MASON CITY, IOWA -- Bruce Weaver knows the type, and with good reason: He's one himself.

"Definitely a salesman," he pronounced, after intently watching Soviet leader Mikhail Gorbachev's hour-long televised interview with NBC's Tom Brokaw Monday night at a neighbor's house. "That's a career I'm in and he reminded me extremely well of the president of the company I work for. This guy's a salesman. He could take on the questions, disagreements or whatever you want to call them, turn them back into a positive and really convince us they were doing the right things. Definitely a salesman."

A traveling salesman metaphor is apt for Gorbachev, who arrives in Washington Monday for a summit with President Reagan. People here, in what residents proudly describe as "the north-central cash grain area of Iowa," and those surveyed by opinion polls nationally, overwhelmingly view the Soviet leader's trip positively, yet approach what he's selling with a certain caution.

Such American ambivalence toward the Soviet Union obviously is not new. It is a product of suspicion and fear born of bitter experience over many generations.

What does seem new, almost startlingly so when measured against the anticommunist rhetoric stirred in political Washington by Gorbachev's impending visit, is the way old fears and passions about communism and the Soviet Union have receded in the traditionally conservative American heartland, replaced, most notably, by fear of other "isms" from abroad: radicalism and terrorism.

"I look forward to Gorbachev's visit being a positive rather than a negative," said Richard Dean, 59, who farms the farm his great-grandfather purchased outside Mason City in 1885. But Dean expressed a typical cautionary note. "That communist religion does not change overnight," he said during a round-robin living room conversation with friends and neighbors at his farm last Sunday night, "not in one decade, or in two decades."

The American Midwest, once the bastion of isolationism and anticommunism, has not undergone an ideological conversion. But if politically moderate, middle-of-the-road Mason City is any guide, a subtle, complex, significant change in American attitudes toward the Soviet Union has occurred.

Two Post reporters last week interviewed Mason City residents in private and public offices, in houses and schools, on their feelings about American-Soviet relations on the eve of Gorbachev's arrival. The interviews coincided with a Washington Post-ABC News poll on the same subject. The poll results reinforced the impressions left by people interviewed in Mason City, population 32,000, the "River City" of Meredith Willson's innocent Broadway hit about midwestern values, "The Music Man."

Two themes emerge from the conversations. Both stem from American perceptions of vastly changing world conditions adversely affecting the United States and the Soviet Union.

First is a matter of practical economic self-interest.

We're in trouble economically and so are they, people here say repeatedly. We're in a global economy, and so are they. We need trade, so do they. We need to reduce spending on armaments in order to resolve other problems, and so do they. We experienced embittering defeat against inferior peasant forces in Vietnam, as it seems they are in Afghanistan. Thus, conditions appear ripe for a new start between old adversaries. Let's deal.

"I have never been opposed to doing business with them, particularly in the area of food," county Treasurer Michael Gandon, 45, said. "If it's good for agriculture, it's good for Iowa. If it's good for Iowa, it's good for me. So those are somewhat selfish motives on my part. I don't confuse doing business with trusting them philosophically."

Second, and by far most striking in its implications for future U.S.-Soviet relations and the question of defense spending and the nuclear arms race, is the belief that the threat of communism has been supplanted by fears of a greater "ism" -- radicalism, causing senseless, indiscriminate killings and terror of the kind commonplace in the Middle East.

"There's a different force at work in the world today, and that force is radicalism," said Thomas E. Jolas, 55, an attorney, former Republican mayor and leading figure in Mason City and statewide economic development plans. "It bothers people a lot more than the threat of communism. The Russians are not as radical as the radicals we see; they're a pretty good decent friend, really. The extremists, you see, are the people you have to fear more. They have no conscience. They're zealots. And, wow, we've not seen that on the world scene

like this in a long time, and I don't know when in

history . . . .

"I'm talking about people like {Iran's Ayatollah Ruhollah} Khomeini and terrorism. It's a whole different type of political concept, very fearful, very disruptive. Everybody knows what they do, everybody knows that that's a real threat to order. It's just indiscriminate killing and who blew up the airplane? This sort of thing. Radicalism has come to be of the greatest concern."

That's not just one person's view. Similar opinions were repeatedly volunteered during the interviews.

Heightening this fear is the belief that the United States and the Soviet Union, despite all their military might, are vulnerable to such attacks and almost powerless to prevent them. Thus, too, their self-interest lies in cooperation instead of confrontation.'I Think Gorbachev Is Sincere'

This fear leads to another, apparently even more pervasive and troubling one: that world events are in danger of slipping out of control.

"Our weapons are useless against these terrorist threats," John E. Anderegg, 39, said during a coffee session early last Tuesday morning with three other farmers at a grain elevator cooperative outside town. "So that makes you vulnerable to any of this. So no matter how much money we spend on atomic weapons or just land-based weapons -- or our Army! -- someone bent on destroying someone with a carload of dynamite is going to do great damage to you. Look at the speedboats in the Persian Gulf right now. We've got billion-dollar ships that are virtually defenseless against them. If they send 20 out to get a ship, they'll get it.

"And look at the lesson that the Russians got when that West German boy flew his plane over the Kremlin into Red Square. They had no idea it was coming. So if that was full of dynamite, there it goes. They might spend the same amount on defense as we do, but it didn't stop that plane. We build high-tech weapons to protect us against their high-tech weapons. Where if you go back to the beginning, the bow and arrow might get you."

"We've got to control our defense budget. It's out of control. Ecology -- we're destroying too much of the world. Economically we've got to control our deficit spending which the defense spending adds to, which the farming subsidies add to. But they have to be controlled or else my kids -- I've got four kids -- the problems are just going to keep multiplying for them. And I think Gorbachev is sincere in wanting to eliminate this friction of the cold war, if you want to call it that. Let's just realize that we've become two nations of people who live a little differently politically but we're still the same people trying to survive."

In Washington, Republican House members threatened to demonstrate and walk out if Gorbachev addressed a joint session of Congress. Here, citizens uniformly express indignation at such tactics -- and none more strongly than Republicans.

"I was interested in the fact that there was a violent reaction when he was more or less invited to address the Congress," said Dave Murphy, 70, a strong Reagan supporter whose manufacturing business makes equipment to dry harvested grain. "Which I thought was uncalled for. I thought it would be a fine thing for him to address the Congress. One of those congressmen, he was a Republican, was violently against it, and I thought that was ridiculous."

Dave Long, Mason City's assistant superintendent of schools and Reagan county chairman in 1980, agreed. "A lot of people are upset that the focus becomes on the protesters rather than on what Gorbachev has to say," he said.

Two other Reagan supporters were equally outspoken. "No one can hurt you by talking to you," said Arlene Gilbert, 50, a real estate agent who was the first woman to head Mason City's Chamber of Commerce. "You're the only one that can be hurt by not listening. I can't imagine why they would walk out. If he speaks to Congress, he's on record. How better than to have him speak directly to us, our people. I don't understand the fear in having him speak."

Ken Kew, 66, now retired and for 12 years mayor of Mason City, said: "Are we afraid of what he has to say? Are we so lacking in confidence in our government that he might persuade us? I'd love to hear what he has to say."

In Washington, the Republican right has launched strong attacks against the proposed treaty to eliminate intermediate-range nuclear forces (INF) that Gorbachev and Reagan are expected to sign this week. Here, approval of the treaty is viewed as a step in the right direction.

The Washington Post-ABC News poll, conducted the same time as the Post interviews here, found that among those with the most favorable views of Gorbachev were Americans identifying themselves as strong Republicans {Details of the poll, Page A29.}.

Results mirrored the feelings of citizens here.

National poll returns from interviews with 1,007 adults showed the American people holding favorable impressions of Gorbachev by nearly a 2 to 1 majority; overwhelmingly (73 to 25 percent) approving Congress affording him a chance to address it; believing (85 to 12 percent) that Gorbachev is more interested in improving relations with the United States than previous Soviet leaders.

Interestingly, 58 percent of self-described "strong Republicans" hold favorable views of Gorbachev compared with 44 percent of "strong Democrats" who had a similar positive opinion.

Feelings about Gorbachev personally here are almost uniformly positive. City Councilman Ralph Watson summed them up after watching him on NBC Monday night by saying that Gorbachev "seems well-informed, intelligent and exhibits real leadership skills. He doesn't hold anything back."

In fact, the image of Gorbachev's strength and decisiveness often inspires a contrary concern. People said they worry that Reagan will not be up to dealing with Gorbachev. A strong sense of Reagan's decline and diminishing power was voiced here; again, often by his own supporters.

"I have to think that Gorbachev is probably a little better of the horse trader," said John Fromm, 52, vice chairman of the county board of supervisors and a Republican. " . . . I kind of tend to think Reagan's age is beginning to tell on him. His style of leadership and his style of management didn't prove out. In the Iran scandal, you know, his style was kind of a loose management. He didn't want to be bothered by the details and you can see what happened. All that arrived because he didn't keep a tighter ship and that came back to haunt him . . . I was for him, he's my party. Now I see a faltering lack of leadership there . . . . "

Nor have all the old inbred suspicions about the Soviets dissipated. Susan Fisher, for instance, president of the Mason City school board and active in peace groups, began by talking about "our paranoia towards communism"; later, however, she remarked that "the men in the overcoats are still standing on the podium."

Nowhere was this sense of a conflicting mental picture more evident than among schoolchildren. Two groups of students, ranging in age from 15 to 18, spoke about the Soviet Union in discussions at Mason City High School and in the local Roman Catholic school, Newman High. Seventh graders, at Roosevelt Middle School, also wrote down their impressions of the Soviet Union in class.

Their ambivalence was summed up by Erica Weiland, an 18-year-old Mason City High senior: "It is not really a friend, but not really an enemy . . . it is just different."

Considered and thoughtful when discussing related issues such as communism and the arms race, the students were far more animated when asked to articulate the image they had of the Soviet Union. Without exception they painted a picture of a dark, unhappy place, the opposite of a "brighter" America.

"{When I imagine the Soviet Union} I think of dark clouds, cold," said Stephanie Graff, 17, a junior at Mason City High, as other students nodded agreement.'I See Things More Fenced In, More Closed'

In the Soviet Union she saw, "the people always look scared. {They} seem like they don't even know where they're going, like they're just following whatever they're told. I don't think they think about life; they just think about living."

America is "greener, brighter," she said.

Newman High's Susan Mousel, 17, used similar imagery: "Whenever I think of them I always see people in dark clothes, I see things more fenced in, more closed. I really don't see any bright colors, I see gray."

Her classmates agreed. But Mousel was also insistent that the Soviet Union in reality could not be that grim: "It's unfair. I don't think that Russia is all dark colors, but that's all I have ever seen," she said. "It's unfair that we are given such a false image of them. We cheat ourselves out of really knowing what other people are like." Classmate Jeff Beach, 16, agreed: "If we are going to try to have peace with them in the future we need to know what they are like."

On that subject, there appear to be no dissenters.

A colloquy between two citizens here, of differing political views -- one anti-Reagan, the other the Reagan county chairman in 1980 -- was illuminating.

"I don't see this visit as being anything more than symbolic," said Robert S. (Chip) Kinsey, second-generation Mason City lawyer who was critical of Reagan. "But it's a very important symbol, hopefully as a willingness to compromise. It comes at a very important time because we have a conservative administration. That's a time when we seem to be able to make inroads in international politics in a way we don't seem able when we have a liberal, Democratic administration. We have a leader of the Soviet Union who seems to want to negotiate. It's a wonderful opportunity. It's symbolic. It's the beginning."

Dave Long, the assistant superintendent of schools and former Reagan political chairman, said, "I agree with Chip" and added:

"I have three sons. I thought about that a lot when they were growing up, the implications for war: We'd been through the World War II and the Korean conflict. I didn't want to go through that with my own sons. So I thought about that a lot. That's why visits like this one with Gorbachev, I think it's a first step. Let's get on with it."

Then, he expressed a note of tempered optimism about the Gorbachev visit, surely shared by most Americans everywhere: "It seems we've had a lot of first steps. I really wish for the sake of the American people and the Russian people that we could see steps 3, 4, 5 and 6, instead of always the first step."