What is driving conservatives crazy about South Africa right now is the idea that, out of all the evils in this world, American liberals choose to focus on apartheid, almost to the exclusion of everything else. The residents of a vast land area -- from Saigon to Vladivostok to Berlin -- live in constant terror, and without any liberty as we mean the word. Their governments are murderous. So are many of Africa's, and many of South America's. When President Reagan condemns apartheid, it's with this feeling -- of course it's terrible, but why this one and not all the others?

It's a fair question. It can be answered convincingly in a direct way, by explaining how South Africa is worse than, say, Mozambique or Nicaragua. But both sides know perfectly well what the real answer is: apartheid resonates in the United States because of our own long, sad history regarding race. If race weren't the most persistently troubling issue in our own domestic life, then South Africa wouldn't be alone at the top of our agenda.

In other words, by imposing our own obsession on another country, we're practicing a kind of moral imperialism. Conservatives who get huffy about this are being a little disingenuous, since they're moral imperialists too, on a grander level, happy to enlist in the Manichean struggle against world communism small nations that care more about matters closer to home. Taking their argument at face value, though, isn't it possible to say, "Yes, we are moral imperalists, and what's wrong with that?"

We became the world's strongest society by being the world's freest too; unlike the Soviet Union's, our government operates by the consent of the people. That means we have to obey a moral code, because it's crucial to maintaining the consent that is our source of strength. A large part of our success in World War II and our failure in Vietnam was the difference between a cause the whole world perceived as not just serving our interests, but noble, and one that seemed at best purely strategic. When we're sure we're a force for good, it's easier for us to do what's necessary to be strong.

As moral consent is crucial to our governance, the feeling that we find racism absolutely abhorrent is crucial to the maintenance of moral consent. Slavery was our original sin, and racism, the slaveholder's justification, has been its durable legacy. In the whole world, it can be argued that 20th-century totalitarianism ranks with racism as an evil -- but totalitarianism didn't happen here, and racism did. For us to have a special concern about apartheid out of all the bad that the world's many governments do is a sign that we want our own house to be in moral order -- which is smart, because when it is, we operate best, even abroad.

But if those are the terms, they would dictate more gazing inward than seems to be going on now. In the sense of laws and intentions, our house is in order -- we're at the opposite extreme from apartheid. On the other hand, in the sense of results, we're uncomfortably less than opposite. There is a big black underclass in America, virtually all poor, virtually all living in urban ghettos, heavily unemployed, poorly educated and with a devastated family structure.

It's an uncomfortable subject. Unlike in South Africa, and unlike in the South 25 years ago, there's no easy-to-spot villain. To be moral isn't simple -- it means hard thought, and long, day-to-day effort. Just to say that the people in the ghettos are victims, that it isn't their fault, and that their lot will improve only when we "fundamentally restructure" our society isn't enough, because it doesn't help anybody right now. The conservative complaint that liberals prefer causes that are faraway abstractions to ones more complicated and nearer at hand, isn't just dismissable with a wave of the hand.

There is a set of emotions that underlies the American obsession with apartheid: an anger about racism, an unwillingness to accept things as they are, an urgency about good causes. Every one of these emotions should lead inevitably to a feeling that the condition of the American black underclass is simply intolerable and must change. Millions of American blacks are just slipping away from the rest of our society, into a life that doesn't connect to everything that makes us feel our country is great. The moral fervor we've worked up about South Africa ought now to wake us up to this, the most terrible of all our domestic problems and the one to which we're paying the least attention.