The French presidential campaign got under way in earnest this weekend with socialist leader Francois Mitterrand's announcement of his candidacy against President Valery Giscard d'Estaing.

Speaking today at the home town of Jean Jaures, the founder of the French Socialist Party, Mitterrand set forth what seems certain to be the general tactical approach of his campaign by launching a double attack against the leadership of the French Communist Party and Giscard's record.

The 64-year-old socialist leader, who came within 300,000 votes in 1974 of defeating Giscard, 54, when discounting the votes registered outside mainland France, appealed over the heads of the communist leaders to the communist voters.The communist electorate was also a large portion of the 45 percent vote that Mitterrand won against president Charles de Gaulle in 1965.

Significantly absent from Mitterrand's first real campaign speech were any attacks on Giscard's nominal partners in government from the Gaullist Party. The Gaullists have also been directing most of their ammunition against Giscard and the communist leaders as the grand strategy for next spring's two-round election becomes increasingly clear.

Barring a major upset, Giscard and Mitterrand are bound to be the contenders in the runoff on May 10 between the top two vote-getters. Whatever formal positions they may take for the first round of voting, France's four main parties are already maneuvering for the runoff.

The real lineup of the two camps is already set, with communist leaders working behind the scenes for Giscard's reelection and much of the Gaullist leadership working for his defeat. Giscard is the founder of the Independent Republican party, but he has gathered support from across political lines in the past.

To win, Mitterrand must hold the bulk of the communist electorate, which has already voted for him twice, and get a significant portion of the Gaullist vote to compensate for his losses on the communist side and to give him the margin of victory.

Mitterrand's announcement that he would run was immediately greeted by an obviously reluctant announcement from his challenger inside the Socialist Party, Michel Rocard, that he would honor his 18-month-old pledge not to be a candidate against the party leader.

Rocard, 50, has been shown to be more popular than Mitterrand in opinion polls. But in the ideological atmosphere of French politics, he also has the image of being a socialist-oriented Giscard. The communist leadership seems already to have persuaded a large percentage of its voters that he is a traitor to the left -- a label that the communist leaders would have far more difficulty pinning on Mitterrand since they have twice called on their followers to support him.

Giscard is bound to be the main target of all the major candidates. Even communist leader Georges Marchais will not be able to avoid attacking the government's economic policies.

The Gaullists, who are apparently going to be split among three candidates -- party leader Jacques Chirac, former prime minister Michel Debre and Marie-France Garaud, a backroom party powerhouse who has decided to go public because she says none of the men in the party are speaking out strongly enough against the Soviet threat -- will be attacking the president primarily on defense and foreign policy.

A new book by Mitterrand indicates he will assail Giscard on a broad variety of issues, including his increasingly monarchical style, his lapses on civil liberties, his attempts at indirect censorship and control of media and various scandals that have come to light such as his close relations with and acceptance of diamonds from the Central African emperor, Jean-Bedel Bokassa, before he was deposed. It seems obvious that Rocard would not have mounted as violent a campaign against Giscard as Mitterrand seems to be planning.

The election of Ronald Reagan in the United States was seen at first blush by commentators here as being unfavorable to Mitterrand since it seemed to confirm righward move by electorates in Western democracies. But the Reagan victory also shows that incumbent presidents are not necessarily unbeatable, that Mitterrand's age is not necessarily as much a disadvantage as Rocard had been saying and that there may be a general mood against leaders who can be accused, like Giscard, of being too accommodating to the Soviets.

This adds up to a picture of a far closer race than most foreign diplomatic and media analysts had been reporting to their home audiences from France. Giscard apparently understands the dangers he faces. Instead of taking a position above the fray that is normally expected from a comfortable French front-runner, he has started dropping some of the aloofness for which he is being criticized.

On Tuesday, without announcing it in advance, Giscard and his wife flew to their home in the Auvergne region on a regular domestic flight instead of the presidential plane. The president of the republic stood on line with the other passengers holding his own suitcase in his hand. The private voyage was a major headline item in the French news weeklies.