The alarming decline in relations between the United States and West Germany at a time when Washington desperately needs big help from its strongest ally was dramatized in a sharp question by a U.S. congressman to top political leaders here at a private dinner one night last week.

"What we want to know is: what are you prepared to do for us right now, in the next 10 days?" asked Missouri Democratic Rep. Robert Young in a refreshing departure from the veiled language of diplomacy. In less direct form, Chancellor Helmut Schmidt has repeatedly been asked the same question by President Carter, by U.S. diplomats here and by leaders of the conservative Christian Democratic Union running against Schmidt in the October election. The question symbolizes a subtle transformation from a quarter-century of interlocking intimacy between the United States and West Germany to a certain distancing resulting from Carter's flawed leadership, geography, Bonn's fear of Moscow and its self-absorption.

Schmidt's hedging is beginning to anger even some of his own leaders in the ruling Social Democratic Party. Indeed, West Germans of all persuasion find themselves today steeped in gloom mixed with genuine fear. For behind the public reproaches hurtlng across the Atlantic between Carter's men in Washington and Schmidt's suspicious policy-makers here, West Germany, in the words of a key Schmidt lieutenant, is "deeply, deeply worried." It sees the shadow of Soviet might, compared with that of the West, enlarged to its greatest dimension since World War II. It sees peril for an investment of 10 years in detente that has yielded enormous economic profit and partly bridged the two Germanys.

"Of course we will back the U.S. in Iran and Afghanistan," one Schmidt foreign policy adviser on West Germany's relations with Eastern Europe told us. "But you should limit your problems in the Middle East to the Middle East and not bring them to Central Europe."

Such wishful thinking recalls the first three years of Carter's own self-deception about the U.S.-Soviet struggle. But even Carter finally accepted linkage after the Soviet invasion of Afghanistan, compelling the president to shelve his cherished SALT II treaty.

The reluctance of Schmidt's government to do the same hints that at least some leading politicians in his inner circle, despite vehement denials, believe that Bonn's investment in detente and its thriving economic business with Moscow and Eastern Europe have become more important than playing follow-the-leader with Washington.

During haggling over their joint statement after Schmidt's Washington visit last month, the German chancellor fought for a paragraph committing Carter to "maintain the framework of East-West relations." But last week, Zbigniew Brzezinski, Carter's national security adviser who is suspiciously viewed here, strongly implied to the American Society of Newspaper Editors that it would not be "seemly" to conduct "business as usual with the Soviets when Afghans are dying by the thousands under the force of Soviet arms." That infuriated Schmidt insiders. Carter's own tough talk to the editors was received with only slightly less hostility.

The split between Bonn and Washington will not begin to cure just because Schmidt in the end acceded to economic and political pressures against Iran (which he will) and goes along with the Olympics boycott (which he will). That Schmidt could show such public resista nce to Carter on Iran -- a crisis that as yet has not even directly involved Moscow -- forbodes far deeper conflicts over future Soviet moves with a direct East-West connection.

Compounding the mutual reproaches between Carter and Schmidt is the fact that both are waging reelection campaigns. Having been left in the dust before by sudden Carter swerves in the road, Schmidt is contemptuous of Carter's alliance leadership. But more than that, he fears that dumping detente, the policy that came out of the womb of his own party, would be political suicide in his own election campaign.

So the deeper impact of the Carter-Schmidt estrangement has by no means been felt as yet. Although their two countries will surely survive the current storm, there is not the slightest doubt here that worse lies in the future.