Paul Mazursky is the most observant and benevolent correspondent covering the middle-class martial front. Nine years ago in "Bob & Carol Ted & Alice" he perceived how conventionally satisfying marriages and friendships could be jeopardized by reckless experimentation with "liberated" mores, i.e., self-congratulatory adultry.

Five years ago in "BLume in Love" Mazursky created an extraordinary personal, affirmative romance from the premises of an adulterous husband struggling to regain the love of his wife. Stephen Blume seemed a representative comic protagonist as well as an inspired addition to the melancholy, foolish suitors of classic dramatic literature. Like many people of his time and class, Blume was an emotional sleepwalker who never realized what he had valued until he had lost it.

Mazursky returns to the aftermath of a martial breakup in his witty new film, "An Unmarried Woman," opening today at four area theaters. Trying to identify predominantly with the injured wife's point of view, he succeeds admirably and enjoyable up to a point - roughly the beginning of the last reel, at which point both the film-maker and his protagonist may be detected sleepwalking their way toward an ambiguously hopeful, insufficiently summarized resolution.

Erica Benton, the heroine of "An Unmarried Woman," is stunned to dis cover that her husband Martin, a New York stock broker, wants out of their happy marriage of 17 years. The opening stages of Mazursky's screenplay create a sense of intimacy with the Bentons, played by Jill Clayburgh and Michael Murphy, and indicate htat there are troublesome signs of restlessness in Martin. Mazursky's feeling for this middle-class domestic milieu is so sure that one can apprehend the danger signals while appreciating the odds against a loving spouse reading them correctly.

The Bentons have crossed that imperceptible line where depending upon one another degenerates into taking one another for granted. Since she detects no threat to herself in Martin's discontent and preoccupation, Erica is completely shocked when he breaks down and announces he wants to marry a younger woman he's been involved with for a year.

The announcement is staged against ingeniously banal backgrounds. Following a lunch date, the Martins stroll along a sunny street. Erica is cheerfully talking about summer vacation plans when Martin pulls up, begins to blubber and lowers the boom on her emotional security.

Clayburgh, in the first of several brilliant moments of acting, the sum of which may establish her as the early front-runner for next year's Academy Award, stares at Murphy and develops an oddly crooked, frozen expression around the mouth. Her mouth still isn't working right as she walks away, accompanied by a some-what overexpressive saxophone cued by composer Bill Conti. Then she can't hold it any longer: The stricken expression is resolved when she heaves her lunch on a street corner.

The reminder of the film chronicles Erica's recovery, which is defined fundamentally as the recovery of a romantic interest in men. Initially so resenftul and hostile that she can't tolerate the idea of her teen-age daughter Patti (smartly embodied by sharp-featured young acress Lisa Lucas, who played the younger daughter of Shirley MacLaine in the "The Turning Point") petting with a boyfriend, Erica eventually contemplates a future which might be criticized for encouraging an almost complacent sense of well-being.

Martin wants to come back. Understandably, Erica rejects that possibility. However, since Mazursky has taken the liberty of introducing a far more attractive man into her life - Alan Bates as a shaggy, amiable abstract painter - he also leaves her perched a bit too advantageously between a cozy single state and a cozier sexual alliance than she had before.

The effect may have been unitentional, but the Bates character, Saul, tends to dominate the film after he makes his belated entrance. He's the equivalent of the Kris Kristofferson figure in "Alice Doesn't Live Here Anymore," prefigured by Kristofferson himself in "Blume in Love."

Unlike the Blume marriage, the Benton marriage is not conceived as a union that can be put back together. However, Saul is so prodigiously likeable and desirable that Erica seems oddly coy or perverse when she resists his offer to put things on a semi-permanent, but still unmarried, basis.

At the very least Mazursky has failed to give his heroine a summarizing speech that would clarify her reluctance to commit herself. Mazursky evidently believes he can resolve her state of mind symbolically, in pretty but rhetorically muddled closing sequence which finds Erica trying to navigate along the streets with one of Saul's huge canvasses. But even if she doesn't want to choose between being on her own and living with Saul, it would be more satisfying if she said as much plainly.

Mazursky supplies Erica with plenty of sounding boards in the early stages of her recuperation: her daughter, a group of cronies dominated by Kelly Bishop as a salty, wise-up divorcee (the immediate pace-setter for best supporting actress) and the seemingly preposterous but evidentally all-too-authentic Penelope Russianoff as a ponderous therapist.

Despite the turgid pressence of Dr. Russianoff, the heroine cane get things off her chest. In the later stages of the movie she gets less talkative, and Clayburgh's performance begins to resolve itself into a series of warming smiles and contented sighs. Her suitors - Bates and Cliff Gorman as a horny sculptor who helps revive the heroine with a splendidly staged, explosive sex scene, in which Clayburgh fashes from shy expectancy to hilarity to passion - express their admiration explicitly. Ditto for her contrite husband. You keep waiting for the heroine to sum it up from her angle, but she never quite does.

The movie seems to drift away in the last reel. By that time Mazursky has sustained so much human interest and built up so much good will that you don't feel cheated. Just mildly disappointed and puzzled. Mazursky's intimate style makes empathy so tangible that it comes as shock to consider that Erica might be a character with fewer resources than one supposed. She's keeping a great guy like Saul at career or a new set of ambitions or even an adequate means of self-support.

Deep down Paul Mazursky doesn't seem to believe in that state of single blessedness. He thinks in terms of couples and the desirability of couples. The theme of "Blume" was "Love is a miracle." It's no great surprise to see that "An Unmarried Woman is dedicated to a married woman: his wife.