"Leave 'Em Laughing" aims to leave 'em weeping, and succeeds, but it's a pity the CBS movie, at 8 tonight on Channel 9, doesn't add up to much more than a good sob. There were sensational or at least potentially satisfying possibilities in this basically true story of a dying, big-hearted old Chicago clown, and Mickey Rooney, who is wonderful in the role, could obviously have handled them.

But the scrpt is spartan and hesitant, and the director, Jackie Cooper, is all too resigned to the alleged limitations of television; the story is kept so emotionally and structurally primitive that it becomes a bleak cartoon. Cooper is competent, but he doesn't exactly have a poet's soul. His style is no different with this sensitve material than the style he used to direct sitcom episodes.

If the actors are going to have to do all the work, let them be as resourceful and magnetic as Rooney, who plays clown Jack Thum, and Anne Jackson, as his wife. The scenes in which they try to cheer each other up are pretty radiant. Mr. and Mrs. Thum found the time in their 34 years of marriage to provide a home for 37 deprived or abandoned kids, in addition to their own daughter, and Thum performed free for years in children's wards of hospitals. Before Thum died of cancer in 1980, Mayor Jane Byrne proclaimed a Jack Thum Day in his honor.

Unfortunately, the film squanders the impact of Thum's discovery of his illness by starting out in a mopey, melancholy tone and staying there. Thum may have gotten great satisfaction from performing his clown act at birthday parties for 50 bucks a pop (a good week: $300, he says), but the lousy script goes for the pathos and neglects the joy.

When hospitalized, Thum caves in to depression more readily than one would expect, particularly of a man justifiably regarded by those around him as a merry hero. The film even has him mulling over suicide on the roof of the hospital while trying to entertain an elderly patient played by Red Buttons.

William Windom, of all people, momentarily sparkles up the story in the role of Smiley, an old friend of Thum's, who's still a pro clown with the circus. Immediately the picture lapses back into sulks, to be only fitfully relieved with ebullience. If Thum's story isn't the story of courage and resilience in the face of death, what is it? The author of the screenplay, 28-year-old film-school graduate Cynthia Mandelberg, isn't ready for the big time. She may not be ready for the same time, either.

The film represents a working reunion for Rooney and director Cooper, who last shared a soundstage in 1936, when they costarred with Freddie Bartholomew in "The Devil is a Sissy." What happened on "Laughing" is that Cooper let a great trouper down, but it must be admitted that the film does deliver a heart-grabbing clincher before the final fade-out. Rooney can take the bows; he's turned into a magnificent old gnome.