FACING a stagnant economy and bellwether elections next spring, France's political leaders are engaged in anti-foreign rhetoric that reflects a growing xenophobia across Europe.After Prime Minister Edith Cresson suggested this summer that "charters" might be engaged to take unwanted illegal immigrants back home, former President Valery Giscard d'Estaing last month described as an "invasion" the number of foreigners arriving in France. This week the rhetorical heat was turned up another notch, when a former interior minister under Mr. Giscard d'Estaing, Michel Poniatowski, called the situation an "occupation." He could have chosen no more emotionally charged word: "Occupation" still conjures images here of Nazi control during World War II. The rhetorical buildup reflects the country's political shift to the right, as well as the preoccupation of leaders with winning back voters who have deserted traditional parties of the left and right wings for the far-right National Front of Jean-Marie Le Pen. Giscard appears to have abandoned his long-held conviction that elections here are won in the center, political observers here say, for a position unambiguously leaning to the right. Latching onto the National Front's drive to safeguard what it calls the "French identity," Giscard called for legislation to change the granting of French nationality from a right of birth on French soil to a privilege of bloodline. Going further, Mr. Poniatowski used the word "occupation" in a speech recommending an alliance between the traditional right wing and the National Front. Without such a union of the right, he said, France was destined "to become an African and socialist boulevard given over to anarchy and decadence." Debate over immigration continued as the French National Assembly adopted tough new legislation Tuesday against employ- ment of what the French call clandestin (illegal) immigrants. The new law proposed by Mrs. Cresson, and strengthened in the Assembly, allows companies found to employ illegals to be banned from public projects, and includes up to three-year prison sentences for employers. The French focus on immigration reflects a deepening anti-foreigner sentiment across Europe. Violence against foreigners stung Germany two weeks ago on the first anniversary of the country's reunification. With more than 200,000 asylum seekers, the German government has proposed setting up camps to house the foreigners safely until their immigration status can be determined. The proposal is causing its own controversy because of historical connotations of camps in Germany. At the same time, anti-foreigner violence is mounting in Italy, Czechoslovakia, and Spain, which until recently was not a common destination for immigrants. The irony of France's situation is that while the country has seen very little anti-foreigner violence, it is the political leadership that has led the assault. The renewed dynamism of the National Front (FN) and an attempt to fill a "troubling ideological void" are the main reasons for this, says Pascal Perrineau, director of the Center for French Political Life Studies here. He warns, however, that using the immigration issue is like playing with fire. "After a certain stagnation during the Gulf war, the National Front is taking off again," says Mr. Perrineau. A string of recent special elections reveal the FN "is the only party, besides the Greens, with any electoral dynamism," he adds, "and that is shaking up both the left and the right." With regional elections set for next spring, before national legislative elections in 1993, political leaders "seem to believe that talking like Le Pen will win back his supporters," says Perrineau. Noting, however, that Paris Mayor and former Premier Jacques Chirac gained nothing in polls after citing in a speech the "noise and the odor" of foreigners, he doubts the viability of that strategy. For Perrineau, the rise of immigration as a political issue also reflects the ideological void left by the collapse of the country's traditional left-right cleavage. On the economy, defense, European development, and the approach to unemployment that may soon reach 3 million, the ruling Socialists and the opposition parties of the center-right differ little. "In poll after poll, however, immigration is cited as the only issue that separates the left from the right," says Perrineau. "The political leadership wants to use the issue to fill the void." The danger, he adds, is that "the clandestine orchestra leader of this debate is Jean-Marie Le Pen" and the far right. "This debate cannot help but provide [Le Pen] with growing legitimacy."