Ruben Zamora is a moderate man in an immoderate time.

Once a teacher, he is now an international spokesman for the guerrillas in El Salvador. He is also a lobbyist, a salesman and ever the teacher--as the Reagan administration, intensely fearful of the spread of communism in Latin America, asks Congress for $110 million more in aid for the U.S.-backed Salvadoran government.

Zamora walks a precarious line. As a former Christian Democrat representing radicals, a politician in the presence of rebels, he does a lot of explaining. Not only to the United States, but to the guerrillas as well:

"There are some tactics that we told them had to stop," he says. "They were blowing up supermarkets. The argument they put forth was that a string of supermarkets was a front for paramilitary groups. It seemed that in the process of blowing up these, they mistakenly blew up the wrong supermarkets. That's another thing they stopped."

He suggested the guerrillas tone themselves down, too.

"The joint statements were using words too heavily charged," he says, "too rhetorical . . . 'the dictatorship,' 'the servant of the military,' 'murderers.' We've used that language for years in Latin America, but for other audiences--Europe and North America--that is not the way. We would say to the guerrillas , 'Let's use fewer adjectives and more facts.' "

If the size of a foreign country were inversely proportional to the global significance that the Reagan administration attaches to it, El Salvador would be the example that illustrated the rule. While Zamora attempts to explain the guerrillas, the United States continues its involvement in El Salvador, dispatching combat advisers and sending money. For Americans, the ghost of Vietnam haunts every statement.

And for the guerrillas, "the U.S. has been just an imperialistic power," Zamora says. "They've come to realize it's a complex political situation. Not all the people here are Reagan."

It is a lesson Zamora is sometimes called upon to teach. Last year he got a phone call from a friend who asked Zamora to come immediately to see him.

Zamora arrived at the appointed meeting place to find a Salvadoran guerrilla commander, a man who had been fighting a long time in the country. They exchanged pleasantries, and the guerrilla explained why he had Zamora summoned. "Explain to me the United States," he said.

"And we started to talk," Zamora remembers. "I explained the Congress, the Democrats, the Republicans, the conservative Democrats, the moderate Republicans, Jesse Helms . . ." Zamora ticks off each category on his fingers as a bemused smile spreads across his face. "I spent three hours talking to him. I wasn't sure if he understood. I said to him, 'Do you get it clear?' He said, 'At least I get clear that it's complex.' "

Ruben Zamora, 40, is a pleasant man, articulate and not given to outbursts of rhetoric. For his work, he is paid $300 a month. He has four children, ages 2 to 12, and a Nicaraguan wife who teaches linguistics.

As a spokesman, he travels the world, particularly the United States and Europe. His goal is to rally support for the Salvadoran guerrillas who have declined to participate in the coming elections and want to negotiate a settlement with the government as a means to ending the civil war.

For Zamora, this task is his last resort. To hear about the circumstances of his brother's death is to understand why.

In January 1980, a new Salvadoran government collapsed after about two months. It had been an the awkward coalition of military men and moderate politicians that promised reforms and delivered little. Zamora was the chief of staff, one of numerous officials who resigned. But he remained in the Christian Democratic Party, as did his brother, Mario, a party official. Mario Zamora was a public defender of sorts. A second government was formed a few days later and a new wave of repression struck El Salvador, remembers Zamora. "It was hitting the Christian Democrats--the midlevel people," he says. "I was receiving day after day people from the interior relating horror stories--the local leaders of the Christian Democrats were being beaten."

Upset by these incidents, the Christian Democratic leadership met with military and junta officials and presented the military officials with a document charging them with repression and calling for it to stop.

"When we finished reading the document, there was a terrible silence in the room," says Zamora. "You could feel it," he says rubbing his fingers together. " Defense Minister Gen. Jose Guillermo Garcia said, 'This document is too serious. We can't give you an answer now.' The military doesn't like to be told they're repressive. I read that as their way of saying they were going to get rid of the Christian Democrats. My brother read it the exact same way. He passed me a piece of paper that said, 'Let's stop the meeting and get out of here.' But then the leadership of the party started talking and backing up and saying, 'Oh, maybe this document is too harsh. We want to be constructive. We want to work with the military.' I thought, 'My God, we are lost.' "

Shortly after that meeting came accusations from rightist leader Roberto D'Aubuisson against both Mario and Ruben Zamora "of having contacts with the guerrillas. It was absurd," says Zamora.

Mario Zamora retaliated by filing a defamation-of-character suit against D'Aubuisson--a rather rare action in El Salvador.

One night late that February, Ruben Zamora stopped by a party at his brother's house, next door to this own. Ruben left early and went home to bed but was awakened later by the sound of his brother's wife next door. "She was talking very strongly, saying, 'Where is my husband? Where is my husband?' I got up and started to walk over to his house when my wife said, 'Think what you are doing.' I came back and dialed his line. It was dead. I started to phone people . . . military people. I said, 'My brother has been kidnaped.' Then someone called and said, 'No, he wasn't kidnaped. He was killed.' " Zamora chokes on the words.

Apparently, men had simply entered the house, asked the room of party guests who Mario Zamora was, found him, took him to the bathroom and killed him.

"And, you know," says Ruben Zamora with a faint smile of surprise, "20 minutes later, my brother's telephone line was fine."

Ruben Zamora resigned from the Christian Democratic party in early March and left the country that same day for his wife's parents' home in Nicaragua, where he still lives.

"In one sense, I've become more radical," he says, "because I saw that to assure democracy and respect for human rights, we have to do more radical things."

"I think he's by far the best spokesman for the Salvadoran left," says Bill Woodward, a staffer for Rep. Gerry Studds (D-Mass.), a member of the House Foreign Affairs subcommittee on inter-American affairs. Woodward has spoken with Zamora on several occasions. "He's the only one who's been to Washington who's articulate, intelligent and capable of reaching an American audience . . . He seems to be able to articulate his position without alienating people."

The State Department isn't so sure. "We met with Zamora at midlevel over a year ago," says press officer Anita Stockman, "saw what he was about and concluded that he has no authority and haven't seen him since. If one accepts the premise that the guerrillas are not monolithic, one then has to ask, 'What is the authority of individuals claiming to be their spokesmen? Who is being represented?' "

Rep. Clarence Long (D-Md.) has similar reservations. "There are five different factions among the guerrillas," says the congressman whose subcommittee on foreign operations will decide whether to recommend Reagan get his El Salvador funds request. "Some of them are communist. Some of them are not . . . I think it's a little like the Salvadoran government. Nobody really can speak for the entire Salvadoran government. Some of them are nice, decent people. Some are the most contemptible people you'd ever want to meet."

Zamora does his part to take the sting out of the Reagan administration's perception of the guerrillas by detailing to officials here what they really stand for.

"Here in the U.S., Marxist equals communist and communist equals Soviet Union," says Zamora. "In Latin America, all those three things are not equal." Of the guerrillas, he says, "they are all socialists.

Some look at the Soviet model. Some look at the Yugoslavian model. Some look at the Cuban model."

As for his own perspective of his role, he says, "I am not representing Marxists. I am representing my people who are Marxist, who are Christian Democrat, who are independents."

Indeed, he does represent a large and cumbersome alliance. There is the Farabundo Marti National Liberation Front (known as the FMLN, its acronym in Spanish), a coalition of five guerrilla groups that until recently spent much energy decrying the tactics of one another. And there is the Democratic Revolutionary Front (FDR is the Spanish acronym), composed of trade unions, professional groups and political groups, including the Social Christians, who used to be the Christian Democrats. In El Salvador there are so many different acronyms that Zamora himself refers to this political spectrum as a "sopa de letras"--alphabet soup.

"I don't represent one ideology," he says. "Our alliance is based on a program."

That program includes respect for human rights, banking reforms and tax reforms. "It's so corrupt," says Zamora of the tax system. "You go to a tax collector and give him $1,000 and that's all. Very rich people don't pay taxes. Here, at least, you have to hire a lawyer and set up a company in the Bahamas."

The guerrillas did not participate in last year's elections. "What did that election solve?" says Zamora. "An important part of the population was not represented in the election. If we had participated in that election, the only thing we would have produced would have been a massacre of our own people."

Similarly, the guerrillas will not participate in the upcoming ones, says Zamora, because the government cannot guarantee security for people representing candidates the guerrillas might put up or support.

"Who is going to protect the people campaigning in the small towns?" he says. "It's just not possible until you go into the process of changing repression in El Salvador. And that's where we need negotiations."

Zamora sees two big points of negotiation for the guerrillas: the role of the military and having all sectors of the population participate in the government. "How it is going to be done will have to be discussed," he says.

The State Department sees all this as vague. "What in specific is their proposal?" asks one State Department official.

As Vietnam haunts the U.S. involvement in El Salvador, the 1979 Nicaraguan revolution haunts Zamora.

"The Nicaraguan process was sort of a honeymoon," he says. "The revolution was so ideal--the nice guy toppling Somoza. Sooner or later, you have divorce. We are not trying to tell anyone we're not Marxists. We're Marxists, we're Social Democrats. This is our program. We don't have any illusions."

But for Zamora, a man who went from being part of his country's government to a man who lives in exile, this is the only way. "What is important is that with the present regime, I have no chance to enact my ideas. We will have disagreement with Marxists, we will have agreements with Marxists. But at least I will have a chance to fight for my ideas."

He says this with quiet determination and some sadness--a moderate man with a radical proposal.