President Carter's uncertain foreign policy is unwittingly contributing to stepped-up contacts between the Soviet Union and left-wing leaders of West Germany's ruling Social Democratic Party (SPD), whohave long opposed Chancellor Helmut Schmidt's devotion to Washington and the North Atlantic alliance, NATO.

Playing the prime role in these very private contacts is Egon Bahr, secretary general of the SPD. Bahr had extensive talks with Soviet President Leonid Brezhnev shortly before Brezhnev's highly successful visit to Bonn in May. He renewed his Moscow talks in July - not as an emissary of Schmidt, but as agent of the SPD's activist left wing and the party chairman, former chancellor Willy Brandt.

Here is the specter of what has always made the Western alliance tremble: a menacing new version of the 1922 Rapallo Soviet-German treaty. Another Rapallo is certainly no possibility in the near future. Indeed, it is unthinkable in Helmut Schmidt's Germany.

Nevertheless, what is clearly at issue in the Brezhnev-Bahr talks is chilling, even though no immediate threat: West Germany leaving NATO with Soviet guarantees against aggression and with the ultimate prospect of German reunification.

Bahr, a fanatical German nationalist, leads SPD's far-left faction, which believes the key to Germany's future reunification is held in Moscow, not Washington. To Bahr and his allies, common political ground will eventually be found between Soviet communism and European socialism.

Political sources in Europe, both West and East, provide the following outline of Bahr's Soviet contacts in Moscow and Bonn, together with collateral talks between other leaders of the SPD's left wing and Eastern European Communist officials.

Bahr attacks the Carter foreign policy, particularly the administration's intention "to play the China card." According to one qualified informant Bahr agreed with Moscow that the Soviet Union "would not accept this" and would retaliate "in Africa, in the Middle East, in Asia and Western Europe," possibly with a "Berlin crisis." Bahr contends West Germany might not survive a new Berlin crisis.

The impact of Carter's human rights policy on the Soviet Union threatens Soviet reaction that could put pressure on West Germany.

Given those alleged dangers to West Germany, Bahr has asked the Soviets what sort of "guarantees" they could offer "in case of a crisis" between the superpowers.

To facilitate such a cataclysmic shift in West Germany policy, a major propaganda operation has been discussed with this purpose: to dramatize perceived dangers of U.S. policy and Carter's "inexperience in foreign policy." It would start by denying the United States "unlimited power" to dictate Western policy to West Germany on strategic arms limitation, Communist China, the Third World and Europe itself.

Bahr and the SPD left wing have exploited Carter's stunning decision not to produce neutron weapons by constantly reminding Schmidt how erratic U.S. policy is today, Schmidt had privately informed Carter he wanted the neutron warhead (widely opposed inside the SPD) and was left holding the bag when the president ruled against it.

Although Bahr greatly influences the SPD's left wing (about one-third of the party), he lacks power within Schmidt's coalition government. Still, Bahr's wide-ranging explorations pointing toward a possible second Rapallo build rising pressure on Schmidt.

Widely noticed in the Carter administration is Schmidt's heavy emphasis on East-West detente since Brezhnev's Bonn visit. The chancellor also rejected an appeal from French President Valery Giscard d'Estaing for French-German "coordination" in blocking Soviet adventures in Africa.

Schmidt is becoming trapped in the continuing uncertainty of Carter's Washington: He bodly opposes his left wing on neutron weapons, and the president pulls the rug out; he yields to his left wing on Africa, and a common Western European policy is sacrificed.

"U.S. leadership is not a realistic factor in the great struggle with the Soviets today," one European experton German politics told us. "There is all too obvious a lack of declared will." That lack of will, and the inconsistency that results from it, is what gives Egon Bahr and his supporters their license to hunt in Moscow for a radical change in West German policy at awesome risk to the West.