As President Carter and West German Chancellor Helmut Schmidt prepare to meet in Venice this weekend, Washington and Bonn appear -- despite public denials -- to be at variance about a key element of European defense.

At issue is the stationing in Europe of nuclear missiles that are capable of reaching the Soviet Union, a decision taken by NATO last December.

With Schmidt scheduled to make a two-day visit to Moscow beginning June 30, Western concerns center on Schmidt's commitment to carrying out this decision.

They were prompted by a proposal Schmidt first floated in April that East and West freeze deployment of new nuclear missiles in Europe for the next three years. Such a freeze would provide time for disarmament talks, a move that Schmidt said in a speech this month "would serve peace."

Chancellory aides were surprised and upset by a letter from Carter to Schmidt last Friday in which the president reportedly took note of the chancellor's forthcoming Moscow visit, restated the NATO decision and specifically opposed Schmidt's proposed freeze.

The tone of the letter, which Schmidt's aides believe was deliberately leaked in Washington, was described by the West German magazine Stern at first as "rough." It was interpreted by the magazine as a warning by Carter to Schmidt not to pursue any new initiatives with Soviet leader Leonid Brezhnev that could be seen as weakening the NATO position.

The NATO decision to begin deploying, in 1983, 572 medium-range nuclear missiles at U.S. bases in West Germany, Britain and Italy, and possibly the Netherlands and Belgium, was a momentous one. Implementing it remains, in the view of many, the most important task now before the Western alliance.

This is because the decision represents the major part of NATO's effort to catch up with what Western military analysts say is Soviet superiority in nuclear forces in Europe. The Soviets have already begun deploying new SS20 mobile multiple-warhead missiles, plus scores of new Soviet Backfire bombers.

A source close to Schmidt called Carter's letter "factual" rather than "rough." The chancellor is said to have regarded the letter as a confirmation of his own thinking. But the fact that Carter was compelled to write a formal letter disturbed some West German officials.

Government spokesmen in Bonn and Washington have been saying this week that no differences really exist between Carter and Schmidt on the need to stand by the December decision. With the two leaders scheduled to meet at the seven-nation summit in Venice this weekend, this may be an effort to avoid any public rift between them. They have differed in the past on military policy.

Schmidt and his aides have tried particularly hard in recent weeks to dispel U.S. doubts about the West German leader's position. They have sought to portray him as still solidly behind the deployment decision.

But the issue has continued to dog him, in part because of the April proposal. Other factors reinforcing allied anxiety have included general U.S.-West German strains, German election-year politicking, and traditional Western suspicion about Bonn's increasingly friendly relations with the communist East.

The Soviet Union has rejected Schmidt's proposed freeze -- as well as U.S. offers to begin talks on limiting the new weapons, insisting that NATO reverse its December decision.

Schmidt, however, reiterated his suggestion in a June 9 speech before a convention of his Social Democratic Party. "I stand by what I said [in April]," he said.

This speech, coupled with concern in some circles in Washington that Schmidt would pose his offer directly to Brezhnev, prompted Carter's letter last week.

At one level, Schmidt's moratorium proposal would seem to be no real variation in the NATO position. Since the West's new weapons will not be ready for deployment for an estimated three years, it would seem there should be no objection to a freeze now, providing the United States could still proceed with missile production and particularly if the result is to get the Soviets to stop their current deployment.

But it is not that simple. Extensive preparations are required before the missiles are actually installed. NATO officials worry that an East-West freeze on installation would bar the West from going ahead with these preliminaries.

That is unacceptable to Washington because it would perpetuate the allies' numerical inferiority -- exactly what the hard-won NATO decision was intended to erase.

Schmidt has never publicly addressed these fine points.

Moreover, NATO officials worry that any talk of amending the allies' position could cause second thoughts in Belgium, where support for the missile decision is already wavering, and in the Netherlands, which has reserved final approval on the NATO action until next year.

NATO's decision was itself very carefully worded. It was a two-pronged action, designed to meet both NATO modernization needs and European calls for arms control. In addition to voting to make and install the missiles, alliance members also proposed beginning new arms control negotiations with the Soviets as soon as possible to limit the number of these new weapons on both sides.

Both measures were to be given equal weight. But influential members in Schmidt's left-center party last week managed to win acceptance for a campaign resolution that seems to give priority to negotiations over deployment. The influence of the Social Democrats powerful left-wing on Schmidt is a continuous source of concern to U.S. officials.

Schmidt himself compounded initial U.S. alarm about his intentions by using ambiguous language.

In a party speech in Hamburg in April -- a speech aides say the chancellor drafted himself -- Schmidt originally suggested on East-West freeze for a certain number of years, rather than the specific three-year term suggested by the NATO decision. Since then, Schmidt has been saying three years and -- privately to American sources -- he excused his first vague statement as a "mistake."

Doubts about Schmidt's real intentions have been fanned by opposition Christian Democrats who, facing uphill national elections in October, have repeatedly posed questions about whether Schmidt's policies represent a loosening of Bonn's traditional ties to the United States and a softening toward the Soviet Union.

Schmidt will be the first Western leader to visit Moscow since the Soviets invaded Afghanistan in December. Chancellory aides have defended the visit as one that can serve to clarify positions and reduce the danger of miscalculations between East and West. The aides reject any notion that Schmidt would try to play a role of go-between for the United States and the Soviet Union and they have sought to squelch expectations for a breakthrough during the trip.

Schmidt himself has appeared to go out of his way at times to allay U.S. concern. Last month he attacked one of the country's most respected newspapers, the Frankfurter Allgemeine Zeitung, for referring in an editorial to a "sudden softening" of his stand on armament questions.

Last week, Schmidt saw Sen. Joseph Biden Jr. (D-Del.) in Bonn. During a two-hour meeting Schmit reportedly declared: "I want you to know that the United States can depend on the bloody Germans." Biden is the chairman of the Senate Foreign Relations subcommittee on Europe.

Yesterday in a speech to the Bundestag, the lower house of parliament, Schmidt said he would follow "a line upon which we consulted with our friends and allies" when he meets Brezhnev, adding, "We know that we have their support."