It may be just a frustrated minority letting off steam. Or it could become one of the major popular causes of the decade. No one is yet sure. What is clear is that a long-forgotten popular movement - comprised of Americans calling for nuclear disarmament - is again snowballing across the United States.
After two decades of dormancy (the last major thrust for disarmament came with the Cuban missile crisis in 1962), the present campaign is rolling with a speed that surprises even its leaders.
It may not reach the fervor of American marches during the Vietnam years. Nor is it apt to match the pitch of activity generated recently by nuclear disarmament movements in Western Europe, where 250,000 people marched on Bonn Oct. 10, followed by similar large-scale marches on Oct. 24 in London and Rome and on Oct. 25 in Brussels, Paris, and Oslo.
Some observers, in fact, are skeptical that the movement will gain much strength at all. Tufts University Prof. Scott Thompson, a former White House fellow assigned to the Pentagon and an advocate of increased defense spending, notes that ''the far more powerful evidence is on the other side'' with what he describes as ''almost a steady upward graph of support for higher weapons spending'' since the mid-1970s. And Harold Brown, defense secretary under President Carter, says that these groups are ''deceiving themselves very badly'' if they think they can influence the Soviet Union toward disarmament.
But leaders of many American disarmament groups contacted point to numerous signs of the movement's spurt of growth. Among them:
* The Council for a Nuclear Weapons Freeze is circulating a petition calling for bilateral negotiations between Washington and Moscow and a freeze on the production and deployment of nuclear weapons. After starting up last March, the council now operates in 48 states. So far it has received endorsements from 18 congressmen and several national church organizations.
''It's becoming mainline,'' says council spokesman J. Malcolm Forbes, adding that the movement is not appealing simply to the ''fringe-radical-peacenik groups.'' And Sen. Paul Tsongas (D) of Massachusetts, although he has not officially endorsed the petition movement, says that at this time ''it is the only way that anyone can make a statement.''
* The Union of Concerned Scientists, a 12-year-old group that looks at the impact of advanced technology on society, now numbers 120,000 members and has an annual budget of $1.5 million. It is sponsoring a teach-in Nov. 11 on 140 campuses across the country. ''We are an educational operation,'' says spokesman Paul Walker, adding that the group's impetus comes from the recognition that ''clearly arms control has failed over the last 30 years.''
* Ground Zero, a Washington-based coalition headed by former National Security Council member Roger Molander, sees itself less as a proponent of nuclear disarmament than as what he calls ''a project in responsive education'' on the problem of nuclear war. The group has contracted with New York publisher Simon & Schuster to do a mass-market paperback tentatively titled ''Nuclear War: What's In It For You?'' written in what Mr. Molander calls ''the Dr. Strangelove mode, with a touch of wry humor.''
Ground Zero is also planning a week of ''intensive activities'' throughout the country April 18-25, hoping to hold educational meetings in perhaps as many as 100 cities. So far, says Molander, they have received endorsements from such groups as the National Council of Churches, the American Association of University Women, and the United Auto Workers.
* American Friends Service Committee spokesman Terry Provance is laying plans for what he calls ''a major mobilization'' next June in New York, when he hopes to bring out 100,000 people to support the start of the United Nations Special Session on Disarmament. His group has just finished coordinating 150 activities nationwide during United Nation's Disarmament Week (Oct. 24-30).
Despite a widely reported shift in American attitudes away from liberal causes and toward a new conservatism, groups in favor of disarmament are proliferating. Physicians, nurses, high-technology professionals, businessmen, lawyers, and family-oriented disarmament organizations have all sprung up. Boston, a hotbed for such groups, even has a new organization of public-relations professionals called Communicators for Nuclear Disarmament, who offer their services to other similarly minded organizations needing such things as brochures, media campaigns, and speech-writing.
These groups generally report significant increases in membership and funding support in the past year. Roger Courtright, executive director of SANE, reports that his organization's membership has increased by 70 percent in the past year. Greenpeace, Physicians for Social Responsibility, and the Union for Concerned Scientists all report substantial growth.
Many draw their members from former student peace marchers of the Vietnam era. Now, says David Stephenson of the Boston Communicators group, these people are ''carrying briefcases'' - and realize that they have professional services to offer.
Why the upsurge at this time?
Most disarmament leaders point to President Reagan. They say his administration has encouraged the arms race by focusing attention on the so-called ''window of vulnerability'' that separates Soviet and US nuclear defense capability and by insisting on heavy military spending.
Proponents of disarmament worry, too, that the administration is increasingly talking as though a limited nuclear war would not only be possible but winnable.
They find particularly troubling the President's comment to newspaper editors Oct. 16 that ''you could have an exchange of tactical weapons against troops in the field without it bringing either one of the major powers to pushing the button'' - a comment later downplayed by the White House after outcries from Europe.
The growth in membership in the disarmament groups, in fact, coincided with last year's election. ''It was hard to criticize the Carter administration,'' says Jeffrey Porro of the Arms Control Association, ''since many of the arms control community were in power.''
Now, however, Walker notes that many of those in power are from the Committee on the Present Danger, a five-year-old organization that has consistently warned that the United States must increase defense spending and modernize its weapons systems to meet the growing Soviet military strength. One of its alumni, widely-respected ''hawk'' Paul Nitze, was appointed last month as chief negotiator with the Soviets over Europe-based missiles.
But while Reagan's election may have prompted immediate support for the disarmament movement, many see underlying causes that are far more complex. Other reasons for joining the movement include:
* Uncertainty. A Gallup poll published in the Oct. 5 issue of Newsweek found that only 30 percent of those polled felt there was a good chance of an all-out nuclear war with the Soviets within the next decade. But 47 percent replied that they tried not to think about the unpleasant question at all. Observers feel that part of the growth in membership comes from that half of the population who suddenly want to know more.
* The economy. Most observers feel that the rapid growth of nuclear weaponry would taper off if the economy were righted. Many of those joining apparently sense a contradiction between increases in military funding and declines in social service spending.
Many see the European demonstrations as motivated by a coalition of concerns far broader than disarmament: unhappiness with the governments in power in Western Europe, dissatisfaction with the economy, concern over nuclear generating plants, and problems with housing and ''squatters'' in many European cities.
* Environmental concerns. Here and abroad, the disarmament movement is often allied to the conservation movements. A New York Times-CBS poll published Oct. 4 found that 67 percent of those polled wanted to ''maintain present environmental laws in order to preserve the environment for future generations.'' Growing doubt that Interior Secretary James G. Watt will honor their wish - and obvious concerns about the effect of a nuclear war on world ecosystems - is apparently bringing these people into the disarmament cause.
So far, most observers see more differences than similarities between the European and American disarmament movements. That is partly a matter of geography: Europe, not America, would be the battlefield for theater nuclear forces.
But the nature of the movements may also be different. Former Secretary Brown says that ''In Europe, I see a lot of groups getting together about the one thing around which they can all rally: their feeling of helplessness in the face of two superpowers.''
Many also note that disarmament is simply not as emotional an issue as the death of Americans in Southeast Asia - except in Europe, where the battlefield seems very close.