"Coming Home" was Jane Fonda's baby. Involved in the movie from its earliest stages, she remained its major sustaining collaborator over a four-year gestation period. Given this devotion, it's a shame she has inflicted the weakest performance of her career on the finished film, which opens today at the new Tenley Circle 1.

Fonda stars as a Marine officer's wife who falls in love with a paraplegic veteran while doing volunteer work at a Veterans Administration hospital after her husband is sent to Vietnam.

The film's greatest asset is Jon Voight, an overwhelming sexual presence in the role of the paralyzed lover. Originally cast as the husband, Voight was allowed to switch roles shortly before production began. The change has renewed his career and enhanced the film's commercial chances but it has reduced the romantic triangle to no contest.

If Voight's performance is the strong point, Fonda's is a sore point. She seems to have substituted a role model for a role. Instead of bringing to life a specific woman with distinctive traits, Fonda stikes poses for an ideological Before and After poster.

The motto on the poster is familiar. - "Make Love, Not War" - but now the admonition carriers as recommencation to make love with war-scarred veterans, since they make the best lovers. Fonda's character, the prim, frigid, innocuous military wife Sally Hyde, never aroused by her gung-ho spouse, blossoms in the embrace of Voight's character, the ex-sergeant Luke Martin, whose legs were left paralyzed when a piece of shrapnel severed his spine.

This romance suggests an inversion of the relationships in "Lady Chatterley's Lover." Voight underscores the joke by projecting more potency than any actor in quite some time. Although confined to a wheelchair, he stands in for Mellors, the sexy gamekeeper. Poor Bruce Dern has been enlisted to deteriorate before our eyes yet another time in the role of Capt. Bob Hyde, and emotionally disabled stand-in for Lord Chartterley.

The movie is rescued form potential disaster by Voight's glowing, robust masculinity and by an unusually clinical, prolinged sex scene between Voight and Fonda, pretty big names to be discovered caressing in the nude and therefore more titillating.

In retrospect, the only aspect of the story development that leaves a vivid impression is the erotic life of the paraplegic hero, especially the Big Cliche when Fonda shudders in ecstasy and announces the arrival of Mrs. Hyde's first orgasm.

Being frank about sex dates "Coming Home" as surely as being discreet dated "The Best Years of Our Lives" and "The Men." Eventually, the new film may seem more dated because of its dependence on sex. "Coming Home" lacks some of the crucial attributes of its famous prototypes: a sense of the present tense, a sound dramatic structure and unquestionably generous motives.

Hollywood is having an awkward time getting an adequate retrospective fix on the war in Vietnam. Judging from the hysterical flailing about in "The Boys in Company C" and the sentimental deviousness of "Coming Home," fictional films are more likely to obscure the subject by fitting it to predictable melodramatic formulas or ideological presumptions.

From the beginning, director Hal Ashby seems to be trying to improvise "Coming Home" into existence. Some of the intuitions and sentiments shared by Ashby and the cast result in affecting interludes, but on the whole the material is too diffuse and complacently wistful to accomplish its ultimate goal of getting you there, breaking your heart, scaling the summit of old Mt. Pathos.

"Coming Home" is set against a hazy backdrop of war in Vietnam. The story begins during the Tet Offensive and ends perhaps a year or so later. The historical landmarks get vaguer as the movie drifts along. Ocassional period, but almost none concerns the war itself.

We see the Smothers Brothers clowning and Bobby Kennedy commenting on the assassination of Dr. King. We don't see the ongoing coverage of the war and protests asgainst the war which gave the period its sense of urgency.

Ashby recalls 1968-69 through a gauzy, distracting audio-visual filter depending on vintage rock hits to carry the period.

Unfortunately, the music is superimposed on the soundtrack indiscriminately. One is jarred by ludicrous juxtapositions ("Hey Jude" as Capt. Hyde strains above his sexually unresponsive wife), distanced by numbers that drown out dialogue or muddy transitions.

This distancing suggests more than a director's stylistic quirk. One detects a conscious effort to divorce the war from a setting of political conflict and controverst, perhaps because so many people involved in the project, from Fonda down, were antiwar activists who now prefer to sound diplomatic.

As a consequence, the movie becomes diplomatic to the point of evasion. It's as if history had been softpedalled in the hopes of soft-soaping a mass audience.

People who considered Fonda a culture heroine for her antiwar stance would have a right to feel sold out by the absence of a political dimension in this film.

People who opposed her would be equally justified in suspecting they were being sold a syrupy bill of goods.

In fact, Fonda's performance merits more resentment, especially from military wives and families, than anything she did during the war. Her antiwar agitation had some self-righteous integrity. Her nation of an Ordinary Sheltered Middle American Millitary Wife Whose Eyes Are Being Opened to Love and True Values smacks of unwitting but disgraceful condescension.

"Coming Home" might have been sentimentally respectable if the triangle situation had been thought out in terms of cumulative, conciliatory pathos. All three principals ought to make powerful claims on our feelings. Now only Voight does.

It would make a world of difference if one could believe that Mrs. Hyde loved her husband and felt truly torn between the two men in her life.

From the outset Capt. Hyde is depicted as an unworthy love object for the heroine. Before departing for the war, he is stigmatized as a warmonger and male chauvinist ignoramus. Upon returning, he is dismissed as a cowardly fraud and suicidal nut.

The filmmakers may be unconscious of their prejudices, but they take a demonstrably, systematically contemptous view of Capt. Hyde and the professional military class they presume he symbolizes. They don't account for his suicidal despair, although Dern expresses it sincerely. They sentence Hyde to self-destruction.

The only discernible change in the heroine over the course of the story is her hairdo. Fonda may imagine that she's holding up a wonderful role model, but she has undermined herself as an actress.

The worst single sequence is a protracted take in which Fonda holds out supposedly comforting arms to Dern as he treatens to go bananas. She looks every bit as human as a department-store dummy, but the gesture is foredoomed by her earlier failurer to express a glimmer of affection for Dern's character.

Mrs. Hyde turns out to be saving it for Mr. Right. But Loves does about as much for her as it did for Ali McGraw's dear Jenny in "Love Story." Fonda's Sally begins as a prig we're supposed to pity and ends as a prig we're supposed to emulate.

If Fonda were making calculations primarily as an artist, she might have been shocked by the emptiness of the character she's impersonating. What Fonda appears to be doing is running for First Lady. In that race she is way behind, and long may she trail.