The political mood in France these days is as sensitive as the stock market, with attitudes among the public and within the parties jumping from euphoria to dejection at the slightest incident.

In March, for example, the coalition of Socialists and Communists known as the Union of the Left scored a resounding victory in the nationwide municipal elections, and that appeared to signal a left-wing triumph in the legislative elections due to be held next spring.

Indeed, French President Valery Giscard d'Estaing seemed so resigned to losing next year's election that he announced the other day that he would remain in office even if the leftists dominated the legislature. That disclosure depressed his own Republican Party as well as the Gaullists, who were already wobbly after their setback in the municipal contests.

But then, three months later, Giscard's prime minister, economist Raymond Barre, performed skillfully in a television debate with Francois Mitterand, the Socialist leader.

This immediately revived the hopes of Giscard's supporters that they and their allies could pull through next year, and analysts began to assert that the leftists had "peaked out" and would fall in the forthcoming confrontation.

It seems to me, however, that these flunctuations have had a divisive effect on both the conservative and the leftist groups, besides complicating and confusing the political picture.

Giscard's former prime minister, Jacques Chirac, the young and dynamic head of the rejuvenated Gaullists, is currently challenging the President rather than cooperating with him.

Chirac takes the view that Giscard ought to launch an all-out attack against the leftists, even though this could polarize political opinions through the country and even split France into two hostile camps.

Giscard, on the other hand, favors prudence in the belief that he can eventually persuade more moderate elements within the Socialist Party to break away from the Communists and join him, either before or after the next election.

With this in mind, Giscard has been promoting Barre, whom he regards as the kind of middle-of-the-road figure capable of appealing to the Socialist moderates. As a consequence, Chirac has been bidding to take over the conservative campaign as a way of advancing his own ambitions.

Meanwhile, the Socialists and Communists have been bickering and their squabble, paradoxically enough, stems from their success in the elections last March.

Georges Marchais, the Communist Party boss, was so heartened by the election results that, anticipating a legislative victory next year, he began to press the Socialists to adopt a platform of rigorous reforms to be applied after the coalition takes power.

These reforms include extensive nationalization of industry and extravagant social welfare expenditure as well as the adoption of a highly-centralized Communist-style economic system. Marchais also believes that the Communists should be an equal partner in the leftist coalition even though, in terms of electoral strength, they trail the Socialist.

But Mitterand, who wants to be the first Socialist prime minister since Guy Moliet governed in 1966 and 1967, is fearful of alienating France's middle class, which is cool to the Communist doctrine.