THE FRENCH Communist Party's denunciation of the Socialists and its abandonment of the union of the left are acts of desperation. The party feared being eaten alive. It understands perfectly clearly that its alliance with the Socialists was its only hope of power. But after three years in a subordinate role in a government run by the Socialists, the Communists dropped out last summer. At its congress last week it went further and denounced the whole concept of the alliance. With that, it consigned itself to a future of perpetual opposition. Inflexible in both its doctrine and its psychology, the party knew no other way to survive.

In the 1970s, the Socialists substantially increased their strength while the Communists remained fixed at one-ffth of the French vote. Then, in the 1980s, the Communists' following began to decline. The most recent demonstration was the election of the European Parliament last June, in which they got 11 percent of the vote. That apparently was the final blow, culminating a long history of disputes and irritations, that led the Communists to give up their four seats in a cabinet dominated by Socialists. They had good reason to think that, if they stayed, they would shortly be a barely visible appendage to a Socialist Party that, the polls suggest, is going to have a difficult time in next year's elections. The Communists want to detach themselves as distinctly as possible from the Socialists before the election campaign begins.

In the same European election last year in which the French Communists suffered their severe losses, the Italian Communists did spectacularly well. For the first time, they won more votes than any other party, and they did it by expanding their base among Italy's growing middle class. Ever since the late 1940s the Italian Communists have demonstrated a degree of independence from the Soviets that the French party has never attempted. In Italy the Communists have been sufficiently flexible in their ideology to be able to seize the kinds of advantages that the French party has left to the Socialists.

It is extraordinary that in France, where so much has changed in the past generation, one major political party has changed so little. The Communists emerged from World War II with two great sources of support, the French worker's deep distrust of the boss and the immense prestige that Communists had won in the resistance against the German occupation. Currently, for many French workers, the government is the boss. And memories of the war are slowly fading. The deep changes in French life -- the sustained rise in standards of living and education -- work against an authoritarian Marxist party with a fixed view of the world.