An effort by France's three leftist parties to draw up a joint campaign platform that could bring them to power here early next year broke up in sharp disagreement tonight over Communist demands for radical changes in the platform.

The Movement of Left Radicals, the smallest and least doctrinaire party in the leftist alliance, rejected Communist demands for sweeping nationalization of French industry and walked out of the conference after a day of open disagreement among leaders of France's Socialist, Communist and Left Radical parties.

Robert Fabre, Left Radical leader, said his party was withdrawing from the scheduled two-day "summit" of the leftist parties "in the interest of the left itself." The Left Radicals would return to the discussions only after the other two parties "deepened their reflection to seek, without useless polemics, the bases for an agreement," he told reporters.

Although Fabre's decision to suspend his party's role in the platform talks did not automatically mean that the leftist coalition face dissolution, it represented a potentially grave threat to the alliance.

Fabre's party commands only 3 to 4 per cent of the vote, according to recent public-opinion polls, but that amount could be crucial in the hotly contested National Assembly elections scheduled for next March.

The three leftist parties collectively get a 53 per cent share of the national vote in current opinion polls, and would require that amount as a minimum to obtain a working majority. The Assembly is weighted in favor of the conservative-centrist coalition that has ruled France for 20 years.

Moreover, the Left Radicals' walkout pushes Socialist leader Francois Mitterrand and his party into a tight corner. The Socialists have been trying to resist the Communist demands without irretrievably alienating the Communists, who can deliver an estimated 20 per cent of the national vote.

Socialist Party aides report that Mitterrand fears losing voter appeal if he appears to give in to the Communists and adopts a more radical program. But following Fabre out of th alliance discussions could spark a revolt within the left wing of the Socialist Party itself.

Evidently determined to keep the pressure on his two partners up, Communist leader Georges Marchais attacked Fabre's "unilateral breaking off of talks" and said the Communist delegation would be present for the scheduled second day of talks Thursday., Marchais said the Communists were prepared to negotiate with the Socialists alone if Fabre's group did not attend.

Mitterrand refused to discuss the crisis with reporters and said he would not make a statement until Thursday. A Socialist spokesman indicated that Mitterrand would not meet the Communists Thursday.

In 1972 the three parties signed a set of political principles and commitments to bring socialism to France. Running on that platform, known as the "common program," Mitterrand came within one percentage point of beating President Valery Giscard d'Estaing in the 1974 presidential elections.

The Communist Party's old guard has never been happy about the deal with the less ideologically pure Socialists and Fabre. By steadily in creasing commitments from the other two parties to radical economic measures, Marchais has stilled much of the restiveness in the party ranks.

In nine hours of discussion that continued through lunch, the three delegations reached agreement on 30 minor points before running into an impasse on the Communist demands to increase the scope of the planned nationalizations of French manufacturing and banking.

The Communists want to add more than a thousand companies, including large steel and automotive manufacturers, to the nine major business groupings targeted by the 1972 platform.

Febre, committed to holding the nationalizations to those agreed on in 1972, said that the Communists "almost systematically refused to take into account our positions" on nationalization.

Other serious differences among the three parties center on defense and nuclear policy, with the Communists fully supporting France's nuclear strike force, and on higher minimum salaries for workers and higher taxes for the affluent.

While the leftist parties were quarreling today, the governing coalition made up of Giscard's Republican Party, the Gaullists and smaller centerist groups signed a campaign manifesto as a symbol of their own new-found unity.

Giscard and Gaullist leader Jacques Chirac have been squabbling bitterly for the past year over the strategy for fighting the left in next year's election. The manifesto is a collection of innocuous principles rather than a program for governing, but it appears to be an important step toward a lasting truce within the coalition.