BETWEEN NOW and Christmas, decisions will be made by the administration and Congress that will influence greatly the effectiveness and the sustainability of the planned defense buildup. Congress will be making final decisions on 1983 spending, and the president and his advisers -- with an eye to what Congress is doing -- will be shaping the budget for 1984.

Military spending decisions cast long shadows over budgets of future years. Major weapons systems take many years to develop and build. Once built, they require huge continuing expenses to man, operate and maintain. After spending on a weapon has been authorized, it is hard to change course, even if subsequent events call into question the weapon's effectiveness or utility. Congress and the Pentagon are reluctant to second-guess their earlier decisions, not only from pride but also because cancelling a project means halting production in various congressmen's districts and writing off big losses.

The Reagan administration's early defense strategy has left both it and Congress with very little comfortable bargaining room. The Carter administration had already planned a substantial defense step-up. The defense totals decided upon by President Reagan shortly after taking office were chosen to signal even stronger commitment to defense. When it came time to fill in details, the military services obligingly served up lists of weaponry they had long wanted with little higher-level review of whether the sum of the wish lists represented a balanced defense strategy consistent with budget resources and available industrial capacity. The totals then became the yardstick against which a critic's commitment to national security is measured.

Since that time, both the Office of Management and Budget and Congress have won minor battles to restrain defense spending -- although thus far they have not cut beneath obvious fat. Last fall, the president cut the Pentagon's 1982 request by $2 billion -- much less than OMB wanted -- and when he came back and asked for more last summer, Congress turned him down. The budget resolution adopted last summer calls for cutting several billion from each of the next three defense budgets, although the president has said he does not feel bound by that agreement for 1984 and 1985.

In a press conference last week, the president's principal defense adviser, Defense Secretary Caspar Weinberger, made it clear that he thinks that the need for defense spending has, if anything, increased. That puts added pressure on the lame- duck Congress to do something for which it is not well suited: carefully review the planned buildup of forces -- the MX missile, B1 bomber, M1 tank, the two additional aircraft carriers and other major procurement decisions--to make sure that they are a balanced and necessary investment of scarce budget dollars. In making that review, Congress should remember that it is doing much more than tidying up a few details in next year's budget -- it is also restricting its future choices both with respect to overall fiscal policy and to the appropriate mix of forces given rapidly changing technology and an uncertain strategic outlook over the next decade.