Washington — As the Reagan administration seeks a buildup of nuclear weaponry, the debate over the purpose of such deadly force sharpens.
Should the United States through its strategic might seek simply to deter nuclear war? Or should it attain a ''warfighting'' capability that presumes such a war is ''thinkable,'' that it can be fought and won? And if it moves in this latter direction, will Americans be more or less secure?
The President last November made what many saw as a bold arms reduction offer: NATO would not deploy 572 nuclear weapons in Europe if the Soviet Union dismantled the intermediate-range missiles it now aims at that region. The idea was rejected by Moscow, and more recently the administration deferred the start of strategic arms reduction talks because of the situation in Poland.
But the administration through its rhetoric and actions also is shifting US policy regarding the use of nuclear weapons. It argues that deterrence is the principal goal, but officials do not hesitate to talk about the possibility of fighting a nuclear war.
The administration reportedly will ask for a considerable increase in spending for civil defense. It is argued that this is necessary because the Soviet Union has a massive civil defense program. Critics say it is one more indication that the administration is preparing for a possible nuclear war.
''We don't want to fight a nuclear war, or a conventional one either, but we must be prepared to do so if such a battle is to be deterred, as we must also be prepared to carry the battle to our adversary's homeland,'' Defense Undersecretary James Wade told the House Defense Appropriations subcommittee.''We must not fear war.''
In a recent study of the US nuclear buildup, the Center for Defense Information warned that such action in fact makes the US less secure.
''I think it's a very dangerous concept,'' said retired Rear Adm. Gene La Rocque, who is a former Pentagon strategic planner and now heads this private defense study group. ''It is our feeling that we are moving closer to a nuclear war.''
Most Americans apparently agree. According to a recent Associated Press-NBC News survey, three-quarters of those polled feel the US is likely to become involved in a war in the next few years. A plurality of 48 percent felt the President was talking too much about the possibility of using nuclear weapons and feared that such talk increased the danger of nuclear war.
Has the US fallen behind the Soviet Union in strategic nuclear capabilities, as the administration suggests? The answer depends on whose statistics one uses.
In its recent publication ''Soviet Military Power,'' the Pentagon argues, ''The Soviet missile forces have moved from a position of clear inferiority in the early-to-middle 1960s to one in which they are generally recognized as equal , or superior in certain measures, to those of the West.''
Counters the Center for Defense Information:''The US, however, has over 10, 000 strategic nuclear bombs and warheads, at least a third of which could survive a Soviet 'first strike' attack. This is far in excess of strategic needs.''
The highly respected International Institute for Strategic Studies (IISS) in London reports that the US has about 9,000 strategic nuclear warheads compared with over 7,000 directed at it by the USSR. But this latter figure does not include the 230 new SS-20 medium-range Soviet missiles aimed at Western Europe (with three warheads each) or the older single-warhead SS-4s and SS-5s.
Even if the Poseidon submarine missiles are included on the NATO side, the IISS says the Warsaw Pact retains an advantage in theater nuclear warheads of about 1.6 to 1.
''The balance is distinctly unfavorable to NATO and is becoming more so,'' warns the institute. ''Nothing has been done to reduce . . . the vulnerability of NATO'S existing nuclear delivery systems or to increase their ability to penetrate pact defenses.''
The administration's answer is to spend $180 billion on new weapons (including the MX missile and B-1 bomber), more missiles and submarines, better defenses, and improved communications. Critics say the total cost could be well over $200 billion.
Congress so far has gone along for the most part, but with skepticism and reluctance from some important quarters. Lawmakers have blocked funding for ''superhardening'' existing silos for the MX missile. Senate Defense Appropriations subcommittee chairman Ted Stevens (R) of Alaska, says there is ''real, substantial conflict in Congress'' over the B-1.
Sen. Sam Nunn (D) of Georgia, one of the Hill's foremost defense experts and supporters, told the Armed Services Committee recently: ''If we allocate so much of our defense budget to strategic programs that we allow our conventional posture to suffer, we will inadvertently decrease our options in protecting our vital interests without resorting to the use of nuclear weapons.''
The debate about the Reagan effort to ''rearm America'' will continue as Congress reconvenes, focusing on the 1983 defense budget about to be released. Conventional and strategic hardware will be highlighted, but so too will the use of such weaponry.
''We are living in a prewar and not a postwar world,'' US arms-control chief Eugene Rostow has said. From this perspective, the shift from nuclear deterrence to a ''warfighting'' capability becomes particularly significant.