When President Carter decided last week to urge the House -- successfully, as it turned out -- to reject the conference committee's compromise budget resolution because it allocated too much to defense, he shifted a layer of the subterranean crust of American politics.

Congressional Budget Office's most recent estimate of the spending implications of the administration's own 1981 defense appropriations request indicated that the compromise resolution exceeded the president's by only about $600 million, or four-tenths of 1 percent. Split the difference -- say, around $2 billion -- and call it close enough for government work. Was this potential difference the real source of the DISPUTE? various executive-branch officials professed, largely anonymously, to be concerned less about the overall amount of 1981 spending than about too much being spent down the road for expensive weapons and not enough for manpower and maintenance.

But such concerns and almost totally irrelevant to the first budget resolution, the one that was being debated. The appropriate vehicles for those fights are the authorization and appropriation bills. There was nothing in this resolution that would have kept the 1981 defense increase from being allocated, for example, entirely to better pay for the career enlisted people who are now leaving the armed services in droves. Carter had said, on board the Nimitz 24 hours before, that he did want to spend more on that problem next year -- by implication an extra $700 million -- but apparently he wants that to come from further cuts in other defense programs.

Set aside the major issues about policy and weapons that are the focus of most of the defense budget debate. You support the draft and don't want to spend more on military pay for first-term enlisted people to attract volunteers under the All-Volunteer Force? Fine, but you still need to keep the career force -- you need able sergeants to run an army no matter how you obtain recruits, voluntarily or involuntarily. You don't like the MX? You want to buy small fighter planes or small ships instead of large ones? Fine, but you still have to maintain what you have and let pilots fly enough to stay proficient. You still need adequate ammunition and communications that can't be jammed.

These sorts of career pay increases and readiness improvements are beginning to draw strong support not only from the Joint Chiefs of Staff but also from a number of legislators across a broad political spectrum -- including, for example, Sens. Tower and Kennedy -- thus raising the possibility of reversing Congress' past penchant for cutting readiness funds. And even if you look solely at the military equivalent of fixing potholes in the city streets and paying policemen a living wage, defense needs in the non-controversial areas dwarf the increase Carter opposed and could not begin to be paid for by other defense cuts.

For example, a one-time expenditure merely to bring the pay of career military people (under half of those in uniform) up to the same real level it was when the All-Volunteer Force was introduced in 1972 -- to make up for their pay having been held 10 percent to 15 percent below the inflation rate for the last eight years -- would cost $2 billion to $3 billion. And bringing ammunition stockpiles, maintenance on existing weapons, fuel for adequate training exercises and communications improvements up to reasonable levels from their currently depressed states would add around another $7 billion to $10 billion.

Nice but unaffordable? The administration's proposed defense budget for 1981 would hold spending to 5.3 percent of the gross national product. During the Kennedy administration in the early 1960s -- before the Vietnam War -- the nation spent 8.5 percent of its GNP on defense. Defense has no automatic and perpetual claim on its early '60s share of GNP and, given some growth in our economy, we do not need to maintain defense spending at that rate in order to defend ourselves adequately -- even in the face of the large Soviet arms surge of recent years.

But the share of GNP is a reasonable measure of the burden defense imposes on the economy. The Joint Chiefs of Staff have recently proposed that we increase defense's share of GNP to 6 or 7 percent. The middle of that range would be only about a third of the way back from the level of military spending we have now to the level we supported under President Kennedy -- roughly an additional $35 billion annually. Adding that much in one year would probably entail some waste, but the much more modest increases that were proposed by the budget conference committee were nowhere close to this. They would have taken defense's share from 5.3 percent of GNP only up to around 5.4 percent, at most -- maybe 1/40th or so of the way back from today's relative defense burden to that of the Kennedy administration.

Could something else have been afoot? The New York Times reported unnamed administration officials' speculating hopefully that President Carter would, by his opposition to the defense increase, undercut Rep. John Anderson's appeal to similarly minded voters. But whatever his objective, by so protecting his left flank on defense spending (if that is, in fact, the flank Anderson ultimately decides to threaten), Carter has created future problems for himself that he cannot easily correct, at least not credibly and quickly. Some polls now show over 60 percent of those who respond favoring real increases in defense spending and over 50 percent doing so even if higher taxes are required to get the job done -- a staggering change from the handful who backed such steps four or five years ago. Chicago Council on Foreign Relations polls have shown, for example, that for some time the people -- the real people -- have been about twice as supportive of increased defense spending as have "opinion leaders" such as professionals, journalists and, apparently, at least some political strategists.

In assuaging such increasingly followerless opinion leaders, Carter may have alienated a rather larger number of simpler folk who believe that weakness invites at least being walked on and, at worst, war. In moving over toward the left-hand shoulder, he has abandoned much of the middle of the road on the defense issue and has given many traditional Democrats -- already unhappy with the economy -- an added reason to vote against him. If he gets the nomination in August and if his subsequent attacks on Ronald Reagan for being too far to the right on national security issues fail to bring a number of Democrats back into the fold, he may remember the week after Memorial Day with deep regret.