"Thinking Twice About Nuclear War" opens with clay animation in which the United States and the Soviet Union are depicted as babies having a spat atop the Earth and reaching for nuclear bombs when the squabble escalates. Obviously what we are not going to get in this program is sophisticated analysis of international politics.

Some issues benefit from boiling-down, however--even from simplistics. "Thinking Twice," at 10 tonight on Channel 26 (most PBS stations showed it April 23), is not precisely pro-disarmament, but it is anti-Armageddon. Despite a slickly self-righteous tone (even the title seems smug and preachy), "Thinking Twice" makes a respectable and thoughtful primer on an ever-present unthinkability: suicide on a scale not previously imagined for this planet.

The narrator-host, alas, is Mike Farrell, the "M*A*S*H" actor who has identified himself with leftish causes in the past, and the fact that he does his narration from the seashore (downstream from his Malibu pad, perchance?) is an annoying little detail. The first segment of the film, in which a Richmond, Va., family agrees to raise its consciousness by watching a month of films about nuclear war, has about as much subtlety as a 1956 commercial for Duz.

"If there was an organization, I'd certainly like to support it," chirps the lady of the house after the month is over and her consciousness has been raised to the rafters. You half expect an announcer to leap in with, "Yes, Mrs. Whosits, there are dozens of organizations! You, too, can help save the world from nuclear annihilation!"

But the program improves after that. Suddenly we see Chet Huntley, of all people, demonstrating a dance craze of the '50s, covering one's eyes and dropping to the floor when a nuclear bomb goes off (this nutty bit of footage doesn't appear in the current theatrical documentary, "The Atomic Cafe"), and some anti-bomb demonstrations of the past are recounted. A man who worked on the neutron bomb says matter-of-factly in an interview, "You have to have a little sanity about it."

Unthinkable things inspire bizarre reactions. Poet Alia Johnson recites a morbid verse, which seems to equate nuclear holocaust with ultimate orgasm, and her husband, identified as "a nuclear physicist turned peace theorist," says, "There's something very exhilarating about war." He is part of a group called Humpty Dumpty that is "working to create scenarios for peace" on the grounds that the human race is bound to develop and maintain larger and larger weapons. They suggest that since conflict is inevitable, we should have an international Super Bowl rather than a nuclear war. They aren't kidding.

The program's credibility is hampered by such fallacious assertions, however incidental, as "Golden, Colo., is a typical American town." But students there who talk about the prospect of nuclear war do so with impressive concern. Though the Public Interest Video Network, which produced this program, claims it to be nonpartisan, some viewers may find it anti-Reagan (the Richmond woman says we can do something about nuclear proliferation at "the next election") and others may just find it naive (the camera zooms in on doctors from the United States and U.S.S.R. shaking hands at a peace confab).

But the simple life-or-death essence of the issue does transcend politics, and it certainly, and rather suddenly, has returned to the national arena. When even Ann Landers turns over an entire column to a call for nuclear sanity, as she did this week, we know that the topic is back in the mainstream. There's plenty to talk about, and brood about, in "Thinking Twice."