The presence of the notorious leftist adventurer Regis Debray as a high official in the Elysee Palace heightens the alram aroused in Washington by Prsident Francois Mitterrand's new Socialist regime, despite his professions of concern deeper than his conservative predecessor over Soviet military might.

Debray was Che Guevara's boon companion and chronicler in his disastrous communist insurgency in Bolivia in the 1960s. High French officials assure the United States that Debray is a mere functionary in Mitterrand's political household, not engaging in policy determination. Nevertheless, his presence symbolizes a troubling mind-set by Mitterrand's French Socialists.

While claiming to be more attuned to Soviet expansionism and more aligned against neutralism in Western Europe than defeated conservative president Valery Giscard d'Estaing, the Socialist stand with Debray in supporting worldwide insurrection -- especially in Latin America. Therein lies the fatal plan of the non-communist left internationally: while opposing the Kremlin, it backs causes whose anti-Western animus is no less intense than Moscow's.

Nor is this the only disturbance beneath the facade of avowed greater anti-Soviet steadfastness here now that Mitterrand is in power. While decrying Soviet SS20 missiles aimed at Western Europe, the new government emphasizes negotiations to get rid of them rather than balancing Western weaponry. Furthermore, the presence of communist members in the new cabinet (even through the Socialists won an absolute majority of the National Assembly) undermines Franco-American relations more than anybody admits officially.

Vice President George Bush in a carefully modulated understatement after meeting Mitterrand June 26, said the presence of communist ministers was "bound to cause concern in Washington."

All this diminishes the importance of anti-Soviet rhetoric by Mitterrand and his foreign minister, Claude Cheysoon, in addressing the Reagan administration. Condemning Giscard for insufficient outrage over the Soviet rape of Afghanistan, they say it is not enough merely to say there must be no more Afghanistans but Moscow still should be pressured to free Afghanistan itself.

Privately Cheysoon goes beyond that, saying: we would return to pre-Gaullist adherence to U.S. leadership in the West if only Washington showed it stands for something. That something, however, embodies the contradictions of the non-communist left.

Cheysson actually is seeking U.S. endorsement of anti-Western, anti-democratic movements in the Third World supported by the Socialist International and its French members, including Francois Mitterrand. The new French president upholds the romantic tradition of the French left that invests an insurgency against authoritarian right regimes with the highest values of mankind. Whereas Chancellor Helmut Schmidt resists such dogma in his German Social Democratic Party, Mitterrand enthusiastically embraces it.

In Africa, where the practical interests of France intrude, Socialist support for the Polisario insurgency in Morocco is being tempered by dealings with King Hassan. The new government's ardent backing of SWAPO guerrillas in Namibia is kept within the framework of a unified Western European approach.

But in Latin America, there are no constraints on transforming the impulses of Debray into policy. French Socialist leaders point to repressive Nicaragua as a model for Third World democracy. Madame Mitterrand's patronage of leftist terror in El Salvador reflects the government's bias. Fedel Castro's warm exchange of letters with Mitterrand upon his election was not an aberration.

Nor can Mitterrand be excused for playing Third World parlor revolutionary on grounds he is measurably superior to Giscard in confronting Moscow. Unpleasant though he was in dealings with Americans (as indeed with Frenchmen), Giscard performed signal backstage services in stiffening West German resolve against creeping neutralism. Whether Mitterrand, while publicly denouncing Soviet expansionism, is similarly useful is yet to be determined.

However annoying Giscard could be, he never posed the prospect of communist ministers. French assurances that any communists will be excluded from international and internal security questions do not mollify Washington, which sees a bad example that finally could push Italy over the edge into a Christian Democrat-Communist Party coalition.

Conversations between France's new leadership and U.S. Ambassador Arthur Hartman (whose expected shift to Moscow conceivably could be cancelled by the change of regime in Paris) are intimate and candid. Hartman's hand in dealing with the French could be strengthened if President Reagan called for arms control talks with the Soviets. For now, the ambassador is urging the Mitterrand regime at least to look at German reports of the true Salvadoran situation before embracing the insurrection.

But the new problem in U.S.-French relations lies beyond the skill of even so experienced a professional diplomat as Hartman. Mitterrand, geniunely pro-democratic and anti-Soviet, represents the unwillingness of the democratic left to see Third World revolutionaries as auxiliaries for Soviet expansionism, a failure that has contributed to the decline of the West since World ywar II. Whether or not seven years in power will change this, that is where the new president of France begins.