy ourselves inventing new sugar-coated breakfast cereals or buying $70-a-pair running shoes.
And what does all that have to do with the pig? Simply this: The pig may be (pardon the shifting metaphor) a kind of scapegoat. Pigs, after all, are supposed to be hogs. They are meant to pig out, gluttonize, live lazy, muddy lives. And we love them for it. Something within us longs for a world in which we could eat our weight in cake and feel no guilt. But something else wants to hold up to ridicule all those qualities we find most ugly in ourselves. So we direct our satire at the pig who stands for us.
That may be what's behind the message on one current bumper sticker. It asks, ''Have you hugged your pig today?''