French President Francois Mitterrand is determined to remain in office even if the conservative opposition wrests control of the National Assembly away from his Socialist Party in elections next March.

Mitterrand, who is beginning the fifth year of his seven-year term, privately acknowledges that it will be difficult for the Socialists to retain power in the parliamentary vote next March.

But in recent weeks he has launched a sophisticated effort to keep control of the political situation in spite of the expected loss.

That decision is important not only for Mitterrand's opponents here, but also for the United States and other foreign governments, which must decide if they will deal with Mitterrand as a lame duck whose power already has begun to erode.

Jacques Chirac, the leading figure in the opposition, also is adopting a strategy to deal with an unprecedented division of power between a left-wing president and a right-wing Cabinet. Chirac will challenge Mitterrand early but will try to create a situation in which it will be the president who must take the responsibility for creating a constitutional crisis.

This equation emerges from conversations with authoritative figures in Mitterrand's Elysee Palace and in the Paris City Hall, where Chirac reigns as mayor.

The major unknown factor is the effect of the guerrilla campaign that is being waged from the small, crowded Left Bank office of former prime minister Raymond Barre, who without the support of a political party or a staff is sharpshooting effectively at both Mitterrand and Chirac and their grand strategies of turning the March elections to their advantage.

Still 10 months away, the elections already are stirring intense activity and comment here. The results will usher in one of the most important and complex periods in modern French political history and are almost certain to present the first severe test of the constitutional system devised by Charles de Gaulle a quarter of a century ago.

Under that system, the Cabinet, headed by a prime minister appointed by the president, drafts and implements all significant legislation. The powers of the Cabinet are clearly defined, while the authority of the president is described in vague terms.

Until now, the president and the prime minister have been from the same side of the political spectrum. But current polls show that the Socialists and their national leaders are losing ground steadily. The opposition leaders estimate at this moment that they will capture at least 320 seats in the 577-member assembly and will have the power to vote out of office any Cabinet that they do not dominate.

Mitterrand and his aides apparently have become resigned to the loss of the strong parliamentary majority the Socialists won with ease one month after Mitterrand was elected in May 1981. Their efforts now focus on limiting the losses and seeking to dominate what could be a confusing two-year period between the legislative elections and the next presidential voting in 1988.

At the moment, Mitterrand projects an air of serenity about all this to visitors. He reflects philosophically -- and with undisguised satisfaction -- on the fact that his is the first leftist government since the French Revolution to wield power for more than a year.

He says that the Socialists should build their campaign for next March around what he sees as a record of expanding civil liberties, bringing new blood into a governing system that had been dominated by a relatively small group and running a scandal-free administration.

In contrast to other Socialist leaders, Mitterrand does not appear to feel that his party should campaign on economic issues, which the conservatives believe will turn the electorate decisively in their favor. The conservatives will emphasize the Socialists' economic record, which they describe as disastrous and which even Mitterrand's supporters characterize as lackluster.

Confronted with the likelihood of a major defeat, Mitterrand already has introduced legislation to change the legislative voting system. The system of proportional representation that the Socialists are expected to push through the National Assembly without much trouble will fragment voting power in the assembly and conceivably could leave no one party or bloc in control.

That would leave Mitterrand with a relatively free hand and with an ace up his sleeve: the president holds the power to dissolve the assembly at a time of his choice and call new elections.

That card also is important in the more likely result, which is that the loose political group led by Chirac, former president Valery Giscard d'Estaing and Barre will have an overall majority and be able to dominate the parliament.

Mitterrand is determined to stay and play out the political game created by such a situation. He would resign only in an extreme circumstance in which he would be compelled to sign laws or take actions that were personally offensive to him.

He is not defining such circumstances. But it does seem clear that one change already announced by the conservatives as necessary -- the denationalization of banks and some industries taken over by the Socialist government -- would not fall into this category.

Chirac's strategy will be to help form and possibly even head a Cabinet that immediately would set to work salvaging the economy and rolling back what he sees as the excesses of the Socialists. He would argue that France urgently needs to find its way out of the crisis the Socialists have created, and if Mitterrand is prepared to stand by and watch this program implemented, the constitutional clash can be avoided.

But Barre, who last week virtually announced his candidacy for the next presidential elections by saying that he was beginning to measure public support for such a move, thinks the effect of such a division of power would be devastating for France and for the chances of the conservative candidate in the 1988 elections.

He is urging Mitterrand to resign immediately after a defeat in March and to seek reelection if he wants. An early election would undoubtedly aid Barre's candidacy by diminishing the advantages of the large party apparatus assembled by his rivals on the right.

Moreover, he sees Mitterrand's willingness to have a conservative Cabinet in charge for the next two years as a clever trap. That government would have to take unpopular austerity measures to revive the economy, in his view, or allow it to stagnate further.

The conservatives then would be vulnerable to Mitterrand's power to dissolve the assembly before the end of his term, call new elections and blast the conservatives for France's problems.