Senate-passed sanctions against the government of South Africa "will not increase U.S. influence on the pace and direction of change . . . and will reinforce those elements most opposed to that change."

That is the official White House reaction to last week's 80-to-12 Senate vote on a bill calling for economic sanctions against South Africa unless it begins to modify its policy of official racism known as apartheid.

It's hard to imagine a more fallacious reading of the bill's potential.

To begin with, it is the administration's own "constructive engagement" approach that has reinforced South Africa's intransigents, convincing them that they were under no real American pressure to change their ways. "Constructive engagement" has watched fecklessly the death of some 400 black South Africans in political violence in the past 10 months.

But, more to the point, the Senate-passed measure -- perhaps even more than the tougher House-passed version with which it will have to be reconciled -- may be almost precisely the way to maximize "U.S. influence on the pace and direction of change" in that white-ruled country.

The threat of sanctions -- like the threat of blackmail -- has always moved South Africa in the direction of at least cosmetic change. The ruling minority there needs the acceptance, the good will and, above all, the economic investments of the West, and of the United States in particular. Threats to interrupt the flow of good will and capital have always elicited a positive response.

But actual imposition of harsh sanctions might well have the opposite effect -- just as a blackmailer loses his leverage once he goes public with his scandalous information. The Senate bill skirts this problem by letting South Africa impose its own sanctions. The legislation initially would prohibit U.S. bank loans to the South African government and prevent exports of nuclear technology and computers used for police enforcement of apartheid. If there is no substantial progress for 18 months, then a tougher level of sanctions automatically comes into play, including a ban on American business expansion in South Africa and on the import of krugerrands to this country.

Unlike the House-passed measure, which would impose tougher restrictions immediately, the Senate version puts South Africa in charge of its own fate. It can increase economic links with the United States or sever them, depending on its own progress toward ending apartheid. (My preference would have been a full range of sanctions, up to and including a complete economic ban -- with South Africa itself controlling the ratchet.)

The White House insists that sanctions are the "wrong way to bring about changes" in South Africa. In fact, they may be the only way for America to have a hand in those changes. Even South Africa's hidebound conservatives, who would be certain to react negatively to any attempt to choke their country's economy, might be inclined to behave a bit differently if the hands about their throat are their own.

That is not to say that the legislation that finally comes out of House-Senate conference will end apartheid. There is nothing America can do, short of military intervention, to accomplish that goal. What this country can do is to enunciate a clear, self- escalating policy that says in effect: If you want to change, we'll help you; if you need outside pressure to facilitate that change, we'll provide it; if you stubbornly refuse to change, then to hell with you.