The heat of the struggle between leftists and conservatives for political control of France is beginning to warp the edges of President Valery Giscard d'Estaing's once-adventurous foreign policy.

Giscard continues to hold the central foreign-policy themes he has enunciated since displacing the Gaullists at the Elysee Palace in 1974. But the strong challence mounted by France's Communists and Socialists to the conservative cololition Giscard heads is altering France's tone on somekey issues.

Giscard's vision of a reshaped Common Market with more Mediterranean members has been shelved while farmers' votes are being sought. His tone toward israel has become more sympathetic. Africa is being handled with extra finesse. And foreign-trade policy is being hardened to show voters that their pocketbooks are being protected.

These adjustments appear to be largely defensive, intended to minimize potential vote losses. But the president's political advisers also expect him to go on the offensive and to use foreign-policy issues to win votes in the campaign, which began unofficially this week with the end of the August summer vacation. The elections for a new National Assembly are next March.

In sharp contrast to the prickly independent stance of the Gaullist toward the United States in past political years, Giscard appears to be calculating that there are more votes to be won than lost in this election by subtly underlining his good ties with Washington.

He is dispatching his prime minister, Raymond Barre, to Washington in mid-September for meetings with President Carter and other officials that are expected to help build up Barre's image at home.

Moreover, Giscard and his principal foreign-policy aides plan to hammer during the campaign on the theme that a victory by the Socialist-Communist coalition could knock France out of the North Atlantic alliance and the Common Market and might lead to a withdrawal of the American nuclear umbrella, which they will acknowledge has helped preserve France's independence since World War II.

Their calculation appears to be that the Communists, seeking to emphasize their responsibility and commitment to Western democracy, are awkwardly placed to play on anti-Americanism in this campaign.

The Communists, who have recently endorsed France's nuclear striking force, have been trying to distance themselves from the Soviet Union in French public opinion and are stressing internal issues in their campaign statements.

The Gaullists, who could attack Giscard's "Atlantic policy" from the right, have also shown little inclination for that kind of campaign. Only an American rejection of New York landing rights for the Concorde supersonic jetliner could give the Communists and Gaullists enough ammunition to make American policy a large issue here.

So far, no single issue has surfaced that is likely to impair the conduct of France's foreign policy as severely as occurred in the United States last year when Ronald Reagan attacked Gerald Ford's support for detente and Panama Canal treaty negotiations.

But the campaign has already taken its toll on Giscard's strongly expressed commitment to Common Market membership for Greece and Spain - in part to support the democratic governments that have taken over from military rulers - and, to a lesser extent, for Portugal.

Giscard had originally hoped to establish France as the new political pivot or "center of gravity" for the Common Market by bringing in Mediterranean countries to balance the economic strength of West Germany and the other Northern European countries.

But the increasing gap between West Germany's economic recovery and the still-troubled state of the French and other Common Market economies has ruled out any immediate expansion of the Market to the south, and appears to have led Giscard to a fundamental reassessment of his original aim.

Almost all of the other French parties, led by the Gaullists, have quickly and strongly denounced the Spanish and Greek applications for EEC membership, which French farmers fear will bring ned surpluses of cheap wine, melons, tomatoes and other Mediterranean crops cascading onto French markets.

In a brief stopover in Athens in July, Prime Minister Barre privately told Greek officials that Giscard's support for Greece's application would have to be shelved for the period of the campaign. But he reportedly left them with the impression that Giscard would resume his efforts once the voting was out of the way.

Giscard gave no such assurances to Spanish Prime Minister Adolfo Suarez when they met in Paris Wednesday, however. An Elysee communique on the meeting stressed that France "will not sacrifice its abundant and productive Mediterranean farming" to bring Spain into the Market. Moreover, Spanish officials report that they were told before Saurez left for the official visit to Paris that France wants a new one-year study on the effect of Spanish membership before commiting itself to new steps on the application.

While France's 600,000 Jews lack the organization and economic power of American Jewry in politics, their votes are coveted in this tight contest and Israel appears to be benefiting.

Giscard has not made significant alterations in France's Middle East policy, which Israeli officials have repeatedly denounced as pro-Arab, but in the wake of the damaging Abu Daoud affair he moved quickly to erase the bitterness of that dispute and to invite then-Prime Minister Yitzhak Rabin to visit France. Now he has assured Israel that the invitation still stands for Menahem Begin, and talks will begin shortly on arranging a date.

The Socialist Party has usually been identified by Israelis as the most sympathetic French group. But the defeat of the Labor Party by Begin's rightist coalition has blurred that identification. Moreover, Gaullist leader Jacques Chirac is cast by many French Jews and Gentiles as the most pro-Arab of the major political leaders, giving Giscard something of an opening on the political spectrum.

French statements on the Palestinian question have become slightly more moderate, from the Israeli point of view, in recent months, moving from periodic endorsement of a "Palestinian State" to a more uniform and generalized support for a Palestinian "homeland" or "country."

To avoid overseas crises that could bounce back in the elections, Giscard is trying to minimize direct French involvement in the Horn of Africa where French troops are stationed in Djibouti, which received independence from France in June. He has also moved to brush up France's image both by shoring up anti-Communist black leaders in Zaire and elsewhere while trying to cut back France's long and profitable military association with South Africa.

Fear that a "protectionist" trade policy could emerge from the campaign are growing in the international fiancial community here.

Two weeks ago, Barre launched a call for "organized free trade" to replace the "ruinous competition" that is hitting French textile, steel, shoe and other industries hard. The government has not yet spelled out what he has in mind, but French manufacturers and workers are taking the pronouncement as a call for a more aggressive tariff policy to protect their jobs and profits.