POSTERS popping up in Paris subway stations say the "festival of humanity," the French Communist Party's annual cultural and political celebration, will go on this month as in years past.This perseverance has taken more than one Parisian by surprise, judging by their comments as they read the posters: The French know that Soviet communism's fall has left little for the beleaguered party here to celebrate. Considered among the most orthodox of the West's remaining Communist parties, the PCF (the party's French acronym), was stung when former Soviet satellites in Eastern Europe shed communist rule. But it has been thrown into a deeper - some say terminal - spin by last month's Soviet communist collapse. Yet, despite the guiding Soviet party's disintegration, the PCF appears willing to choose its own demise over reform. At a two-day central committee meeting called this week at the insistence of party dissidents, PCF hard-liners controlling the party stated, "Our party has no intention of ceasing from being itself." The problem is that the number of party faithful continues to shrink. Once France's largest party, winning more than a quarter of the vote after World War II, the PCF now gets well under 10 percent. Party dissidents, especially vocal since the fall of the Berlin Wall, were aggravated by the party's official response to Soviet events, which called the attempted coup "unacceptable" but at the same time criticized Soviet President Mikhail Gorbachev and his reforms. That reaction caused some of its best-known figures and central committee members to call again for an overhaul of party rules to make it more democratic, and for the ouster of Georges Marchais, the party's general secretary since 1972. But with reformers holding only a dozen of the central committee's 139 seats, they say they have few illusions and talk of a new party for the French left. All of this might hold little more than historical interest, if it were not that the governing Socialist Party still has electoral ties to the PCF. Denied an absolute parliamentary majority in 1988 elections, the Socialist government counts on the Communists' 25 National Assembly members - out of 577 seats - to pass legislation, and keep from falling. France will hold bellwether regional elections next year before national legislative elections in 1993. Socialists know that the traditional Communist vote will be one key to winning. Victory in 1993 for the country's conservative parties would mean a replay of the "cohabitation" period of 1986-88, when a conservative legislative win forced President Francois Mitterrand to name a conservative premier. Some French political analysts say Mr. Mitterrand, in office until 1995, might resign rather than relive that arrangement. Still, the Socialists disagree on how to approach the Communists' disarray. Some are ready for a final break with the PCF. "We have to know when to turn a page," says Bernard Poignant, Socialist member of the National Assembly and mayor of Quimper, in the Paris daily newspaper Le Monde. The Socialist Party's first secretary and former Prime Minister Pierre Mauroy announced flatly, "If the Communist Party wants to survive, it must change." But he was careful not to burn all bridges with PCF leaders. What keeps the Socialists from dancing at the Communists' disarray is that those who have voted Communist do not automatically move to the Socialist camp. A surprisingly large percentage of former Communist votes have embraced the far-right National Front of Jean-Marie Le Pen, which polls about 15 percent of the national vote. And that is a source of worry for both the left and the right.