Slashing attacks on nominal friends and erstwhile allies are all the rage in Paris this week, as France's combative political parties open what promises to be the country's most hectic and divisive campaign season since the political turmoil during the Algerian war of 1954-1962.

The Communist and Socialist parties, which have formed an alliance to try to take power from the conservative and Centerist coalition headed by President Valery Giscard d'Estaing, are fighting bitterly over the program they will favor in their campaign for the National Assembly elections next March.

The governing coalition's gleeful appreciation of the fighting on the left was cut short today, however, by a serious challenge to its own recently found unity. In what appeared to be a desperate bid to outflank his more conservative rivals in the coalition, reformist leader Jean-Jacques Servan-Schreiber disclosed the existence of an anti-Gaullist "front" within the group.

Most observers regard the intramural sparring as a warmup for the National Assembly elections campaign. They do not believe that either of the two ideological blocs that dominate France is on the verge of breaking up.

But the sparring, particularly between the Socialists and Communists, appears to have drawn more blood than anyone had intended. On both sides, new fears and tensions that could determine the outcome of the expected close race have come out of the spate of campaigning.

A poll published today by the conservative newspaper Le Figaro showed that 53 per cent of those questioned would vote for the leftist alliance. It was the third consecutive such finding in eight months.

The Communists broke the relative calm of the vacation season Monday. The party's newspaper, L'Humanite, carried a two-page attack on the Socialists for not accepting Communist proposals to update the campaign platform the two parties agreed to support in 1972.

The Communists want the Socialists to commit themselves to a comprehensive program of nationalizing industry, raising wages and soaking the rich with new taxes. The Socialists want to go into the campaign with a blurred profile on these issues.

In part, the Communist attack was seen as tactical maneuvering for a meeting next week at which the Communists, Socialists and their minor ally, the Left Radicals, will adopt their platforms.

The criticism suggested that the Communists are still afraid that the Socialists will dump them if the left wins the election and will make a deal with Giscard to run the country. The Communists seem to be trying to get the Socialists to lock themselves into a binding, radical agreement.

Socialist leaders took the attack seriously. The Communists, said Claude Estier, Socialist national spokesman, "show a permanent suspicion that the Socialists will betray the interests of the workers." Charles Hernu, another Socialist leader, said te Communists are behaving as though they want to "prevent a victory of the left."

The Gaullists, Centerists and members of Giscard's own small Independent Republican Party agreed this week on their own "manifesto" a set of campaign political platitudes and attacks on communism.

In part because of their antipathy toward Gaullist leader Jacques Chirac, Servan-Schreiber and his Radical Party have not taken part in the consultations on the manifesto. After meeting last night with Prime Minister Raymond Barre, Servan-Schreiber told newsmen that he had been meeting privately with leaders of the Centerists and Giscard's Republicans for four months to choose common candidates to run against the Gaullists.

Servan-Schreiber's declaration, containing new criticism of Chirac, clearly embarrassed Jean Lecanuet's Social Democratic Centerists and the Republicans. Servan-Schreiber appeared to be trying to pressure Giscard and Barre into allying themselves with him and excluding Chirac by suggesting that the Radicals could run candidates on their own and plunge the governing coalition into chaos just before the elections.

Barre, however, has reportedly reestablished good working relations with Chirac after a period of coolness, and is not likely to want to lose the Gaullists, the largest party on the right.