In "The Seduction of Joe Tynan," the only politician who exhibits any redeeming qualities is a senator with approximately 10 seconds of screen time. He uses those 10 seconds to proclaim his utter disgust with Washington and his decision to return to private life at the learliest opportunity.

Most previous novels, plays and movies about American politics have been written by buffs -- men like Alan Drury and Gore Vidal and Patrick Anderson whose cynicism was tempered by a fondness for political lore and an itch to believe in the possibility of honor in government. Alan Alda, the star of TV's "M*A*S*H" and the star/screenwriter of "Joe Tynan," has peopled his government with a shallow collection of drunks, make-out artists, mental cases, hatchet-persons and megalomaniacs.

And one of the shabbiest public servants of them all, alas, is the senator played by Alda himself.

"Joe Tynan" takes a substantial theme -- the conflict of ambition and family -- and treats it gracefully and entertainingly, with the help of two dynamite performances by Meryl Streep and Barbara Harris. So there are plenty of good grounds to see this movie. But the question of the hero's seduction or nonseduction is never a matter of enough moment to hang a plot on, a fact that renders the whole considerably less compeling than its parts.

Alda has said, legitimately, that "Joe Tyanan's" focus is its hero's private life, not politics and not Washington. This being Washington, however, it should hardly surprise anyone if the political side of the movie, however incidental, comes in for unusually close scrutiny here.

Sensitive members of Congress may feel they have been defamed. The rest of us can take the calmer view that Alda, despite a generally high level of technical accuracy, has painted the city with a needlessly broad brush.

But the problem with "Joe Tynan" is not the stark context into which its hero is thrown. It is the hero himself, as Alda has written and acted him.

This telegenic, well-tailored senator from New York doesn't have an idea of his own in the movie. When a curmudgeonly colleague, played by Melvyn Douglas, asks Joe not to fight a reactionary Supreme Court appointment, he promises not to. When his eager administrative assistant asks him to break that promise, he breaks it.

Taking a phone call from a man whose name means nothing to him, Joe comes on the line with a hearty "Henry! Henry, how are you?" And when an aide hands him an index card, he adds briskly, "How's Florence?"

Joe Tynan, in short, is such a spineless fellow from the start that it looks like he was seduced long before the movie got underway. Either of the two women in his life would make a more absorbing protagonist by far.

As the hot-blooded daughter of an old southern pol, Streep takes Alda and the audience on one of the most dizzying power trips in motion picture history. From the moment she first enters the action, to offer her professional help in the Supreme Court appointment battle, she runs the show as few actresses have since the generation of stars who passed their prime in the 1940s.

Streep is an actress who can give new, understated life to an old line like "When I want something, I go get it," adding, softly, "just like you." She can make a simple "no" sound anything but simple. And in bed with the senator (a foregone conclusion if ever there was one), she can rock with vicarious delight as she tells him. "Oh my God, you're going to be fantastic in this hearing! Do you know how far you're going to go with this? When you get to the Ross Garden, grab me a rose, will you?"

Streep could hardly be a stronger contrast to Barbara Harris as the wife back home (in New York) -- a high-strung, intelligent woman oppressed by the burden of raising two children and establishing herself as a therapist. When Alda critizes her for mentioning her own history of psychological treatment to a magazine interviewer, Harris tells him off -- impressively. But later, dragged along as an ornament at a fund-raising dinner, her fragility is frightening, and evokes images of many real-life political wives on public exhibition.

The confrontation between troubled teen-ager and distant parent has become an old movie standard by now. The child storming off to her/his room, slamming the door and refusing to let the parent in or, on relenting, refusing to discuss the problem -- all this has been done scores, maybe hundreds of times. But it has rarely been written as well as Alda has written it, or acted as well, on the child's end, as Blanche Baker acts it here.

The only problem with this scene and others is Alda's brittle, sardonic style of acting. The same style that works so well on "M*A*S*H." -- in which he virtually narrates his own adventures -- has a hollow ring to it in this longer, more earnest form. Alda does a fine job of playing the sleazy insincerity of his character but woefully poor job of convincing us that this isn't the whole man.