Although the gloom is naturally concealed at the fortress-like Communist Party headquarters here, the inner mood these days is one of beleaguered worry tinged with desperation.

The reason is not hard to find: The spectacular Communist decision to break its five-year union with the Socialist Party of Francois Mitterand has forced one of those 180-degree reversals that litter the Communist landscape back to the Russian Revolution.

Party leader Georges Marchais (actually a closet opponent of the party's decision to break the Union of the Left) had made Mitterand the shining symbol of the Socialist-Communist union and its Common Program. Now Marchais's job is to convince disillusioned party cadres that Mitterand is some sort of anti-Marxist, anti-worker traitor. These are the same cadres who were promised by Marchais for the past year that sweet victory awaited the Union of the Left in the election now scheduled for next March.

"It is becoming clear that the leaders of the Socialist Party have heard the siren song and are setting forth,r Marchais told his central committee in a battle-cry speech last month. Setting forth where? For a new political union with "national and international forces opposed to genuine political change" - in other words, the hated bourgeois Center - just as "the Social Democrats are managing the affairs of capitalism" in Britain, West Germany, Austria, Portual and other West European states.

The sin of the Mitterand Socialists, says Marchais, is their refusal to agree to an economic revolution that includes nationalization of major holding companies and their affiliates, a 50 per cent increase in family allowances, a tax on capital and wealth, and worker elections of board chairmen in national companies.

Marchais's political dilemma is profound. Even with a $2-million kitty to hold the party cadres in line and whip up new members for the March election, can he justify sacrificing such other economic goals as higher minimum wages, agreed to by the Socialists, just because Mitterand will not buy the whole new Communist package?

These other, lesser parts of the Common Program did indeed appear to be within reach before the September break of the Union of the Left. Today, however, the prospect appears remote of either a healing of the breach or of enough informal Communist support for Socialist candidates in the election to give the Socialists control of a new minority government. Indeed, the reverse may occur, a Communist decision to cut key Socialist candidates in selected districts in the runoff (second) balloting, a process that could cost Mitterand between 50 and 60 seats.

In short, the economic goals for workers seem far from Marchais's mind today, even the goals that formed the Common Program before the Communists escalated their demands and forced la rupture of the Left. The Communists' real goal may be more complex: to whittle down the fast-growing Socialist Party, no matter how much the Communists damage themselves in the process, and then move into a rebuilding process of the Left after the election.

That would mean hibernation time for the Communists for several years. But given the present preeminence within the Left of Socialists over Communists - a reversal of roles since the Union of the Left was formed in 1972 - temporary self-destruction of the Left in the March election followed by rebuilding may be the best of bad choices for Marchais's Communist Party.

It would free the cadres for serious party rebuilding from a base of ideological commitment far stronger than the Socialists'. It would also free them from an unacceptable position as No. 2 party in a leftist government - something both the Communists here and in Moscow seem unwilling to accpet.

A step backward by the Communists followed by a longer step forward caries grave risks for France. It could confront President Valery Giscard d'Estaing with this predicament: an election in which the disunified Left captured more popular votes but fewer deputies than the tenuously united Center-Right.

That would probably doom Giscard's cherished desire for a coalition government between the center and the Socialist Left. Leaving the Left out of the new government would have dangerous results in politically volatile France, results that might greatly profit the Communist Party.