Just before a veil of secrecy descends on a new round of negotiations opening here Thursday between the United States and the Soviet Union to limit medium-range nuclear weapons in Europe, the propaganda war to win the hearts and minds of the European public on arms control has approached fever pitch.

Fearful that Moscow's strident complaints about U.S. intransigence were affecting Western European allies, President Reagan has tried to emphasize a more conciliatory theme by voicing his willingness "to explore every kind of proposal" that would lead to an equitable agreement with the Soviet Union.

Soviet arms expert Rostislav Tumkovski, meanwhile, was quoted in an interview with a Hungarian newspaper as dismissing the new U.S. optimism as "exaggerated and baseless" and saying that the Soviet Union now harbors "serious doubts about prospects" of reaching an accord.

Each side has sought in the past to appear to be more flexible than the other, and it was impossible to know whether these public positions reflected only preconference maneuvering or would be maintained in the talks.

The United States' more amenable posture has brought it closer to a growing consensus in Europe that an interim arrangement to limit the number of missiles on both sides may be the only feasible possibility for compromise this year.

Displaying signs of a new flexibility, chief U.S. negotiator Paul H. Nitze arrived here Tuesday following consultations in Brussels and Bonn and said the United States "is certainly not locked into zero option." That position calls for the North Atlantic Treaty Organization to cancel deployment of Pershing II and cruise missiles, scheduled to start at the end of this year, only if the Soviet Union agrees to dismantle its arsenal of nuclear-tipped rockets aimed at Western Europe.

On the Soviet side, Moscow's lament about U.S. reluctance to bargain seriously has changed in recent days to warnings that any deployment of NATO missiles could bring the world closer to nuclear confrontation.

Stopping in East Germany after a three-day state visit to West Germany last week, Soviet Foreign Minister Andrei Gromyko abandoned the soothing appeal that he had sounded in Bonn and warned that new missiles deployed by the West would cause "a serious aggravation" and compel the Soviet Union "to take countermeasures to ensure its security."

Gromyko's trip to Bonn served as a reminder that Moscow intends to direct its greatest propaganda barrage at West Germany, where mounting public clamor to halt installation of the nuclear missiles has overshadowed all other issues in the campaign for national parliamentary elections on March 6.

The opposition Social Democrats' candidate for chancellor, Hans Jochen Vogel, was warmly received in Moscow earlier this month by Soviet leader Yuri Andropov. Vogel, rejecting the U.S. zero option, would be willing to delay stationing new missiles on German soil if the Soviets agree to reduce the number of their rocket systems rather than dismantle all of them.

The Christian Democrats, led by Chancellor Helmut Kohl, have continued to stress support for the U.S. position but also speak of a compromise that would establish "concrete results" in the arms talks this year.

One U.S. diplomat said the conflicting campaign positions have posed "an inherent dilemma" for Washington, which would like to show progress "by moving toward compromise" while seeking "not to ditch the guy Kohl who has been fighting to support our position all along."

A bold move toward a more flexible negotiating line could harm Kohl by making him appear duped by a U.S. shift and provide his Social Democratic rival with the opportunity to claim that his views managed to nudge Washington.

The uncertainty over the German election is a key reason why many analysts predict that positions are not likely to change much until after the vote.

A Christian Democratic victory, it is believed, would reaffirm support for the tough U.S. position adopted so far, while a Social Democratic victory would place pressure on the United States to accept a deal with the Soviets that, at least initially, would achieve less than the zero option.

Regardless of the election outcome, very little information is expected to seep out of the negotiations unless Moscow or Washington decides to divulge bargaining positions for public relations purposes. Nitze and his Soviet counterpart Yuli Kvitsinsky have pledged not to break the confidentiality of their talks while they are in Geneva.

The starting point of Thursday's session from the Soviet side is expected to resemble the position outlined by Andropov in a Dec. 21 speech in which he offered to cut Soviet intermediate-range nuclear missiles to the total of 162 now deployed by the British and French deterrent forces in exchange for cancellation of the West's plans to deploy other medium-range missiles.

The United States, Britain and France rejected the notion of counting the two nations' nuclear arsenals in the negotiations, considering them to be independent forces. The Bonn government also expressed its disapproval, but the Social Democrats contend that the French and British forces must be counted in any overall assessment of the nuclear balance in Europe.

The Soviets may also voice their longstanding request to count aircraft as well as missiles in the negotiations if a broad agreement to limit weapons on the basis of warheads, as the United States suggests, is to be achieved.

The half dozen American delegates arrive Thursday morning at the sprawling Soviet compound to take their seats inside a gray, antiquated residence known as Villa Rosa.

The United States will host its share of the sessions in its arms control offices commanding a breathtaking panorama of Lake Geneva.

While the U.S. delegates gaze across the table at the opposing team with a soothing view of the lake in the background, the Soviets will be staring up at a triptych of Leroy Nieman paintings of track, baseball and basketball scenes.

Every Tuesday and Thursday, the delegations will alternate their meeting sites, shuttling along a winding street that is fittingly, or perhaps ironically, called the Avenue of Peace.