French Communist leader Georges Marchais today declared an end to a 25-year-old political strategy of attempting to reach power by forming a "union of the left" with the Socialists.

Addressing the opening session of his party's 25th congress, Marchais accused the Socialist government of failing to implement a joint program agreed upon with the Communists following a left-wing election victory in May 1981.

The strong criticism of the Socialists, which was combined with a personal attack on President Francois Mitterrand, reflected the deepening rift between France's two leading leftist parties over the past seven months. The Communist Party pulled out of the Socialist-led coalition government last July.

Marchais said the strategy of the union of the left had in practice favored the Socialist Party much more than the Communists. The Socialists have succeeded in overtaking the Communists as the largest left-wing party in the country over the past 15 years.

The Socialists "posed as the guarantors of the union of the left in order to divide and demobilize the working-class movement and reduce the influence of the Communist Party," Marchais told the 2,000 Communist delegates from France and abroad gathered in a sports hall in the Paris suburb of Saint-Ouen.

Describing the congress as "a turning point in French political life," he read a passage in the draft resolution that referred to the completion of a 25-year period marked by the formulation of a left-wing program and the victory of a left-wing government.

Marchais depicted Mitterrand as a cynical political operator who had benefited from the introduction of the "quasi-monarchic" Gaullist constitution in 1958. The constitution strengthened the powers of the French president but also, according to Marchais, built Mitterrand up as the candidate of the unified left-wing opposition and the key to any alternative majority.

By turning his back on the strategy of a united left, Marchais appeared to be preparing the ground for a prolonged period of political opposition. Political analysts here say they believe that the Communist Party is now banking on the failure of the left in parliamentary elections next year and hoping to recoup its fortunes by picking up the votes of disaffected Socialists.

Marchais did not, however, rule out ad hoc electoral alliances with other parties in his 5 1/2-hour keynote speech. Socialist Party strategists predict that the Communist Party may be forced to come to some kind of electoral understanding with them in the 1986 elections in order to prevent a fragmentation of the left-wing vote.

The Communist Party's share of the vote in France has dropped to a 50-year low of 10 percent, compared to between 20 and 28 percent in the three decades following World War II. This contrasts with the success of the Communist Party in neighboring Italy, which last year for the first time collected more votes than the dominant Christian Democrats.

Unlike the Italian Communist Party, the French party has refused to distance itself from Moscow. Marchais today ruled out breaking with the Soviet Bloc countries "in the hope of avoiding the negative consequences for ourselves."