The historic decline of the French Communist Party on the eve of its 25th congress is captured in the political transformation of Jean-Claude Ferment.

In the early 1970s, it seemed the most natural thing in the world for Ferment, a trade union activist in a working-class district of Paris, to become a Communist. The French economy was reeling from the first of a succession of oil-price shocks, unemployment was beginning to rise, and the Communist Party was still accepted as the natural voice of discontented labor.

Today, Ferment is a vociferous supporter of the extreme right-wing National Front, which campaigns for "putting the French first." To explain his passage from one end of the political spectrum to the other, he cites unease with the Communist Party's unconditional loyalty to Moscow and its suppression of democratic internal debate.

"It was like a comedy. The Communists kept on saying that they represented the objective interests of the workers but never took any notice of what we said. Eventually I got sick of it," explained Ferment.

The scale of Ferment's political conversion may be more spectacular than most. But the process of disillusionment that he describes is one that has been shared by hundreds of thousands of working-class voters all over France. Once the strongest, best organized political force in the country, the Communist Party now enjoys less popular support than at any time in the past half century.

In European elections last year, the party's share of the vote fell to 11.2 percent, just a fraction ahead of the National Front. This compares with a record 28 percent of the vote in 1948, and 22.5 percent as recently as 1979.

Party leaders have sought to blame the decline on public frustration with President Francois Mitterrand's left-wing administration. The Communists pulled out of the ruling coalition last July after accusing their senior Socialist partners of reneging on 1981 election promises to expand the economy and reduce unemployment.

The rift with the Socialists has helped party leader Georges Marchais crush a rebellion by reform-minded Communists who had called for greater internal democracy and a more independent attitude toward Moscow. Despite growing rank-and-file dissatisfaction with his leadership, he is likely to be reelected secretary general at the congress, which opens in an indoor sports stadium in the Paris suburb of Saint-Ouen Wednesday.

"Any other political party that had suffered the kind of defeat that the Communists have just experienced would undoubtedly throw their leader out. The Communist Party, however, operates according to different rules. Marchais has led it from disaster to disaster -- but he remains secretary general as long as he is loyal to Moscow," said Philippe Robrieux, the author of a critical history of the party.

The party has remained one of the most ideologically subservient to Moscow of all West European communist parties. After dabbling briefly with the liberal variant known as Eurocommunism in the mid-'70s, it endorsed the Soviet invasion of Afghanistan in 1979 and the suppression of the Solidarity trade union movement in Poland.

"We were led to believe that everything in the Soviet Union and the socialist countries in Eastern Europe was perfect," said Ferment's wife, Irene, now also a member of the National Front.

The identification with Moscow cost the Communist Party the support of leading intellectuals during the '70s as French society became sensitized to human rights violations in the Soviet Union through the writings of Alexander Solzhenitsyn and other Soviet dissidents. So negative has the image of the Soviet Union become in the eyes of the French that, according to a recent opinion poll, it is shared by 39 percent of Communist voters.

In an interview last year, one of the party's leading reformers, Pierre Juquin, complained that a majority of young people were making the equation "Soviet Union equals the Gulag equals the French Communist Party."

Support for the party has been further squeezed over the past decade by Mitterrand's success in building the Socialists into the dominant political force on the left.

By offering the Communists four relatively junior posts in his administration, Mitterrand succeeded in depriving them of their image as the natural party of protest -- which is one explanation for why at least some former Communists suddenly found the National Front attractive.

The party leadership succeeded in isolating Juquin and the other reformists by invoking the Leninist principle of "democratic centralism" in preparatory meetings. The mechanism obliges dissidents to unite around positions accepted by the mainstream of the party. It enabled Marchais to pack the congress with his own supporters.

Rank-and-file disillusionment with the leadership was reflected in the lack of interest in the preparatory conferences for the congress, which have been going on for the past two months. In one district of Paris, only 87 out of more than 700 party members showed up for the preparatory debates.

According to most estimates, 25 percent of party members sympathize with the reformists. But Robrieux says "the miracle of democratic centralism" means dissidents at the congress are down to 5 percent of the delegates.