THE DECLINE of the Communist parties is changing the politics of Western Europe.

Through the 1960s and 1970s, in parliamentary elections, the French Communists consistently got 20 percent of the vote or a little better. In 1981 they fell to 16 percent, and in the election this month they were under 10 percent. They have become isolated, no longer fashionable, no longer the mainstay of the left, but instead a sect living in the shadow of a Socialist party that is now three times as large and, you would have to say, three times as vigorous and interesting.

Western Europe's other mass Communist party, in Italy, has been going through a similar erosion. It rose to a peak of 34 percent of the vote in the 1976 election, and it seemed inevitable that Communists would shortly be in the government. But the party seems deliberately to have drawn back from the ideological compromises that power imposes. In the last two elections its voting strength has slowly dropped. Meanwhile, the country is being governed with great flair by a Socialist prime minister, Bettino Craxi. With nearly three out of 10 Italian voters still supporting the Communists, the party remains much more of a force in Italy than in France -- no doubt because in Italy it has been less rigidly obedient to the Russians. But the prominence of the Socialists raises, for the first time in Italy, the possibility of a broadly based non-Marxist left.

Why is it happening in the 1980s? The passage of time accounts for some of it. For the generation of Frenchmen and Italians who lived through World War II, the genuinely heroic achievements of Communists in the underground conferred vast standing on the party. It is generally true that the Communists are strongest today in those places where they led the struggle against the Germans more than 40 years ago. But the actuarial tables are catching up with the people who remember those times of great danger and sacrifice. Their children no longer draw the connection so clearly. What they have seen is the repression, cynicism and economic stagnation that prevails throughout Eastern Europe and the Soviet Union itself.

Poverty and turbulence feed Communist movements, and most of Western Europe is now as comfortably prosperous as North America. A new social stability has been established. For many Europeans, getting rich was not a pleasant process. The Communist Party drew strength from the fears and resentments of, say, the southern Italian peasant who suddenly found himself working in a northern factory and living in a slum. Rapid economic growth extracts its own kind of costs. But the extremely high growth rates of the 1960s and early 1970s have not continued. The economy is changing much more slowly now, and for the past decade life in Europe has been remarkably tranquil. The Communists, as the party of crisis, are having trouble adapting to placid times.