A Reagan administration campaign to moderate its views on nuclear war - and how it can be prevented - has been building for weeks. Designed to ease the concerns of West European allies as well as many Americans, the campaign exhibits a thrust and tone perceptibly softer than earlier Reagan administration statements and policies.
The new approach can be seen in Defense Secretary Caspar Weinberger's annual report to Congress, recent speeches, press conference responses, the Pentagon's secret five-year defense document and its report on Soviet military power.
The subtle but significant shift continues here on the south coast of Portugal as NATO defense ministers gather this week to discuss allied nuclear strategy.
While holding to the administration's ''zero option'' on intermediate-range nuclear weapons as the preferred goal for Europe, the American delegation, according to sources traveling with the Weinberger party, is here to listen and not preach. It wants to let West Europeans know that Washington is paying close attention to their concerns, and in fact is quite willing to consider other options.
''The ultimate result is to get rid of these weapons,'' said a senior official aboard the Weinberger flight to Portugal. ''Maybe there are other ways to reach that goal. That's one of the reasons why there's discussion going on about it in Washington.''
''One can imagine interim steps that would be in our interest,'' said another high-level official.
Observers find such comments significant, particularly since the Pentagon has been taking the harder line in resisting any movement away from the administration zero-option proposal, which would halt the planned deployment of 572 US-built Pershing II and ground-launched cruise missiles in NATO countries beginning this December, in return for the dismantling of 351 Soviet SS-20 rockets and other missiles aimed on Western Europe.
Talks in Geneva between the United States and the Soviet Union on the intermediate-range nuclear forces will recess next Monday for two months. Any breakthrough is not expected before then, but evidence grows the US will agree to some interim position.
Such a step is being urged increasingly by the European allies and should be the prime topic for discussions, which begin here today, within NATO's Nuclear Planning Group.
In recent days, government leaders in West Germany, Italy, Britain, and the Netherlands have publicly counseled President Reagan to be more ''flexible'' at Geneva. NATO Secretary-General Joseph Luns, over the weekend, went so far as to describe the American zero option as ''not obtainable.''
At the same time, discussions at what one official calls ''a rather exalted level'' have been taking place within the administration on how to break the Geneva deadlock without sacrificing the ultimate goal of removing the triple-warheaded Soviet SS-20s, which recently increased to 351.
''There are ways to do it,'' says one official known to be a hard-liner.''There may well be some interim positions that would have the effect of dramatizing our flexibility.''
Among those under discussion: reducing NATO's full-deployment goal to something less than 572 missiles in response to a Soviet counteroffer; broadening the basis of discussion to include nuclear-capable aircraft; and deploying some NATO missiles while the Soviets reduce theirs, then holding a ''dismantling party'' at which all such weapons are taken apart in plain view.
''There are a lot of possibilities,'' said a senior US defense official, describing additional instructions likely to be given to US negotiators during the hiatus before talks resume in June on intermediate-range forces. But none of this is expected to happen quickly or easily, and political volatility on both sides of the Atlantic is an important factor.
* The US is holding firm to the four principles outlined by President Reagan: equal levels of weapons on both sides; no inclusion of French and British strategic forces in the talks; not allowing the Soviets merely to move the mobile rockets east of the Urals; and an ability to verify an arms agreement.
To these, a senior US defense official added a fifth principle: Don't diminish NATO conventional forces by bargaining away aircraft which are ''dual-capable' (able to carry nuclear or nonnuclear arms) and in which the Soviet Union has a distinct advantage.
''It would be a bitter irony if, in order to achieve an agreement affecting nuclear forces, we were to so impair our conventional capability that we would become even more dependant on our nuclear forces than we are today,'' said a high-level source.
* There still is serious question whether the US will deploy its MX strategic missile on home soil, a prospect noted with some irony by West Europeans. Meanwhile, the American nuclear freeze movement is being watched with keen interest here.
So too is the continuing flap in Washington over the Senate's resistance to confirming Kenneth Adelman as Reagan's choice as director of the Arms Control and Disarmament Agency (ACDA). In the view of some West Europeans, this reinforces the impression that the Reagan administration is less than fully enthusiastic about arms control.
The feeling among US officials is that the Soviet Union stalled in Geneva before the West German election March 6, which reaffirmed Chancellor Helmut Kohl's relatively conservative mandate. European antinuclear rallies planned for Easter may be a key staging point for US-Soviet negotiations.