THE SEDUCTION OF JOE TYNAN -- AMC Academy, AMC Skyline, Americana, K-B Studio, Landover, Roth's Manor, Roth's Tysons Corner, West End Circle, White Flint. AMERICATHON -- Beacon Mall, Beltway, Roth's Seven Locks, State and Wheaton Plaza.

The senator's wife in "The Seduction of Joe Tynan" has a stiff hairdo with simulated bouncy flip-curled ends, and a stiff upper lip, which she also keeps upwardly curved.

On a night that she is conned into making a speech about "how proud I am to be Mrs. Joe Tynan" -- and then accidentally discovers in public that 1) the senator is having an affair with a stunning labor lawyer, 2) he forgot to inform her that he is now running for president, and 3) his staff doesn't trust her to speak to the press, even about her own life -- her only immediate protest is to slip quietly away, leaving him physically stranded, but with his image of domestic bliss intact. She will do her screaming at home, to avoid political ramifications.

What makes this scene touching as well as devastating is that the stiff form of a perfect political wife has been fitted onto a warm, sexy, resourceful and inventive woman. Barbara Harris' superb and rounded characterization is only one of many deliciously funny-sad and authentic touches in this amazingly unstereotyped political satire.

What political satire has come to elsewhere can be seen in "Americathon," a gross comedy that depends for jokes on President Carter's teeth, Governor Brown's California psychoculture and other nationally recognized targets that anyone can hit blindfolded. Mostly, that film just whacks crudely away, although now and then it hits its mark with an impressive smack.

"Joe Tynan," directed by Jerry Schatzberg, is a rare form of subtle political satire -- political not only in the sense of being about Washington, but in showing our society its hero material. The film first holds up its targets, displaying their worth, making us recognize them as something we consider valuable. After that, its feat of hitting them accurately is extremely impressive.

The senator, his wife, his colleagues, his extramarital love, his staff -- all are reasonably good, intelligent, well-meaning and sophisticated people, managing impressively in enviable but demanding situations. They represent success. This is not the old story of lusting for success, but the more complicated one of managing it.

We see endless caricatures of the hypocritical, dishonest, dumb political animal; Senator Tynan, as played by Alan Alda -- who also wrote the script -- is both charismatic and effective. One can understand how he got the job, and how he has risen in it. He seems to be a pretty good senator, too, provided you accept the idea that a senator's first duty is not to weigh good against evil, but to weigh such questions as whether it's wiser to placate a powerful member of the club or to go against him for the sake of the publicity.

What makes him the quintessential senator is, most of all, his desire to keep happy as many people as possible, and his skill at juggling many factors for the maximum result. The most fascinating thing about his glory-ridden campaign to defeat a southern Supreme Court nominee is his casual admission, in the privacy of his office, that the man is, in spite of usable and damaging evidence, not really a racist. The immorality of his dual love for wife and mistress is precisely that he does love them both, and is not truly torn so long as he can keep giving each enough to keep her support.

We also know too well the standard caricature of the crusty old southern Senate leader, but Melvyn Douglas' version suggests an interesting and cultivated person gone slightly mad with that combination of age and rank called seniority. When he commits political suicide on national television by an ancient habit of lapsing into bad but gentlemanly French, the downfall is more shocking than any lurid scandal could be.

Meryl Streep, who is singlehandedly keeping the old movie-star word "luminous" appropriate, plays a political groupie with a difference -- that of reaching out from her own power base to an equal, rather than up toward a superior.

It is only in occasional excesses that the film falters. The drunken party, the lecherous senator played by Rip Torn -- no doubt these may be easily authenticated, but showing them has none of the effect of the low-keyed satire:

The look on a boy's face as he watches his father, on a TV talk show, make cute material out of his losing a class election.

The form letter being signed by a signature machine: "I've been giving a lot of thought lately to the state of our nation . . . "

The realization, by one administrative assistant, of the true meaning and political consequences of the superficial reassurance offered by another administrative assistant.

The first mutual groping to remove each other's clothes by a pair of high-ranking lovers each wearing a three-piece dark suit and prim white shirt.

The outstretched hand of a politician on the telephone waiting for his aide to give him the name of the wife of his caller, so he can end the conversation with "And how's Florence?"

The cheers and flag-waving of national convention delegates as yet another generation of politicians roars, "We have a new vision in this land of ours!"

The mutual deal, in the exchange of a glance between a husband and wife who agree to stay together, largely because they love each other, but just a bit, perhaps, because he knows how a divorce might hurt him, and she knows that it's unreasonable to compete with the roarings of a crowd.