Gaullist leader Jacques Chirac today complicated French President Valery Giscard d'Estaing's already difficult reelection effort by issuing a tepid personal endorsement of the president in the coming runoff vote and leaving his own followers free to vote as they choose.

Chirac, who helped Giscard to victory in 1974 but angrily broke away from him two years later, suggested at a press conference that the centrist president could get full Gaullist backing only by pledging now "to reform profoundly his policies and methods." The deep personal bitterness that exists between Chirac and Giscard makes that an unlikely prospect.

The results yesterday of the first round of the two-stage election placed Giscard slightly ahead of Socialist leader Francois Mitterrand for the runoff May 10 between the top two vote getters. Both of the remaining candidates are dependent on the tactics of the runner-up in their respective center-right and leftist camps.

The final results gave Giscard 28.3 percent of the nearly 30 million votes cast, Mitterrand 25.8 percent, Chirac 18 percent and Communist leader Georges Marchais 15.4 percent.

The two big losers were Giscard, who lost more than 4 percentage points compared to his showing of 32.6 percent in 1974's first round, and Marchais with 6.1 points less than the last Communist presidential candidate in 1969. Marchais scored even lower than the previous postwar low point in Communist fortunes, when the party got 18.9 percent in the legislative elections of 1958 after Gen. Charles de Gaulle's return to power.

Communist weakness is likely to make it harder than it has ever been for Giscard to invoke the traditional incumbent's appeal for moderates to rally together against the Communist threat. Mitterrand already has said he would not grant the Communists the Cabinet posts they have been demanding unless they loosen their ties with Moscow, denounce the Soviet invasion of Afghanistan, drop their appeals to working-class racism and reaffirm a commitment to democracy that they have been deliberately soft pedaling.

The rise of the Socialist Party as the dominant force onthe left poses a major dilemma for the Communist Party's leadership, which is expected to try to cut the Socialists down to size by sabotaging Mitterrand's election, despite the massive desertion of Communist voters disenchanted with the Communist Politburo's efforts to scuttle leftist unity. Presistent Communist attacks on the Socialists aborted a widely predicted leftist victory in the 1978 parliamentary elections. a

In a front-page editorial, Rene Andrieu, editor of the Communist newspaper L'Humanite, gave a broad hint today that a special meeting Tuesday of the party's Central Committee can be expected to be about as lukewarm in its endorsement of Mitterrand as Chirac was today toward Giscard.

"Our party's decline in this election," Andrieu said, "does not create the best conditions to achieve the necessary political changes."

However opaque that may seem to outsiders, it will undoubtedly be understood by Communist militants to mean that the party is in no position to allow the victory of a Socialist president whom it cannot sufficiently influence.

Chirac today also used opaque language to tell his supporters that Mitterrand's past associations with the Communists made it impossible for Chirac to vote Socialist, whatever his feelings about Giscard.

"Whatever his intentions," Chirac said, "Mr. Mitterrand, with an economic program that has failed wherever it was applied and with political alliances from which he did not want to untangle himself, would be unable to succeed. Indeed, I cannot ignore the consequences for France that would be involved in the Communist Party's sharing of power."

After dismissing Mitterrand's chances of success, Chirac was harsh toward the president: "There remains Mr. Giscard d'Estaing as an alternative. Can he succeed when, as the incumbent president, he only attracted little more than a quarter of the vote? To do so, he would have to reform profoundly his policies and his methods, to take measures to increase production to reduce unemployment, reduce the state's tax burden on the nation, conduct foreign and defense policies that are firm and clear."

Referring to the Gaullist complaint that they were systematically ignored and frozen out by Giscard, Chirac added that nothing would be accomplished by the president "if mutual respect and dialogue and not restored."

Since he was automatically eliminated from the race, there is no reason for him to make a formal endorsement of anyone, said Chirac.

"On May 10," he said to loud cheers from his party workers, "everyone should vote according to his conscience." Only after having said that did he add, "In a personal capacity . . . and following the political line I have always followed in favor of a certain type of society, I have no choice but to vote for Mr. Giscard d'Estaing."

This frees left-leaning Gaullists, many of whom are former Socialists who rallied to De Gaulle as the symbol of wartime resistance, to vote for Mitterrand. It also absolves in advance Gaullists who are considering staying at home or casting blank ballots in the runoff.

Giscard's campaign headquarters immediately expressed its satisfaction and said it would welcome Chirac's participation in political rallies for the president. But a Chirac spokesman said privately that he thought it was doubtful that the Gaullist leader would do anything to help Giscard.

Chirac's statement, said his spokesman, "is not an appeal to vote for Giscard. If it is interpreted as such, an official correction will be issued. And that comes from the horse's mouth."

Mitterrand already has said that, if elected, he would dissolve the present National Assembly dominated by the nominal Gaullist-Giscardist coalition, and hold new parlimentary elections. Chirac would then become leader of the center-right opposition to the Socialist president's party. But that would happen only if Giscard is eliminated.

The president served notice that he intends to fight the second round of the campaign energetically. Without bothering to take the traditional breather between the two rounds, Giscard tonight went back to the campaign trail with speeches in Dole and Dijon, the fief of former Premier Edgar Faure, one of the shrewdest observers of the Franch political scene. Echoing the Giscard campaing organization's view that every vote counts, Faure predicted that Giscard would win by 50.5 percent.