The trickiest argument now running in our foreign policy debate goes to the double standard in respect to South Africa. If the United States backs freedom fighters in Afghanistan and Nicaragua, why not also in South Africa? If economic sanctions are the right tool to use against Cuba and Poland, why not impose them, again, against South Africa? For that matter, if "constructive engagement," the name given the Reagan policy, fits that particular anti-communist goose, why should it not also fit communist or Soviet-leaning ganders? Why indeed?

In this way does the nagging issue of moral consistency promise to move nearer the heart of foreign policy controversy in a second Reagan term. The main directions of policy have already been set by the president, politically validated in the November elections and absorbed into the bureaucratic mainstream. But there is plenty left to argue over here.

South Africa may not be more prominent in our foreign policy, which is still necessarily centered on the great nuclear and political issues relating to Soviet power. But it is becoming more prominent in our foreign policy debate. There was some loose political space there, room for new people to come in and start pushing. Certainly there was moral slack -- a gulf between the monstrousness of apartheid and the modesty of American policy. Furthermore, the Reagan policy was not paying off.

Most countries live so close to the margin of security, stability or prosperity that, in their foreign policy, they cannot afford flights of idealism unrelated to a fairly strict definition of national interest. By contrast, most Americans demand a good measure of morality or, if you will, ideology. And once that element becomes strong in the mix, all of us become vulnerable to appeals to apply it not just in one place but in others. Our foreign policy politics are value- laden, guilt-edged.

In practice, we do not often let our moral instincts overwhelm our political judgment -- Vietnam was perhaps the most conspicuous recent exception. That is why -- at this point, anyway -- it's uphill going for those who want Americans to take greater account of racial injustice in South Africa. Notwithstanding the recent hints of conservative interest, it is still regarded largely as a black and liberal-white cause.

Furthermore, South Africa is still widely seen as a weight on the anti-communist side of things worldwide -- this at a conservative time, when the worldwide view of things dominates.

There is a good argument that effective long- term anti-communism requires us to come down more emphatically against apartheid, to anticipate the wave of the future. Certainly this is no less reasonable an argument than the administration's contention that the changes requisite to deterring radical or Soviet influence in South Africa can best be pursued if the ruling whites see them as being in their interest. Politically, however, the heavy artillery is on Reagan's side.

But there is another, in a way weightier, explanation for the American hesitation to start fighting apartheid as vigorously as our government and popular majority have been willing to fight communism. It goes beyond the influence that the people who feel most strongly about apartheid can mobilize in the political arena.

Who speaks for South Africa's blacks? There is, in much American consideration of South Africa, an unspoken presumption that outsiders of good heart must divine the will of the oppressed and unenfranchised black majority and then try to reinforce it. But divining this majority's will is a difficult political and intellectual exercise. There are different voices; they are not easy to interpret. The diverse black reactions to Sen. Edward Kennedy were sobering, or should have been.

Then, what sticks and carrots will work best in South Africa, and what groups will be affected by their application? I am aware that putting the question of who pays and who benefits can become an evasion of the insistent moral requirement to oppose apartheid. The struggle cannot be allowed to be diverted into an arid argument over competing theories of social change.

But it remains essential to keep in mind that it is other people who will bear the consequences of American policy. It is South Africans whose suffering deepens if the United States carelessly gives aid and comfort to the practitioners of apartheid, or if the United States takes passionate but ill-advised steps ostensibly in their behalf. The single standard that most needs to be applied in respect to South Africa is whether the people we want to help are actually helped by what we do.