The era of Gramm-Rudman is not going to continue much longer before attention starts to focus on the federal government's second-biggest welfare program. Social Security costs about $200 billion a year. Estimates of the direct cost of our part in defending Europe -- troops and weapons stationed there, plus troops whose sole mission is to reinforce them -- begin at a conservative $100 billion (a third of the defense budget). The cost of defending Japan is fuzzier to calculate, but it would be hard to figure less than $150 billion for Europe and Japan together. And guess what? That's just about the size of the deficit.

The numbers that call this expense into question are familiar. We spend about 6.5 percent of our gross national product on defense, the NATO allies spend an average of 3.5 percent, and Japan spends 1 percent. The United States, with less than half the total GNP of NATO plus Japan, forks out for almost two- thirds of the total defense bill.

Even under a Defense Department calculation figuring, on the principle of the progressive income tax, that richer countries should pay a more-than-proportional share, we're carrying 20 percent more than our fair share and Japan is carrying one-fourth of its own.

Yes, yes, there are rebuttal points in this "fair share" debate. Most European nations spend more on foreign aid than we do. They also have conscription, which we don't, and their manpower costs are lower, skewing the results a bit. Figuring in reserves, the Europeans have a larger fraction of their population ready for battle than we do -- though figuring active-duty soldiers only, we bear this burden disproportionately too. (And none of these caveats applies to Japan.) Of course, people say, we're the superpower after all. True enough. ut they, not we, are the ones at the most immediate peril.

Oddly, the Defense Department sees the allies' larger peril as a mitigating factor for their sloth. In his most recent report on the burden-sharing question, Secretary Caspar Weinberger tolerantly notes that "there are understandable differences among the allies" about defense spending, "not only by virtue of history and culture, but also because of geography. . . . The Europeans' sense of the risks of conflict is more immediate than our own, and the public desire for an easing of East-West tensions is more widespread." Needless to say, Weinberger doesn't show this remarkable understanding when the lily-livered desire for an easing of East-West tensions surfaces in his own country.

Whenever the matter of this patent imbalance is raised, Washington professionals go all- worldly, invoking history, performing national psychoanalysis and cautioning against naivet,e. Current arrangements are protected by a dense network of foundations, seminars, studies and conferences on "the future of the alliance," most often funded by the Germans and the Japanese. Together, these boondoggles ensure that almost everyone involved in the debate gets at least one trip abroad every year to mingle with fellow chinwaggers and reject oversimplifications.

Of course, the future of the alliance needn't be in danger. The subject here is cash. The allies can shoulder more of the defense burden directly, or they can find ways to reimburse us for some of our expenses. It doesn't matter, and either way the underlying mutual commitment can be preserved. The first solution might make more sense in the case of Europe, the second in the case of Japan, where it wouldn't hurt to have another century or two for healthy antimilitarist sentiments to take root.

It's true that the main leverage we have in raising the matter of relative defense costs with our allies is the threat, at least implicit, to withdraw our enormous subsidy. And it's also true that we'd be crazy to do this without being sure that the allies would take up the burden themselves. Failing to take it up would be suicide for them, but the suicide of Europe and Japan would be catastrophic for us as well. That doesn't mean, though, that our leverage is worthless or unusable. Their implicit suicide threat is inherently less convincing.

In any event, in this negotiation we're a long, long way from anyone's ultimate bluff being called. A bit more astringency in the air, a bit more frank discussion of the relative costs and benefits of present arrangements, could concentrate our allies' minds and produce some marginal reforms. And marginal reforms could make a big difference to our budget. If Europe and Japan were to devote just 1 percent more of their respective GNPs to defense, in a way that spared us a like amount, this alone could slice $50 billion off our deficit. Yet we would still be bearing the superpower's share and more.

Here is an issue tailor-made for the Democrats. To be blunt, the vast majority of Americans who don't get invited to conferences on the future of the alliance think we're being had.