Former president Valery Giscard d'Estaing seems anxious to correct what he considers a mistaken impression: the idea that his Socialist successor, Francois Mitterrand, is turning out to be a more reliable ally for the United States than he was.

It is a perception that is largely based on Mitterrand's willingness as president of France to endorse publicly the deployment of American cruise and Pershing II missiles in Europe to counter the buildup of Soviet SS20s. While he was in office, the centrist Giscard never took such an unambiguous stance, and he now explains that he feared that doing so would encourage the Kremlin to lump France's independent deterrent together with U.S. nuclear forces.

"We didn't accept that French strategic forces should be included in the negotiations. I thought that if we took a formal stand on this, we were opening the door for the Soviet Union to count us in. This is exactly what has now happened," he said.

The former French leader invited a Washington Post reporter to his family chateau in the Loire Valley Easter weekend for a lengthy conversation in which he defended his position on Euromissiles and outlined what he believes is a way to break the current negotiating deadlock between Washington and Moscow.

Welcoming President Reagan's decision to move away from insistence on the zero option proposal, he suggested that the U.S. administration and its West European allies announce and implement a carefully phased program for deploying cruise and Pershing II missiles.

The advantage of such a plan, Giscard argued, is that it would allow the West to begin deployment at fairly modest levels while gradually increasing pressure on Moscow to negotiate. It thus would enjoy greater chances of success than either an attempt to get the Soviets to scrap all their SS20s, SS4s and SS5s at once (which is what the zero option amounted to) or an interim agreement on scaled-down arsenals of Soviet and U.S. missiles.

Last week Reagan proposed that the United States would "substantially reduce" the 572 Pershing II and ground-based cruise missiles scheduled for deployment in Western Europe from the end of this year if the Soviet Union reciprocated by dismantling part of its stock of SS20s. The offer was described as "unacceptable" Saturday by Soviet Foreign Minister Andrei Gromyko, partly on the grounds that it failed to take into account French and British missiles.

The choice by Giscard of the Euromissiles issue as the theme of the conversation--he refused to be quoted on other subjects--was perhaps significant. Foreign policy is a field in which he can justly claim to have much more experience than his most prominent rival for the leadership of the opposition, Jacques Chirac, the neo-Gaullist mayor of Paris who emerged as the main victor in municipal elections last month.

Giscard has been making an effort recently to get back into the limelight following a period in which he maintained a low profile after his election defeat in May 1981. By contrast with the aloof indifference to the idea of political image-making that he presented during the 1981 campaign, he is now seeking effective ways of projecting himself as a "modern" politician.

In private conversation, he displays some bitterness at what he evidently sees as the preference of the American news media for Mitterrand during and after the last presidential election. And, while he is careful not to criticize his successor directly, the strong implication of his remarks is that Mitterrand's declarations of solidarity with the United States have only complicated attempts to negotiate a compromise in Geneva.

The former president said his policy while in office was to avoid making public statements about the deployment of U.S. missiles in Europe since France does not belong to the military wing of the North Atlantic Treaty Organization. In private, however, he made it clear that he supported NATO's plans to deploy the missiles if there was no negotiated settlement by the end of this year.

Giscard said that during his term of office the Kremlin never seriously raised the issue of including French missiles in negotiations for balance in the European theater. The connection was first made in public late last year when the Soviet leader, Yuri Andropov, offered to cut the Kremlin's force of about 250 SS20s deployed in Europe to 162 to equal the total number of French and British nuclear missiles.

The Soviet proposal was rejected by both France and Britain. It has, however, become a valuable propaganda argument for Moscow in winning over public opinion in Western Europe. Gromyko raised it again last weekend when he asked sarcastically whether a missile labeled "I'm British" would not kill people just as any other missile would.

Giving his view of Soviet negotiating strategy, Giscard said he believed that the Kremlin had been gambling on the belief that it was possible to prevent the deployment of the new U.S. missiles altogether. The victory of the Christian Democrats in the West German election last month had represented a setback for the Soviets, but they had not entirely given up hope, he said.

Elaborating on a theme that he touched upon in an article for the Paris newspaper Le Monde in February, Giscard said the West should now make it clear that deployment would definitely go ahead.

"I think the West must say, 'We want to negotiate with the Soviet Union. We know that the chances for such negotiations succeeding at present are practically nonexistent, so we will create the conditions in which negotiations can produce results.' This implies, first, that there will be deployment and, second, that the method of deployment will induce the Soviets to negotiate," he said.

Illustrating his argument that the Kremlin at present has no interest in serious bargaining, he said it was impossible to imagine a new leader like Andropov agreeing to "unilaterally destroy" an SS20 force that had been built over a period of four years. There was no chance of negotiations succeeding as long as Moscow thought it was still possible to prevent deployment, he said.

Giscard said it was regrettable that Western European leaders had not taken what he called "a historic opportunity" to propose an initiative for breaking the impasse. Since there was no chance of agreement on the zero option formula, he said it would be "an act of statesmanship" to present to the Soviets a program for phased deployment of U.S. missiles that could be stopped at any intermediate level.

"If you just mention a single level of deployment, then what does the Soviet Union have to negotiate about? Nothing . . . because the first stage in such a negotiation would be to reduce its own level of deployment. If you only propose one level of deployment, then it has to be rather high in order to be credible. But if you phase your deployment, then you can start low," he said.

Giscard said that a program of phased deployment also would be easier to sell to western public opinion since it could be presented as a transitional step toward an eventual "zero objective" involving the dismantling of intermediate-range weapons on both sides. graphics/photo: Such a plan, Giscard d'Estaing said, would allow the West to begin deploying missiles at modest levels while gradually increasing pressure on Moscow to negotiate.