Who has raised voice against apartheid policy

President Reagan may finally have recognized that quiet diplomacy is not the most effective way to persuade South Africa to abandon the contemptible system of apartheid. His proclamation for International Human Rights Day declares that Americans ''strongly object to and seek to end such affronts to the human conscience as . . . the manifest injustices of the apartheid system.''

Only last week, while receiving Bishop Tutu of Johannesburg at the White House, Mr. Reagan was clinging to his policy of ''constructive engagement'' with South Africa. It seemed to rule out vigorous condemnation as well as hostile action. The change in tone was undoubtedly forced by public revulsion at Pretoria's latest crackdown on dissent. That revulsion extends to the conservative members of Mr. Reagan's own party. And South Africa's release of some dissidents is the best argument yet for continuing loud protest.

''Immoral, evil and totally un-Christian.'' That's how Bishop Tutu, the winner of the 1984 Nobel Peace Prize, characterized the Administration's insistence that gentle persuasion could gradually move a minority of white South Africans to cease the exploitation and suppression of more than 22 million blacks. Bishop Tutu insisted that the soft line only emboldened Pretoria to become more repressive. And indeed, over recent months, South Africa's Government police invaded the homes and randomly raided the offices of suspected dissenters, arresting scores.

This crackdown prompted a wave of protest in several American cities, not only against apartheid but also against the Administration's ''constructive engagement.'' The cruelest blow may have been the letter from 35 conservative House Republicans to South Africa's Ambassador Bernardus Fourie. It warned of economic sanctions unless his Government could demonstrate a commitment to changing its racial policies.Surely those protests, more than any quiet diplomacy, account for the release in recent days of more than 20 black political detainees. (Upward of 100 are thought to remain under arrest.) Now the demonstrations appear to have changed Mr. Reagan's tone as well.All the more reason to keep up the pressure. Congress ought to revive the measure proposed by Representative Stephen Solarz, to prohibit nearly $400 million in United States bank loans to the South African Government. The Republican conservatives ought to be encouraged to describe the kind of sanctions they have in mind, proving that opposition to apartheid is no longer a partisan or even tactical issue in America.

President Reagan has shown that he understands that speaking out won't do him any harm. It may also do millions of black South Africans a world of good.

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