For the past seven months, long before President Carter began considering an American boycott of the 1980 Moscow Olympics, the Soviet Communist Party has been telling its cadres that July's games pit "decadent capitalism" against "socialism, which is growing stronger every day."

"The history of the Olympic movement is characterized by a constant struggle between the progressive forces and the forces of reaction," senior party theorists wrote in a small, authoritative, paperback book for newly elected party activists in factories, collective farms and institutes across the country.

The book, entitled "Little Book for the Party Activist" and available at kiosks for about 50 cents, provides general information for party members about the ruling Politburo, party structure, ideological work and economic plans. But the 1980 edition of the annual "catechism" as it is sardonically known here, contains a special 13-page section that bluntly lays out Moscow's highly politicized view of the 1980 games, the first ever to be scheduled in a socialist country.

The party handbook's section on the Olympics passed largely unnoticed here until the Carter administration began talking about the possibility of an Olympic boycott to protest the Soviet invasion of Afghanistan last month. Soviet Olympic officials and the controlled news media in recent days have denounced any move to "politicize" the games by such a boycott, quoting international Olympic Chairman Lord Killanin as saying that sports and politics must be kept separated.

But the little book makes clear in flamboyant, unfettered language the political meaning of the games always held by the Kremlin.

"The forces of reaction are trying to use the Olympic movement and the games in the interests of the exploiting classes, in the interest of commerce and business, of the propaganda of the bourgeois way of life, of the capitalist system and its ideology, to divert youth from the political class struggles," declares the book, which was published here in June by the Political Literature Publishers.

"the forces of [socialism] strive to propagate far and wide noble Olympic ideals, to put the Olympic movement and sports at the service of education and bettering man's health, using them as a means of greater understanding of friendship between peoples of all countries and nations . . .

"The acute ideological struggle between the two opposed social systems also directly affects the choice of cities for the Olympic games, the competition programs, reporting of the preparations and the conduct of the games."

The party booklet glorifies the triumphs of Soviet Olympic teams in "putting an end to the unshared hegemony of the United States" since the post-war Olympics began in 1952 at Helsinki. "By every proof, Soviet sportsmen were first five times except in 1968, when they were just a little below the U.S.," the party writes. They record that at the 1976 Montreal Olympics, the Soviet Union and its ally East Germany stood first and second, with the United States third in number of gold medals won. Important capitalist countries like England, France and Italy "weren't even among the first 10," it says.

"The socialist countries were six of the top 10. These facts are convincing proof of the change of social conditions in socialist countries, where physical culture and sports serve humane goals," declares the handbook.

Part of the 13-page section seeks to explain and justify in this housing-short nation why the party has reserved so much energy, manpower and money for the Olympic village, new sports complexes and other facilities will be used for Moscovites after the games, contrasting that with what was said to be the problems Munich and Montreal had in using their Olympic facilities.

It carefully explains to the cadres that the October 1974 decision of the International Olympic Committee to award the games to Moscow has been accounted for in the 1976 to 1980 five-year economic plan.

"Everything that is going to be done toward the Olympics is fully included."

The book notes the games' preparation "will be an important step in fulfilling the goal set forth by Leonid Brezhnev," of establishing Moscow as "the model communist capital." But it complains about alleged anti-Soviet propaganda from the West about the games.

"The friends of our country try to show every support and cooperation for the 1980 games," declare the theorists in telling the party activists how to present the Moscow games to the people.

"The nonfriends try to defile our work in preparing the competitions, undermining the authority of the Soviet Union. The fact that Moscow [was chosen] the 1980 capital called forth the most furious attacks of our enemies. Bourgeois propaganda tries to show that we allegedly are not in a technical condition to guarantee the holding of the Olympics, that Moscow is unable to put up all the guests and so forth.

"These efforts to blacken the Moscow Olympics clearly show their second, more global aim -- the attempt to discredit the socialist system itself, her potential, her achievements, Soviet democracy, our way of life."

Such pugnacious hyperbole is common to everyday party pronouncements, and aims at playing on a traditional Russian -- and Soviet -- sense of embattlement. But the exaggerated language also underlines how much the ruling party wants success for the Olympics to justify its policies and indicates the impact a boycott by the Americans or other Western countries could have.

The handbook notes that about 20 percent of the equipment needed to host a successful Olympics has been supplied by members of Comecon, the Soviet-dominated East bloc economic group. They say that about 5 percent of the equipment has come from capitalist countries, "a concrete expression of international detente."

There is no mention that Moscow is busily merchandising special Olympic coins and souvenirs in the West, or that private firms of the capitalist countries have paid millions of dollars in hard currency precious to the Soviet economy for rights to participate in the Olympics. For example, at the main press center in Central Moscow, where most Western sports reporters and photographers will work, film processing will be handled by Kodak and camera repair by the Japanese Nikkon firm.

The "Little Book" was printed in an edition of 1 million and is specifically for communists who have been chosen by higher party circles to be "activists" proselytizing the ruling tenets and precepts. Frequently, these workers, technicians and collective farmers lack clear knowledge of party directives and intentions, but they are expected to learn them and spread them. There are about 16 million enrolled party members in a population of nearly 263 million, and only a tiny fraction are privy to, or consulted by, the ruling circles. Every directive comes down from above.

The "Little Book for the Party Activist" is one way official attitudes and perspectives are instilled in the cadres, and that is why its polarized political views of the 1980 Olympics reveal something of the nature of the Soviet Union as it faces the controversy over its Afghanistan invasion and the possible retaliatory boycott of the Moscow games.