French President Valery Giscard d'Estaing formally launched his reelection campaign today with a gloves-off speech warning that the election of his main rival, Socialist Francois Mitterand, would create a serious crisis for the nation.

The president's approach was apparently deliberately intended to recall former president Charles de Gaulle's traditional campaign theme that the choice for France could be summed up as "Me or chaos."

Giscard's combative style contrasted radically with his statements in 1974 during the campaign for his first seven-year term as president. Then he asserted that France needed to get away from turning politics into "a kind of civil war conducted by other means."

The difference is clearly a reflection of the judgment at the Elysee Palace that Giscard's reelection is by no means automatically assured in the two-round election April 26 and May 10.

"I welcome this campaign not as a trial but as a deliverance," Giscard said. "For seven years, reasons of state and the reserve imposed by my position forbade me to express myself with total freedom. Finally, I will be able to open my mind and my heart to you as man to man."

The first round of voting is often described as the French equivalent of an American primary election in which the challengers to the frontrunners inside the two main U.S. parties are eliminated. The two candidates securing the most votes then compete in a runoff two weeks later.

Giscard, representing his centrist alliance called the Union for French Democracy, dismissed his major rival in the governing coalition, Gaullist candidate Jacques Chirac, with a turn of phrase: "All the indications show with certainty that no other candidate [besides Giscard] has any chance to defeat the opposition."

This offhand dismissal of the Gaullist leader was an evident attempt to cut Chirac down. At the same time, others, including key Gaucardists, have been expressing their surprise about what an effective campaigner Chirac has become, especially in his attacks against Giscard, which have themes deliberately similar to Ronald Reagan's rhetoric such as the need to get the economy moving again by dramatically reducing the tax burden.

By concentrating his fire from the start on Mitterand as a danger to public order, Giscard apparently intends to try to rally the many Gaullists who have seemed tempted to vote for the Socialist. In a slick-paper campaign biography distributed today and containing previously unpublished interviews purportedly dating to 1974, Giscard is quoted as recalling how much De Gaulle always distrusted Mitterand.

"The opposition," Giscard said today, "remains what it has always been with the same leaders relentless in their struggle since 1958 against the Fifth Republic." De Gaulle founded in Fifth Republic in 1958 in what amounted to a legal overthrow of a government that could no longer cope with the Algerian war.As the Giscardists never tire of recalling, Mitterand denounced the Gaullist takeover in a book entitled "The Permanent Coup d'Etat" and ran against the general in 1965, forcing him into a runoff election when it was assumed that De Gaulle would win on the first round.

In his opening speech, Giscard struck a number of other traditional Gaullist themes. A Mitterrand victory, he said, would mean "farewell to nuclear independence and to the rank of France in the world." Mitterand would have no choice, Giscard said, but to govern with the Communists "or to betray their voters after having profited from their vote -- which would not take place without grave shocks." That was a barely veiled reference to the Giscardist view that the Communists would not hesitate to start a prolonged strike wave to enforce their will on a Socialist president.

Communist candidate Georges Marchais seems to be going out of his way to make that Giscardist theme as credible as possible by insisting at every opportunity that his party would demand communist ministers in any Cabinet that Mitterand might form. This sudenly introduced communist demand followed a period of several years in Marchais was accusing Mitterand of having turned himself into the secret ally of Giscard or Chirac.

Marchais and Giscard have a clear mutual interest in Mitterand's defeat, and Mitterand and Chirac share a similar interest in the elimination of Giscard.

The incument showed that he recognized that the biggest single danger to his reelection is the huge rate of youth umployment, with half of the country's steadily mounting pool of more than 1.5 million unemployed consisting of young people under 25. These people were not eligible to vote in 1974, when Giscard defeated Mitterand by 300,000 votes in the runoff out of a total of 26 million cast in mainland France.

Giscard is responsible for lowering the voting age from 21 to 18, with the result that 8 million more voters are eligible.

Both the Socialists and Gaullists have been hammering away at the unemployment issue and offering programs that are strikingly similar for the creation of new jobs despite rhetroric that sounds radically different.

Giscard has been hampered in the search for solutions by the insistence of his prime minister, Raymond Barre, that priority must be given to the fight against inflation. But Barre is being ostentatiously kept out of Giscard's campaign, which has been largely entrusted to Administrative Reform Minister Jean-Francois Deniau, who is already openly touted by the Giscardists as Barre's successor if Giscard wins.

"The central commitment of my campaign," Giscard said today, will be the solutions he offers for youth unemployment. The president indicated he would spend the next month campaigning primarily through television appearances and press interviews. Then, in the last month before the first round, he has an already elaborately planned program of 12 mass rallies throughout the country, starting March 28 in Paris.

The slow start and gradual buildup, Giscard strategists say, reflect a recognition that overexposure for a man who has been president for seven years in one of Giscard's worst risks. It is a concern that he shares with the old political warhorse, Mitterand, who put off the start of his campaign with a recent long visit to France's communist voters that aligning themselves behind a Kremlin that seems to favor Giscard's reelection is not the only option for a West European communist party.