Dr. Helen Caldicott, the slender, spirited Australian-born doctor who heads Physicians for Social Responsibility, wants House Speaker Thomas P. (Tip) O'Neill Jr. to show a a film called "The Final Epidemic" to a joint session of Congress.

It runs for 29 harrowing minutes. Doctors and scientists explain in wipe-out detail what will happen in the event of a nuclear attack. The longest segment shows Dr. H. Jack Geiger, director of community health at City College of New York, telling, in matter-of-fact tones, what San Francisco could expect if a one-megaton nuclear bomb were dropped on "a clear, dry day, say a Monday, at 3 p.m."

In the hell that would follow--radiation, firestorm, 500 mile-an-hour winds, collapsing buildings--780,000 people would die instantly, and 382,000 would be seriously injured.

"Among survivors," he points out, will be tens of thousands with third-degree burns, a number that would "exceed by a factor of 10 or 20 the capacity of all the burn care centers in the United States."

To bring such grisly, although irrefutably relevant data, to the attention of our legislators might be difficult for Speaker O'Neill to arrange. Traditionalists would object to "movies" on the floor, and hawks would be outraged at official sanction of negative thinking on the Reagan doctrine of "peace through strength."

But there is nothing to prevent O'Neill and his fellow Democrats, who flinch when labeled members of "the war party," from taking up the end of the world as a political issue. Democrats are intimidated by the popularity of Reagan's arms buildup. But they might take courage from the ultra-conservative American Medical Association, which recently passed a resolution saying starkly that it is "incumbent on the AMA to inform the president and the Congress of the medical consequences of nuclear war and that no medical response is possible."

On Dec. 14, a group of scientists and physicians, which included two Nobel laureates and was led by the papal delegate, carrying the pope's warnings on nuclear war, called on President Reagan to tell him face-to-face about the horrors ahead.

Dr. Howard Hiatt, dean of the public health school at Harvard, appealed to the president in compelling personal terms. He recalled for the president the medical marvels, the trained teams, the drugs, plasma and skills mobilized at George Washington University Hospital to save Reagan's life last March 30.

"In the event of a nuclear attack on Washington," the doctor gently told the president, "there would be no George Washington University Hospital."

Reagan "stepped back" under the force of Hiatt's message, according to one witness. During the 15-minute session in the Oval Office, the delegation was not invited to sit down. That tells you something about the president's hospitality to those bearing anti-nuclear arguments.

What tells you more is that he presides over the greatest nuclear weapons buildup in history and has requested an additional $100 million for civil defense in his 1983 budget.

The precautions against annihilation are to be taken by the Federal Emergency Management Agency and center on a Crisis Relocation Plan (CRP). The evacuation of people from "risk" areas to "host" areas--they would bring their own lunches--depends on something that cannot be guaranteed, a warning of three to eight days. Such "survivability" thinking strikes scientists and physicians as madness on the order of the nuclear arms race itself.

U.S. politicians, like many Americans, are subject to what Caldicott calls "psychic numbing" on the subject. In a powerful series on "The Fate of the Earth" in The New Yorker magazine, Jonathan Schell calls it "a denial of the most important reality of our time."

But there are signs that, without prompting by their leaders, people are rebelling against such a fate. Three state legislatures have called for a nuclear moratorium. Californians have collected enough signatures to put the issue of a bilateral arms freeze on the November ballot. Half of the town meetings in Vermont will vote on the question one week from next Tuesday.

It is a cosmic issue looking for a leader. But the Democratic presidential candidates have yet to see it. Fritz Mondale has not been heard from. Gary Hart spoke at a nuclear symposium but frets that a mass movement might be taken over by the left wing. John Glenn wants us to truck nuclear weapons across our highways. Edward Kennedy concentrates on survival under the Reagan budget.

As it happens, nuclear dread has been tested in the field. In 1980, Sen. Patrick J. Leahy (D-Vt.) was running 10 percentage points behind his conservative Republican opponent. In the last week of his campaign, he opened up on the criminal lunacy of a nuclear arms race and won by one percentage point.

Caldicott may not succeed in making members of Congress watch "The Final Epidemic." Their constituents may force them to, however, and could even make them understand that nuclear war is suicide for the human race.