On Aug. 26, 1980, more than two months before Ronald Reagan won the presidency, a young woman in Brookline, Mass., wrote a memorandum that began: "A national effort to win public support for stopping the nuclear arms race is gaining momentum."

It seemed an unlikely analysis on the eve of a presidential election campaign that would end with the landslide victory of an unabashed hawk who was calling for new rounds in a race that had already produced nearly 50,000 nuclear warheads in Soviet and American arsenals.

The conventional wisdom then and since has been that the country was in a hawkish mood, anxious to build up American military capabilities, including thermonuclear forces. But Randall Forsberg, the author of that memorandum, was one of a small band of activists who rejected that conventional wisdom, and who now feel they are on the verge of refuting it.

For them, the big news of the 1980 election was not Ronald Reagan's electoral landslide, but the results of a non-binding ballot initiative in three state senate districts in western Massachusetts. The initiative instructed the three senators to introduce a resolution in the state senate asking the president of the United States to propose to the Soviet Union a freeze on the further testing, production and deployment of nuclear warheads and the rockets and bombers used to deliver them.

The small cities and towns in that part of Massachusetts have names borrowed from England--Cheshire, Windsor, Savoy, Richmond and New Marlboro among them. In many of those small towns Ronald Reagan beat President Carter handily, by as much as 2 to 1, while the freeze initiative won by equal margins. In Hancock, for example, Reagan won 175 to 97 and the freeze won 166 to 113. In the three districts together, Carter barely defeated Reagan but the freeze proposal won handily.

The referendum campaign in western Massachusetts was organized by Randy Kehler, a 36-year-old Harvard graduate who had served two years in federal prison for his refusal to cooperate with the Selective Service System during the Vietnam war. The hurt is still audible when Kehler describes the way the national news media ignored the triumph of the pro-freeze referendum in 1980.

"That vote gave me personally so much hope that all wasn't lost just because a pro-military president had just been elected," Kehler said in an interview last week. It proved to him, he added, that anxiety about nuclear weapons runs deep in America, and that a national campaign to try to freeze the arms race could actually work.

In the 17 months since, the campaign has brought together a coalition of veteran antiwar activists, church groups and ordinary Americans of all kinds to press local and state governments and officials in Washington to seek a freeze on new nuclear weapons. There are organized freeze efforts of varying degrees of effectiveness in 43 states and 279 House districts around the country. Pro-freeze resolutions have been passed by 309 New England town meetings, 33 city councils around the country, 10 county councils, seven state legislatures and one house in the legislatures of four other states.

In the past few weeks, the stubborn optimism of Forsberg, Kehler and other early crusaders for the nuclear freeze has begun to look like inspired foresight. The news media, Congress and the Reagan administration have all been jumping, in response to a new popular phenomenon that they did not anticipate and still find difficult to understand.

The sudden emergence of this public pressure was particularly unwelcome for the Reagan administration, whose first reaction was to attack it sharply. More recently, though, both President Reagan and Secretary of State Alexander M. Haig Jr. have sought to ally themselves with those concerned about nuclear weapons, and to look for ways to head off the national freeze movement.

The country has surprised its opinion leaders and political leaders, just the way Randy Forsberg and Randy Kehler said and hoped it would. In fact, this ambush has been more dramatic than even those optimists predicted. When she wrote that memorandum in August, 1980, Forsberg circulated a "national nuclear-weapon freeze strategy time line," written by hand on graph paper. It predicted that, by early 1983, 33 senators and 150 members of the House would introduce a resolution supporting a mutual Soviet-American nuclear freeze. In fact, such resolutions were introduced in the Senate and House last month, and already have 24 and 166 cosponsors, respectively.

The surge of popular sentiment to do something about nuclear weapons goes far beyond the freeze campaign. At the same time Forsberg and Kehler were sowing its first seeds, Dr. Roger Molander, a member of the National Security Council staff in the Ford and Carter administrations who was responsible for the Strategic Arms Limitation Talks, began to organize Ground Zero, a campaign to educate the public about the dangers of nuclear war.

Molander left the government at the beginning of 1981 and, with his twin brother Earl and others, has now organized Ground Zero Week, which begins April 18. It will feature educational activities in 150 major metropolitan areas and more than 500 smaller communities, and on 330 college campuses. More than 8,000 people are helping to organize these programs, most of which will include the planting of a 3-by-5-foot Ground Zero banner declaring: "If this were ground zero, a one megaton nuclear explosion would instantly destroy virtually everything within two miles of this spot."

The Molanders have raised $285,000 to finance Ground Zero so far, and local groups have raised thousands more. The Molanders have written a book called "Nuclear War: What's In It For You." Published by Pocket Books, this primer on nuclear weapons is in its third paperback printing a month after it came out; 225,000 copies are in circulation.

Physicians for Social Responsibility, a group composed primarily of doctors that has been raising the hair of Americans in many big cities by describing what a nuclear attack would be like, has been growing like crabgrass. Ten months ago PSR had 3,000 members; today it has 20,000, each paying $30 a year in dues, and its 1982 budget will be about a million dollars.

Now new groups of lawyers, educators, nurses and businessmen patterned on Physicians for Social Responsibility are being formed.

The strength of this phenomenon is probably easier to grasp not from any organization's growth, but in the story of a 34-year-old cattle rancher and electrician from Charlo, Mont., named John McNamer. He is a Vietnam veteran with a Bronze Star who is just starting out with an 80-acre "cow and calf operation" which he finances with construction work and his wife's salary as a first-grade teacher. McNamer has taken on the MX missile and the nuclear arms race. From his perspective he is winning.

It all started one night last August. "I was concerned with the nuclear situation in general, like a lot of people here," he said in an interview. But when he heard that the federal government might try to base MX missiles in Montana, that pushed him into action.

"Driving home one night I just thought about what I could do," he said. "I thought up the idea of a 'people's petition,' I called it, which said, 'We the undersigned express our opposition to the placing of the MX missile system in Montana, and to the escalating development and deployment of nuclear weapons by the United States.' My wife and I signed it first."

The McNamers passed their petition on to friends, who signed and passed it on again. In four months, without any organization whatsoever, they had 11,000 signatures on the petition. Then they sent it to a lot of local and federal officials.

In February, McNamer started a second petition drive, this one to put a resolution on the state ballot next November. This resolution declares that the state opposes deployment of the MX in Montana, and also opposes "further testing, deployment or development of nuclear weapons by any nation." To put the proposition on the ballot will require 18,024 signatures, and 11,000 have already been collected. McNamer said he is certain of having enough.

"It's basically just a question of getting the petition out in front of people and they are willing to sign it. It's just an amazing turnaround in attitude in the last six months."

How does he explain it?

"People are scared to death, for one thing," he replied. "It's a great moral issue, too . . . the MX is an immoral waste of our resources."

McNamer insisted that this is no country-bumpkin movement; Montanans know, he said, that the MX would give the United States the ability to strike Soviet missiles inside their silos, and they don't want their country to have such a first-strike capability. "Basically," McNamer said, "the Pentagon has usurped our right to think about the nuclear situation," but he is determined to reassert that right. And he is doing it without any outside help, and without any connection to the freeze movement, Ground Zero or any other national organization.

The antinuclear-weapons phenomenon seems comparable to a chemical reaction that could only have taken place after a fortuitous combination of necessary ingredients. Activists from all branches of the new phenomenon agree that the Reagan administration's big defense budget, its harsh rhetoric about nuclear weapons and the possibility of limited nuclear war, and the bad economic situation have contributed substantially to the changing public mood, but these alone were not enough to cause what has happened.

At least half a dozen elements went into the chemistry of this public reaction, starting with an event seven years or more in the past: the Vietnam war. McNamer cited his Vietnam experience as "part of my own personal thought process" that led him to fight nuclear weapons. All over the country people who fought in or against the Vietnam war are playing important roles in the new movements.

Vietnam gave this phenomenon more than officers and foot soldiers. It also legitimized the idea of challenging the government and the experts on a "national security" issue. The challenge was legitimized further by the collapse of the SALT process that every administration from Johnson to Carter invoked to demonstrate its desire to control nuclear weapons, according to several activists.

This movement also needed one or more simple, graspable ideas around which to mobilize, and the organizers found them. Apparently the most appealing is the notion that it is time to stop the arms race right where it is--freeze it in place.

This idea percolated up from at least three sources in 1979. At the suggestion of friends in an evangelical Christian group called the Sojourners, Sen. Mark O. Hatfield (R-Ore.) incorporated it in an amendment he offered that year to the SALT II treaty calling for a "moratorium" on all future deployments of new weapons.

Richard Barnet of the Institute of Policy Studies in Washington also proposed a cap on the arms race at 1979 levels. And in December, 1979, Randell Forsberg suggested a freeze in a speech to a conference in Louisville of the Mobilization for Survival, a peace group.

Forsberg, now 38, said in an interview last week that many of those in the Louisville audience urged her to put the idea into a formal proposal. This led to a series of drafts of a "Call to End the Nuclear Arms Race," which Forsberg circulated within the peace movement and among experts at Massachusetts Institute of Technology, where she had been a Ph.D. candidate before giving up her studies to devote full time to the Institute for Defense and Disarmament Studies in Brookline, which she founded.

Meanwhile, Randy Kehler heard about the freeze idea from Jim Wallis of the evangelical Sojourners, one of the people who had helped Hatfield formulate the idea. He took it home to western Massachusetts and began proselytizing there.

There were national meetings of people interested in the freeze in September, 1980, and again last month, and a gathering in Denver the month before. By now a rich variety of people from church groups, old-line peace groups, labor organizations and many walks of life have been swept up by the freeze idea.

The Physicians for Social Responsibility and Ground Zero both have somewhat more complex but also readily graspable ideas around which to organize. Essentially, both groups are preaching a need to understand more about the potential effects of nuclear weapons. The physicians argue that a nuclear war would create medical horrors that would be untreatable--"the last epidemic," they call it. Ground Zero seeks to raise consciousness generally about the devastation nuclear weapons could cause.

Another element in the chemistry of this new phenomenon is the opportunity it gives people to follow an idea that many obviously consider to be religious--literally or figuratively. Marta Daniels, a freeze activist in Connecticut who has worked for the American Friends Service Committee, described this element in an interview.

"The challenge," she said, "was to overcome the tremendous despair and cynicism" felt by most people. "The freeze," she continued, " . . . has unlocked hope and convinced people that their participation can make a difference."

The Rev. Jan Orr-Harter, 27, a Presbyterian minister in New York City and freeze activist, was asked where the freeze movement had come from. "Jesus," she replied.

A final crucial ingredient this chemical reaction required was a continuing sense of progress and momentum, and this has come from a coincidence of external events and the hard work of the people who signed up a year or two ago.

In Europe hundreds of thousands of citizens turned to the streets to demonstrate against nuclear weapons, a sign of international solidarity to many of this country's activists. Last June, the CBS television network broadcast an unprecedented series of five documentaries on defense issues, the first of which simulated a nuclear attack on Omaha, Neb. This program had the biggest audience of any documentary in television history--perhaps 40 million people. The conservative American Medical Association, prodded by the PSR, formally acknowledged that nuclear war posed unsolvable medical problems.

During the last Christmas recess, Sen. Edward M. Kennedy (D-Mass.) discovered how strong the freeze idea had become at home. He decided to adopt this cause as his own, and with Hatfield introduced a pro-freeze resolution in the Senate. By stagecraft and political maneuvering, Kennedy generated more public attention for the idea last month than it had received in its entire previous existence. He also helped sign up an august body of endorsers, from W. Averell Harriman to George Ball and Edmund S. Muskie.

All of this is deeply satisfying, if also somewhat amazing, to the battalions of mostly young activists who got the whole thing going. "A lot of people think this grew up suddenly, like a mushroom in the rain," observed Randy Kehler, now transplanted from western Massachusetts to the national freeze's "clearinghouse" office in St. Louis. "But there was an awful lot of hard work going on for more than a year . . . ."

Sandy Scarlet, a "Kelly Girl" secretary temporarily employed in Kehler's scruffy, second-floor St. Louis office, is a little awed by her new colleagues. "It's really neat," she told a recent visitor, "especially nowadays, to see the dedication they have. It's infectious. These are people who care about something other than the almighty buck."