ONE IMPORTANT casualty of the current budget quarrel may be the national consensus supporting a substantial buildup of American defense capability. As the prospect of record-setting budget deficits and the painful reality of further cuts in the domestic budget have become widely apparent, public attention has naturally turned to the buildup proposed for defense. There is now the danger that hasty, poorly planned cutbacks will lock in place a pattern of spending that is not consistent with the country's main defense needs.

To some extent the administration has invited this result by its dollars-first-plan-later approach to the military budget. President Reagan inherited from his predecessor a commitment to a defense buildup of already very substantial proportions--a five-year, 5 percent annual increase above inflation. The first hastily prepared Reagan budget raised that target to 8 percent, adding money for almost every weapon system suggested by the military in recent years. The resulting budget calls for a step- up in weapons buying that many authorities believe cannot be sustained by the defense industry. Other observers suggest that the choice of what to buy was dictated more by the familiar preferences of the military services than by strategic considerations.

One such questioner is the MIT professor, William Kaufmann, whose analysis of the defense budget was published this week by the Brookings Institution. In developing his detailed defense alternative, Kaufmann--a top Pentagon adviser to both Republican and Democratic administrations for 20 years--put himself through the sort of exercise that one hopes would guide Pentagon planning. He started with an assessment of the likely threats facing the United States and its allies over the next decade, determined the best available combination of weapons and personnel to meet that threat and added up the costs of producing the needed forces over the next few years.

Mr. Kaufmann, of course, for all his expertise, is not a soldier; his plan must necessarily be taken as a relatively abstract work, one that can hardly be expected to foresee or comprehend all the turbulence of relations among nations and governments; and his defense budget is admittedly enormous--more than $1.4 trillion over the next five years--and the procurement rate it calls for may still be unsustainable. Even so, it would spend almost $130 billion less than the Reagan budget over the same period.

The major contribution of this analysis is not that it can be accepted as "the right" statement of the country's defense needs, but rather that it talks about the choices in the right sorts of terms--what do we really need, how can it be reasonably bought? This is the debate that Congress should now be having.

Without such hard discussion, it is likely that the choice of what to cut will be left to the Defense Department. If the past is any guide, this could mean that down payments will be made on expensive systems of lower priority rather than long-overdue measures to improve the readiness of current forces. It may also mean that when the full consequences of these decisions become apparent in future years, the reaction will be a return to the start-and-stop pattern of military spending that has disrupted defense planning and inflated weapons costs over the last decade.