The French Communist Party, after a dramatic setback in last spring's elections, has opened a timid public debate on what went wrong, carefully avoiding direct blame of the party leadership.

The published criticism, leading up to the party's 24th congress in February, represents great disappointment and blame-laying, much of it directed at party leader Georges Marchais and his aides, according to party dissidents, who hold Marchais' pro-Soviet leadership responsible for the disastrous electoral results.

The tactic of allowing a little steam out to reduce pressure is part of an attempt by Marchais and his team to contain dissatisfaction, the dissidents say.

Although even opponents predict Marchais will succeed, the tone of the congress nevertheless will strongly affect party attitudes toward cooperation with President Francois Mitterrand's ruling Socialists and continued Communist participation in his government.

"They have to get by the congress," said a party official critical of Marchais. "The leaders have their backs to the wall. If they really have the militants debate the defeat, the debate will necessarily be critical of their leadership in the last few years."

The rumbling in the ranks grows in part from significant electoral reverses, including Marchais' poor showing in the presidential election last May and a drop from 86 Communist deputies in the National Assembly to 43 after last June's legislative elections.

The showing, representing a loss of about 1 million traditional voters, marked the worst defeat since the party was founded 60 years ago. It reflected what one dissident called "a profound political crisis" that has seen estimated membership drop from 700,000 several years ago to 500,000 now. Circulation of the official party newspaper, Humanite, is said to have sunk from about 140,000 to less than 100,000.

In addition, Communist sources say, militants have been confused by the party line: that it was impossible to work out a common platform with the Socialists during the election campaign, but that now it is good politics for the four Communist ministers to join in applying the Socialist platform.

Communist leaders, for example, recently handed the U.S. Embassy a long petition denouncing American neutron weaponry only one day after Prime Minister Pierre Mauroy announced France's decision, in which the Communist ministers participated, to go ahead with its own neutron weapons.

"The coherence of party policy is being questioned," said the dissident party official.

Against this background, the party published a long draft resolution for February's congress. It analyzes the setback at the polls as as the result of "strategic lateness" in marrying party doctrine with modern times. But the resolution, which Marchais described as the basis of a "big debate," steers carefully away from major decisions taken since Marchais took over in 1972.

Chief among the decisions -- and, according to dissidents, a prime reason for last spring's defeat -- was the break with Mitterrand's Socialist Party during the campaign for the 1978 legislative elections. Before the break, opinion polls had given the "union of the left," joining Communists, Socialists and a small center- left party, a good chance of winning a parliamentary majority. As it turned out, forces loyal to then-president Valery Giscard d'Estaing stayed in power with a comfortable edge.

Since then, Mitterrand gained strength largely by increasing his support among civil servants, young executives and others who had refused to accept his party while it was allied with the Communists. The Communists, as rivals to the left of the Socialists, had little chance of winning those votes, and ended up losing about a fourth of their traditional electorate in a bandwagon effect stirred up by Mitterrand's gains.

The draft resolution says, "With its 24th congress, the party has the will to look clearly at the reasons for its setback, the better to resume moving forward; to point out all the consequences and all the implications of its last two congresses, and to deepen reflection on the political practices that will permit them to be put to work."

The document lays the blame on historical errors rather than Marchais' tactics, however, citing a slowness by Marchais' predecessors in jettisoning such doctrines as the dictatorship of the proletariat long after they were out of step with France.

Since publication of the draft, the party newspaper Humanite has begun a regular "discussion tribune," printing contributions from Communist militants commenting on the resolution and adding their own explanations for the reversals. Most conform carefully to the resolution, reaching back into history and avoiding Marchais' break with Mitterrand.

Georges Poincon, a militant from the Paris suburbs, wrote, however, that the resolution skipped too lightly over party strategy of the last several years, saying:

"While it is always useful to analyze what happened in the last decades, history's lessons always shedding light on the present, it seems to me at least necessary to profoundly analyze what happened in a more recent period, which the resolution does not do, or so little."

Poincon took care, however, to avoid mention of Marchais or the break with Mitterrand. Despite what he called "fundamental disapproval," he also avoided citing Marchais' expulsion of a group of Communists who opposed the break.

"In simple terms," said the independent leftist newspaper Liberation, "If they do not want their prose to land in the wastepaper basket, Communist militants must carefully stay away from two taboo subjects: the leadership's policies since the breakup of the leftist union and the internal strains caused by exclusion of" the dissidents.