President Valery Giscard d'Estaing expects that a long and at times tense testing period between Moscow and Washington will dominate world politics for much of the year, but he thinks the testing will bring an eventual renewal of dialogue and strategic negotiations between the two superpowers.

The French president, who has remained in close contact with Soviet President Leonid Brezhnev and who is favorably impressed with the first month of Ronald Reagan's presidency, is believed to be considering several initiatives that would facilitate a new and broader East-West dialogue once the testing period is over.

He does not expect such an opening for several months. He believes that the Soviets, stung by Reagan's initial verbal attacks, have pulled back and are considering how to shape their own code of conduct toward Reagan rather than thinking about ways to seek a new international code of conduct.

He apparently is hopeful, however, that continued Soviet nonintervention in Poland and some movement in Afghanistan and other problems could create the atmosphere late this year for the convening of an East-West summit conference -- that would include not only Reagan and Brezhnev, but also the leaders of the world's other major powers -- to discuss global accords.

For the moment, the French president, who is facing a surprisingly spirited reelection struggle in which foreign policy will play an important role is saying nothing publicly about these impressions and his ideas on breaking the impasse that detente has reached.

He retreated into diplomatic silence when the subjects were broached for comment during a conversation in his Elysee Palace office yesterday.

He did conform that after receiving a response from Brezhnev, he is pushing forward with the proposal he made last month for an international conference on Afghanistan that conceivably could bring the United States and the Soviet Union together at a bargaining table.

Brezhnev's previously undisclosed Feb. 4 letter contained a reaffirmation of traditional Soviet positions on Afghanistan and other subjects, Giscard said, but did not constitute "a formal negative response." Instead, "it cited a number of problems such a proposal raises," falling short of a rejection, he said.

He said positive, responses have come from the Islamic Conference, his European partners and now from Pakistan, which had reacted with initial irritation when Giscard made the proposal on French television Jan. 27.

The Reagan administration has endorsed the plan, which Giscard said on television should bring together the Soviet Union, which has put 85,000 troops into Afghanistan, those countries that are accused by the Soviets of actively supporting the Afghan rebels and the permanent members of the Security Council.

The Soviet Union backs direct bilateral talks between the Kabul government of Babrak Karmal and Afghanistan's neighbors, an idea rejected by Pakistan.

Giscard's proposal on Afghanistan appears to be part of a much broader preoccupation he has with the troubled state of East-West relations and the outlook he has for the coming months. It is an outlook marked by the fact that when he speaks of the period of detente, he uses the past tense.

For him, the most reassuring spot on a cloudy horizon appears paradoxically to be the slow pace that the Reagan administration is taking in setting policy initiatives.

Associates say that Giscard is impressed that Reagan is "taking his time," in contrast to Jimmy Carter's initial burst of policy proposals. Giscard is known to feel that the welter of proposals led to four years of vacillation and confusion in American foreign policy.

The French president, who regularly receives lengthy letters from Brezhnev, senses that the Soviet leadership was in some ways relieved to see an end to four erratic years under Carter and was ready to define a new relationship with the Republican administration. If that disposition existed, in the French view it has been chilled by the accusations from Reagan and Secretary of State Alexander Haig that the Soviets are treacherous and support international terrorism. This view holds that it will be several months, while the Kremlin also continues to weigh and balance events in Poland, before that chill lifts.

Such a projection does not appear to worry the French leadership greatly. Giscard has no intention of saying so publicly in an election year, but recent statements do hint at a strong approval of Reagan's apparent commitment to build up American military strength before bargaining with the Soviets on strategic arms and other matters.

Those statements have centered on the need for a new "equilibrium" in superpower relations, which Giscard sees as having tipped in favor of the Kremlin since 1976 as the Soviet Union moved to fill a vacuum that, in Giscard's eyes, was created by Carter's policies and that he now hopes will be filled by reassertive America.

But Giscard also continues to caution against the dangers of triggering a major new arms race by an effort for American military superiority, which he feels the Soviet Union will not accept. For him, the threshold that should be passed appears to be the antiballistic missile treaty. Renouncing the treaty would raise expenses and risks of nuclear confrontation to unacceptable levels, he feels.

Giscard appears remarkably relaxed for a leader who suddenly is running neck and neck with Socialist Party candidate Francois Mitterrand in the most recent polls taken on this spring's presidential election. He has not yet announced, but Giscard is expected to launch during the first week in March what will be a 50-day campaign for a new seven-year term.

Earlier, widely held expectations that Giscard would be an easy victor in the two rounds of voting on April 25 and May 10 have been diminished by accusations that Giscard once accepted gifts of diamonds from deposed emperor Jean-Bedel Bokassa of the former Central African Empire and by portrayals of Giscard in the French press as an increasingly self-absorbed, monarchial ruler.

He has not yet responded in detail to the accusations, which appear to have personally wounded him, but it is likely that he will do so during the campaign.

Giscard's advisers expect foreign policy to be an area they can use to great effect in the campaign. They will argue that it would be dangerous to change leaders at a time when an air of crisis hangs over the superpowers' relations. This strategy could be particularly effective if Giscard can persuade the voters that he has proposals that will help break the impasse.

The French leader appears convinced in any event that the shocks over the last 18 months of Afghanistan, Poland and the failure of the United States to ratify the second strategic arms limitations treaty (SALT II) have brought an end to detente hopes that Soviet-American relations would continue to improve and world tension could be reduced.

The best that can be hoped for now, in his view, is a period of "stabilization," in which Moscow in particular will restrain its activities in the Third World and the two superpowers will accept an amended version of SALT II.

Poland is the major threat to such a formula, he acknowledges. But he is somewhat encouraged by his assessment that the Soviet Union continues to balance off the sharp reaction from the West that an invasion would bring against its perceived security problems, and continues to choose nonintervention. Economic help from the European Community and the Soviet Bloc gradually is helping to stabilize the Polish conflict, Giscard feels.