In a move that could boomerang against President Valery Giscard d'Estaing, the Soviet Union has publicly praised his performance as France's leader with an article in the Communist Party newspaper Pravda that virtually endorsed his reelection bid.

In a Pravda analysis of the upcoming French election, Giscard was described as having "gained personal authority as a prudent and careful politician, especially in the international arena, where French positions have been consolidated these last few years." The criticisms that he is responsible for unemployment, inflation and the trade deficit are laid by Pravda at the door of Giscard's rivals.

The Soviet expression of preference has made Giscard's supporters nervous, the Communists furious and the Guallists and Socialists gleeful for the opening it has provided to charges that the incumbent president was too soft on Moscow.

"Brezhnev Votes for Giscard" headlined the pro-Gaullist newspaper Le Quotidien over a front-page photograph of the Soviet leader placing his ballot in a voting urn. "Brezhnev Chooses Giscard," headlined the pro-Socialist Le Matin. The Presidential Palace maintained an obviously embarrassed silence.

"This was not a good turn by the Russians," said an important official privately. "How maladroit. They couldn't have chosen a worse way to help him."

Goerges Marchais, the Communist Party leader and presidential candidate, told a provincial campaign rally, "I recognize no one's right to mix in this country's affiars. French policy is set in Paris, nowhere else." He denied that Pravda had in effect endorsed Giscard, however.

Marchais is described by the Pravda analysis as a good candidate with no chance due to the "anti-Communist campaign" of "the bourgeois mass media" helped by the pro-Socialist press. Socialist candidate Francois Mitterrand is said to have shifted to the right, to be favorable to the Atlantic Alliance and to have an unclear and contradictory program opportunistically designed to catch the votes of the discontented.

The fourth major candidate, Gaullist Jacques Chirac, is dismissed by Pravda for being more attached to free enterprise than Giscard and advocating a "firmer" foreign policy.

A Gaullist Party spokesman said that the Pravda article shows that "the Soviets prefer a weak president" in France. A Socialist spokesman called the Pravda article an "endorsement" of Giscard and predicted that French voters would punish this "unacceptable interference" in French internal affairs.

The Soviet Union has always made known its preference in French presidential elections, tipping its hat in turn to Charles de Gaulle and to his two successors, Georges Pompidou and Giscard. But this was the first time the Soviets moved at the start of the campaign.

When Giscard ran in 1974 for his present seven-year term, the Kremlin waited until just before the runoff election between the top two vote getters to signal its choice by sending Soviet Ambassador Stefan Chervenenko to make a highly publicized call on the then-finance minister at his office in a wing of the Louvre Palace.

Then as now, Giscard's main opponent was the Socialist Mitterrand. Marchais then protested the Soviet action in far stronger terms. But 1974 was the heyday of East-West detente, and Giscard may actually have been helped. This time, Giscard must defend himself against accusations that he broke the Western ranks with his surprise trip to Warsaw last May to see Leonid Brezhnev after the Soviet invasion of Afghanistan.

Meanwhile, there were news leaks in Brussels that France had asked the European Community to rule that French farmers could export 600,000 tons of wheat to the Soviet Union without being considered to have violated the U.S.-requested embargo. Britain and West Germany made it known that they would veto the French request, and a U.S. official in the Belgian capital said Washington would consider the sale a violation of the embargo.

Pravda's praise of Giscard could have the perhaps unintended effect of neutralizing the negative effect on anti-Giscard Guallist voters.

Mitterrand's main argument to attract Gaullist voters this time was that, since the Communists sabotaged the Socialist-Communist leftist unity pact by acting to prevent a leftist majority in parliamentary elections three years ago, he could no longer be regarded by moderate voters as being a prisoner of the Communists. The Communists in turn tried to undo that advantage for Mitterrand by conceding that they have no one but him to vote for in the runoffs but stressing that a man who wins with Communist votes would have to pay off the party with a portion of power.

The dominant reaction here is tht the Soviets were simply hamhanded in their attempt to influence the complex French situation. But it cannot be excluded that they knew what they were doing and wanted to destabilize France just as it appears, from the Reagan administration's perspective, to be the most effective advocate of a strong defense in Western Europe compared to troubled Britain, Italy and Spain and a West Germany listening more closely than ever before to neutralist siren songs.

There are also strong indications that, whether the Soviets were out to help or harm Giscard, an interesting byproduct of their move was most probably to help the old Stalinists in the French Communist Party to undercut the position of Marchais. There is an alliance of circumstance between the pro-Soviet wing of the French party and the Communist liberals and intellectuals to dump Marchais over the unorthodox racist and strong-arm tactics he has been using in an apparently desperate attempt to avoid a rout at the polls. The general assumption is that, if he does as badly as expected, his party rivals will move in for the kill soon after the elections.