Efforts by France's three leading leftist parties to agree on a plan for nationalizing French industry and banking if they come to power failed today as the Communist Party continued to demand a sweeping program of state takeovers.

After 4 1/2 hours of talks the leaders of the Communist, Socialist and Left Radical parties agreed to continue their discussions Thursday on a joint electoral program for next March's National Assembly Elections.

But Communist leader Georges Marchais told reporters that "It is impossible to say now that we will come to agreement." Marchais renewed his sharp attacks on the Socialists, who he claimed "have not only retreated from their former position" on nationalization "but have submitted proposals that do not even resemble" the original 1972 agreement between the parties on nationalization.

A two-month campaign of attacks on the Socialists by Marchais and the Communist demands that hundreds of medium and large firms be added to the nine major industrial groups targeted for nationalization in the 1972 program have raised the threat of a formal split in the three-party leftist alliance. Public opinion polls show that alliance winning 53 per cent of the projected vote next March.

The three-party leadership meeting to update the joint electoral program broke up Sept. 14 when Left Radical leader Robert Fabre stalked out to protest the Communists' tough position.

Despite a week of lower-level contacts and indications from each of the three parties that it was ready to seek compromises, today's discussions were "very difficult" and made little progress, Marchais and others present said.