FRANCE'S ELECTION campaign is off to a highly uncertain start. President Valery Giscard d'Estaing is running for reelection against a fragmented opposition, and a month ago it seemed certain that he'd win easily. But the election is still five months away and the president, pursued by the scandal of the African diamonds, has reacted to it in a manner that is evidently beginning to dismay French voters.

It is a revealing example of politics in a system that, more than any other Western democracy, concentrates enormous prerogative in the hands of its president. It's the constitution that Gen. de Gaulle devised in 1958 to bring the country forceful and unified leadership, at the height of the Algerian crisis.

During his brief reign, the self-proclaimed Emperor Bokassa of Central Africa gave some diamonds to President Giscard and his family. A writer named Roger Delpey was doing a book on that interesting relationship when, earlier this year, the French state security court slung him into prison. The court, which works behind closed doors and exercises unusual authority, was also originally designed to deal with the threat of civil war over Algeria.

Last month the newspaper Le Monde published a report that the court had illegally dropped some of the documents from the Delpey file -- documents relating to the diamonds and the president. A week later, the government indicted Le Monde on criminal charges of insulting the judiciary in articles over the previous several years. Why such obviously vindictive retaliation? The answer isn't clear. Perhaps it's simply that the president's tolerance for serious criticism was declining.

And perhaps the government then sensed that it had gone too far. In any case, it suddenly released Mr. Delpey from prison. But if that was a political gesture, it came too late. Two weeks ago, by-elections were held for seven seats in parliament. Mr. Giscard's party was beaten in all of them.

But popularity and power remain two very different things in French politics. The president continues to hold, firmly, the center of the spectrum. On the left, the Communists are mainly devoting themselves to ensuring that the Socialists lose. On the right, several major candidates are sniping at each other. It's still probable that, in the final runoff next May, Mr. Giscard will once again confront the Socialist candidate, Francois Mitterrand and, as in 1974, defeat him. But it also seems probable that the campaign will reawaken all of the constitutional questions about the extraordinarily broad and unchecked power of a French president.