SENTIMENTALITY goes with the winter holidays, but the sentimentalist role that heart-warming TV programs once played has been largely usurped by television commercias, which spread out their appeals to emotional vulnerability all year long. Sometimes they are so fantastically effective at this that TV dramatists can scarely hope to compete.

Each week television flashes thousands of images of the American home, and many of these images are the idealized but still empathetic vignettes in commericals for insurance companies, soft drinks, fast-food spas, and, that champ of the seductive soft-sell, Ma Bell and her long-distance machine.

Certainly there must be almost universal snaps of recognition, however reluctant, when viewers confront such familiar spectacles as papas teaching their little boys how to ride a bicycle without training wheels, or tiny tots being licked and tickled by squirming pupies, or scenes of grandma and grandpa welcoming broods to their comfy-cozy old house for a ceremonial dinner. You can be as cynical as Caligula and still find resistance failing when faced with sights like that. Television is no art form but TV commericals might be - insidious art, perhaps, but often awesomely efficient.

The kind of unabashed sentimental appeal that works in commercials, though, no longer really functions in entertainment intself. Jack Paar used to weep over everything from obituaries of the stars to Jose Melis solos; Johnny Carson apologizes hurriedly for even the tiniest display of emotion. It would be too facile to trace the decline and fall of wholesome or even maudlin sentimentality to one social phenomenon - or one Far Eastern war, or one naughty presidential administration - or another, but the fact is the only place you can really get away with it today is in commercials, because there people know just what you are up to, that the sentimentality has a justifying practical function.

Dramatists have had to re-invent pop sentimentality and as part of that adjustment have given us a new genre of holiday greeting card - the bittersweet nostalgia of what might be called the upper-downer. This takes the form of a TV movie or play or series episode about someone who is dying and, in facing this fact, rediscovers the meaning of life. So you get a downer situation redeemed with an upper ending. The most striking example this season may be the ABC special that airs tonight, "The Gathering," at 9 o'clock on Channel 7.

Edward Asner stars as Adam thronton, who lears in the film's opening scene, while snow falls on a rushing river, that he has, his doctor says, only "30, 60, maybe 90 days" to live. Instead of breaking down in the manner of the old style tearjerk artist, he says "Too bad" and resolves to tie up the extremely loose ends of his upwardly mobile middle-class life. He summons his four estranged children and one estranged wife to one last Christmas convocation; only his wife, played by Maureen Stapleton, knows about doc's bad news.

For a holiday special, this may seem unusually downbeat material - even in a sophisticated age of cooled -off heartwarmers - but the gloominess is resolved through Thornton's ability to redeem himself in the eyes of those he had alienated. Writer James Poe doesn't go so far as to say Thornton's predicament is a symbolic middle-class materialist dilemma, or even a sign of the times, but this is the kind of drama that allows one to read in whatever personal signaficance one wants.

As directed by Randal Kleiser and filmed last winter in and around Chagrin Falls and Hudson, Ohio, "The Gathering" may be all the more moving for its restraint, and for Asner's as well. This was still the fat Asner, the pre-"Lou Grant" Asner, and he makes a very touching figure - so much so that he wisely refrains from overt melodramatic flamboyance. His most demonstrative letting-go occurs, justifiably, when he welcomes home the son he exiled years ealier because the kid dodged the draft of the 1960s.

Then, he had said, "Get the hell out of this house." Now, in the eleventh hour, he says, "You were right and I was wrong."

There are lots of scenes and moments and even remarks in this special that will undoubtedly hit responsive chords in viewers, even beyond the crucial classic theme of absolution through confession. "Gathering" may represent the best kind of television drama, since the medium itself is by definition domestic and has the inside track over all other forms when it comes to reaching us intimately - where we live in both the literal and figurative senses. A dramatist can be cheap and glib in exploiting this capability or, as Poe has done in this case, he can do his very best to maintain a sense of leavened veracity.

We don't want the whole truth on every subject, after all, at least not from television. At the moment, a number of TV commercials are providing us with the wish-fulfillment and sentimental escapism that movies and later early TV shows used to provide - shows like "I Remember Mama" and its annual Christmas pageant, "The Night the Animals Talked," which never failed to induce an epidemic of sniffles at my house in the '50s.

There is a special, if seasonal pleasure in giving oneself over to gushing emotionalism beyond all reason, and holiday entertainments used to give us ample opportunity. Those opportunities have grown less ample and the entertainments less naive, which is not to say better or more cherished.

In TV drama, we have a new, qualified, even slightly mordant sentimentality that tells us, subtly, how things have changed and how certain aspects of what might be called innocence are lost. The new sentimentality seems less organic than the old, and more corrupted by reality, but the emotional rewards can still be substantial. Shows like "The Gathering" may not be unflinching realism incarnate, but they are benignly distorted reflections of how life is; those who have learned how to read television can see through the filters and behold a kind of truth.