Remarkably appealing but perhaps a shade too wistful for its own good, "Melvin and Howard," opening today at area theaters, confirms the affinity for authentic Middle American social comedy demonstrated by the young director Jonathan Demme.

"Melvin and Howard" is in touch with a rich, wild vein of comedy, but it feels rather tentative and ends abruptly, breezing by the most eventful and fascinating aspect of the factual story that inspired the movie -- the belated public consequences of a chance encounter between eccentric American originals at opposite ends of the social and temperamental spectrum. These accidental, sublimely satisfying soulmates are, of course, the improvident but lovable loser Melvin Dummar, played by Paul Le Mat, and the legendary reclusive billionaire Howard Hughes, played by Jason Robards.

The movie opens with a sustained fictional elaboration of the meeting recalled by Dummar, the only available authority for what may or may not have taken place. Of course, the pretext itself may be rejected by people who insist on believing that Dummar is at once a stupe, a liar and one of the great amateur forgers in the history of handwriting. The controversial

"Mormon Will," which named Dummar as one of Hughes' beneficiaries, was judged a forgery by a jury in 1978, but that vercict is likely to be remembered as a classic, socially lamentable dumb call.

In their hearts the filmmakers know that the so-called Mormon Will was genuine and that Dummar was telling the truth about giving a lift to a stranded, injured, scroungy, hostile old coot who must have been Hughes. Dummar never filled out his sketchy recollection, so Goldman has tried to imagine what might have imprinted the encounter on Hughes' mind and persuaded him to become the silent benefactor of a humble obsurity full of high hopes and prone to repeated failure.

The answer: To know Melvin is to like him. The movie suggests that Hughes was tickled by Melvin in spite of himself, that in a peculiar way he perceived something of his own drives in this opimistic young goof and decided to return the favor in the fullness of time.

Dummar discovers Hughes, injured riding his motorcycle out in the desert, when he pulls off the highway to answer a call of nature. He helps the old man into his truck and offers to take him to a nearby hospital, but his passenger insists on going to Las Vegal.

Naturally friendly, Dummar tries to make conversation, is initially rebuffed but gradually breaks through the grim-faced, suspicious-eyed defenses of Hughes, in part by threatening to expel him from the cab if he refuses to join in singing "Santa's Souped-Up Sleigh," one of many unpublished compositions. By the time Melvin drops his mysterious passenger in Las Vegas and consents with a sigh to lending him a quarter, a magical camaraderie has been achieved by Robards and Le Mat.

The story jumps from Tonopah, Nev., to Los Angeles to Willard, Utah, following Melvin's tireless but boobytrapped efforts to get ahead. Whenever luck seems to be running his way, he does something foolish and plunges straight back into debt.

Comic intimacy has become Demme's forte. Observing Melvin's conflicts with his first wife, Lynda (Mary Steenburgen, finally blessed with a role that puts her fluttery, squirrelly characteristics to appropriate and likable use), Demme sustains a sympathetic yet penetrating viewpoint, close enough to appreciate their fundamental decency and affection yet detached enough to perceive the incorrigible behavior patterns that drive them apart. They're deeply attached and utterly incompatible, and the contradiction seems to make perfect sense.

We see Melvin established in Willard with his second wife, Bonnie (Pamela Reed, a dynamic young actress who made an impressive debut last fall as Belle Starr in "The Long Riders"), a sensible, straighforward divorcee who recognizes an energetic family man when she sees one and also a potential screw-up who needs disciplined looking-after. Having come a long way with characters you've grown to like, foibles and all, the movie really needs to take on the climactic challenge of the Dummars' troubles after the will is delivered and made public.

Unfortunately, Demme and Goldman treat this incredible circus, which promises comic turmoil in the dizziest Preston Sturges tradition, with scant attention. Just when you hope the movie might kick into a faster humorous gear, it begins sputtering out. Sweet as it is, the story feels incomplete without a fuller account of the events tht transpired after Howard Hughes had made Melvin Dummar a household name.