Francois Mitterrand's hopes for victory in France's presidential election were heightened today when the Communist Party issued an unconditioned appeal to vote for the Socialist candidate.

A surprisingly supportive statement by the Communist Party's 150-member Central Committee stressed that Mitterrand has refused the party's demands to include Communists in the Cabinet, a move that can only serve to help Mitterrand in his efforts to reassure moderate voters that his victory would not also be a victory for the Communist Party.

Coming after Gaullist party leader Jacques Chirac's statement yesterday that he would personally vote for President Valery Giscard d'Estaing but that Gaullist voters were free to follow their own consciences, the Communist move makes Giscard's position tha much more difficult. Mitterrand and Giscard were the top vote-getters in last Sunday's first-round balloting, and will face each other in a runoff May 10.

The Communist endorsement was followed this evening by a state television interview with Communist leader Georges Marchais in which he reinforced the impression that his party has finally decided to help Mitterrand get elected.

In response to progovernment interviewers who kept trying to get him to repeat his previous statements that a Matterrand victory would be followed by a wave of strikes and factory sit-ins, Marchais steadfastly refused to say anything that could be interpreted as embarrassing for the Socialist's election chances.

Marchais said his party would distribute 8 million copies of its appeal to elect Mitterrand. This dramatic shift in the party's attitude, after having given the impression for about two years that it was in fact working for the reelection of Giscard, was apparently dictated by the Communist Party's weakness after its poor showing Sunday in the first round of the presidential elections and by the desire to avert an even deeper plunge in party fortunes.

Marchais got 15.4 percent of the vote, the party's lowest level of support since 1963 in a national election. There has been a wave of complaints from Communist mayors and other local officials that the party line of sabotaging a Mitterrand victory was leading the Communists to disaster.

While the Communists dropped about 5 percentage points from their traditional national voting level, this was translated locally in party working-class bastions like the Normandy port of Le Havre to drops of as much as 20 points. In the Paris Red Belt area of Seine-Saint Denis, the most solidly Communist region in France, the party fell 10 points.

A former head of the Communist youth movement said old comrades described the scene to him in one city hall where the Communist mayor, elected with Socialist votes in the heyday of the Communist-Socialist alliance, said with tears in his eye as the reports of Marchais' poor showing were coming in Sunday night: "What even Hitler didn't manage to achieve, that bastard has managed to do. He is destroying our party."

Since Mitterrand has pledged to call legislative elections if he wins, the Communists would have a chance to raise their national percentage back to a level that maintains their influence. Giscard has pledged to call no new legislative elections until the expiration of the normal term of the National Assembly in 1983. That would leave plenty of time for the impression created by the Communist Party's low score to become permanent and perhaps even dip lower.

The party's apparent decision to go all-out for Mitterrand thus appears to be an attempt to save what can still be rescued from Sunday's disaster. The Communist dilemma was to risk losing its electorate by not following it or to risk the danger that, as president, Mitterrand could carry out his stated intention of working to reduce the Communist hold over French workers.