WHAT CAN YOU SAY about a movie which promotes itself by sending newspapers a piece of plywood which proudly announces itself as "a plank from the political platform of Manfred Link, President of the United States of America?"

Not much that is good.

When Buck Henry started to write the script for First Family during the midst of the Watergate scandal, he must have known he'd hit upon a great idea. Every day brought fresh evidence into the homes of once-trusting Americans that the President--and those who helped out around the White House--were just like you and me, only worse. Scandals, illicit relationships, incompetence, cheating, profanity--this was the stuff of the modern Commander in Chief.

As the years wound on and the film was postponed, Henry could have only grown more satisfied with his concept. Even the best scriptwriter couldn't have produced a funnier character than Gerald R. Ford. There he stood, simian-like in profile, first skidding down the steps of Air Force One into the waiting arms of the Secret Service and then, in the ongoing battle against inflation donning a big red button with the letters WIN embossed in white. And who can forget Ford's classic performance in the debate with a onetime peanut farmer, when in the century's greatest diplomatic coup he simultaneously liberated Poland and lost an election.

As the idea festered in the screenwriter's mind and the former governor of Georgia moved into 1600 Pennsylvania Avenue, Henry's smile could only have been matched by the new president's. There stood James Earl Carter--a veritable personification of incompetence--garbed in v-neck sweater to save energy, hiring an adviser on drugs who pushed quaaludes, and journeying to Poland to declare, through the mouth of an interpreter, his true lust for the Polish people. Meanwhile, as the director of the Office of management and Budgets stole millions from Atlanta banks, the president's right hand man was spitting Manhattans on women he didn't know and making rude comments to ambassadors' wives.

And then there was the Carter family. Sister Ruth, who hobnobbed with Billy Graham and his ilk, and, like her brother, was a born-again Christian. And of course, brother Billy, the greatest filial embarrassment to hit the White House since Sam Houston Johnson moved into the White House so Lyndon could keep an eye on him. Billy, who declared his dislike for Blacks; Billy, who loaned his name to a (bad) brand of beer; Billy, who picked up a stack of money from Col. Khadafy and the Libyan gang. Billy, a loud, obnoxious, good ol' boy who pissed on airport runways while his brother the President piously declared: "I have never had an occasion to be embarrassed by Billy. I'm proud he's my brother." And then there was the First Mother, Miss Lillian, who after examining the products of her marriage questioned whether she should ever have abandoned her virginity.

THIS IS the stuff of very funny movies but there is something very sad about First Family. After an hour and a half of inanities, you leave the theater feeling like you had been to what was billed as the greatest Washington dinner party of the year--catered by the finest restaurant, with the most interesting and "in" people in attendance--and the hostess' five-year-old son threw up on the Pakistani ambassador and the English wolfhound did likewise on the wife of the Secretary of Defense.

Finding scapegoats for failure seems to be a popular preoccupation in Washington and in the case of First Family, the blame rests squarely in Henry. Given his past credits--The Graduate, Heaven Can Wait and Get Smart--and his cast's talents, everything should have come up roses. But the script...

Bob Newhart plays Manfred Link, the President of the United States, who is simultaneously seeking reelection and some way to control his daughter's (Gilda Radner) irrepressible urge to "do it" and his wife's (Madeline Kahn) alcoholic intake. The story also concerns the efforts of the Link administration to gain the United Naitons' vote of a country called Upper Gorm. When the Upper Gormese ambassador visits the White House, he announces that Link and the first family have agreed to visit his country. Unfortunately, the president and his staff don't know a thing about it.

But Link and Co.--looking for U.N. votes and natural resources--visit the volcanic island where, along the way to securing a treaty which trades 1500 average Americans for fertilizer to produce scallions the size of telephone poles, Link's daughter is defiled by a stone statue and the president goes quietly nuts. Mixed in with the infrequent stabs at humor (The Upper Gormese president tells Link: "We have everything a civilized country needs but a repressed minority.") are dull sequences, foolish jokes and a handful of racist portrayals. Back in the good ol' U.S.A., Link's advisers kill him off (well, not really) and the vice president (Bob Dishy) dies of a heart attack from the shock of becoming president.

SO POOR is this "satire in the broadest comic sense" (the producer's label) that it manages to embarrass even some of America's funniest actors and actresses. Newhart, who must cope with a ridiculous scene about dreams, Kahn (who says about three words) and Radner (who goes to waste) hold up adequately under the assault of Henry's script but there is little they can do to salvage any humor or grace. As the ambassador to the U.N., Harvey Korman (who deserves a film of his own) stands out as particularly funny; perhaps he adlibbed his lines. The rest of the actors stagger around poor sets under the weight of very un-funny lines, looking like they'd rather be out ringing doorbells for the candidate of their choice. The movie's music, written by John Philip Sousa, is its most redeeming feature.

First Family would be something to laugh about if it didn't completely kill any chance that a funny movie about the ridiculous people who inhabit the White House will ever be made. When the critics and studio accountants finish with this one, no studio will be foolish enough to bankroll a similar project until the memory of this film is dead and buried.