When the Rev. Jerry Falwell called Bishop Desmond Tutu a "phony," it seemed as if the depths of offensive know-nothingism had at last been plumbed. But President Reagan has gone his unofficial chaplain one better by claiming, in a radio interview, that segregation in South Africa, as we once knew it in this country, has been virtually eliminated.

Falwell's smear was too much even for the racists in his flock, and he sent a telegram to the bishop in which he accused himself of "an unfortunate choice of words." Similarly, after the president had made a remark that compares with President Gerald R. Ford's hapless insistence that Poland was not under communist domination, a flurry of clarifications came from the White House press office.

Falwell, who purports to do the Lord's work, is plainly doing Reagan's. They want to kill sanctions legislation and to rehabilitate the government of South Africa. It is such uphill work it is no wonder that they have been driven to excesses.

They see the current agony as a communist plot. Falwell warns of "a red river of communism" engulfing the country; Reagan thinks "we would be very innocent, naive" if we didn't believe that the Soviets are "stirring up the pot and waiting in the wings for whatever advantage they can take."

The Reagan-Falwell defense team is getting no help from their clients in South Africa. They are being routed on the field of public relations. President Pieter W. Botha combines the charm of a southern sheriff at Selma with the appeal of a Chicago alderman losing a sewer contract. On the other side are the radiant, committed Tutu and Winnie Mandela, a woman of infinite sorrow and dignity.

For weeks, the American people have been regaled by the sight of South African policemen wading into crowds wielding whips, recalling their use by overseers of slaves.

Falwell, a man of infinite brass, is not the least embarrassed by any parallel to slavery. In fact, he invoked it in one of the many speeches he has given since he launched his squalid campaign to make mush of a moral issue.

"We were 180 years as a nation before segregation was eliminated here," he said. "Thank God the world was more patient with us than we have been with South Africa."

It is true that the world was tolerant of slavery as a U.S. institution. But the country at last was not and took arms to end it. The Civil War is not an event for which Americans are grateful.

Tutu has done more than any other person on the scene to avert the civil war that sometimes seems inevitable if black Africans -- who, despite Reagan's rosy view, cannot live where they want, must carry passbooks wherever they go and cannot vote -- are to achieve their liberties. Far from claiming to represent his people, Tutu has humbly -- and pointedly -- noted that he is amazed that, considering the situation, they listen to him at all.

The South African government and the Reagan-Falwell team do not realize how lucky they are to have him. It is impossible to imagine what horrors would be occurring without his valiant, principled presence. Perhaps Falwell and Reagan missed the pictures of the bishop intervening, at the risk of his life, to save a black man who was being menaced by a crazed mob, and warning that he would leave South Africa if blacks resorted to violence.

Yet when Tutu declined to attend a meeting of religious leaders called by Botha, the White House did not hesitate to express disapproval. For Falwell's squalid assault on Tutu, however, there was a presidential pardon. Reagan commended Falwell for his "apology" and explained his offense. The bishop, he said, was only reporting what he had heard from South Africans -- not passing a pygmy's judgment on one of the nobler figures of our time.

In making his apology, Reagan's favorite divine explained he had merely meant to say that Tutu was not speaking for the South Africans "any more than I speak for all Americans."

It was magnanimous of the leader of the Moral Majority to suggest he does not speak for all of us. What was most egregious about his smear was the suggestion that he was speaking for the South Africans, putting credentials acquired in 5 1/2 days on a government tour against Tutu's lifetime scars.

But what is most strange is that Reagan and Falwell, both of whom have benefited so mightily from public opinion expertise, have failed to impart any of it to their clients in South Africa.

If they really want to succeed, they should whisper to Pretoria that arresting schoolchildren, the new and repugnant tactic of the "reformist" government, is hardly the way to win friends and influence people in behalf of "constructive engagement."