Now that an actor is living at 1600 Pennsylvania Avenue, are we in for a spate of White House spoofs? If so, let's hope they fulfill their campaign promises better than this one. "Remember when comedy was King?" declare the ads. "Now he's President." The idea of Bob Newhart as Chief Executive is funny; spending 104 minutes watching him act presidential is only moderately so. True, Gilda Radner is delightful as the sex-starved First Daughter, and it's amusing when Cabinet members dress up as pumpkins, rabbits and turkeys at a party. But where does one go from there? Buck Henry directed his own screenplay, which disintegrates about halfway through the film. That's surprising, since Henry wrote the scripts for Catch-22 and The Graduate. Maybe Bonzo wasn't so bad after all. (R)