One of the great open secrets of this town is that Edward Moore Kennedy, senator from Massachusetts, is running for president. His kids are grown, ambition still burns and this time around there will be no incumbent to face in either the primaries or the general election. It is, therefore, with some satisfaction that Kennedy's critics watched him use South Africa as Buster Keaton did banana peels: it sent him for a fall.

He was denounced by whites and blacks. Speeches had to be canceled. Spokesmen for both races accused him of using South Africa, its agony and its obscenity, as a prop -- yet another photo opportunity in a life full of them. In the South African press, Chappaquiddick was mentioned -- even the nearly legendary cheating at Harvard. A man who cheats at college, a newspaper said, lacks the moral standing to denounce apartheid. South Africa, so abundant in diamonds, apparently has a critical shortage of subtle minds.

In Washington the sounds you could hear were not of silence but of chortling. Here was Kennedy trying to re-create the South African trip of his late brother, Robert, and it didn't work. Bobby's 1966 visit was a triumph. He was mobbed by blacks, whites and those of mixed race alike. Robert talked of human rights, and it seemed that people listened. Maybe not much changed in South Africa as a result, but even so, a politician should be credited for telling evil to its face that it is evil. What's true for Bobby should be true for Teddy too.

Ah, but something really has changed in South Africa, and the Kennedy trip points it out. Instead of concentrating on whether the trip was advanced properly (a Democratic obsession) or Kennedy hurt himself as a candidate (a Republican obsession), we ought to pay some attention to what the reaction to the trip says about the effect of the Reagan administration's constructive engagement policy. It has almost certainly wrapped racial oppression, capitalism and the United States into one neat little package for every black to hate.

It's true, of course, that the blacks who shouted Kennedy down and who threatened to disrupt his speech, represent a small minority -- but it is probably larger than anyone thinks and probably growing to boot. Whatever constructive engagement is supposed to either mean or do -- and it is not without its black defenders in South Africa -- it nevertheless sends the message that the United States is sympathetic to and supportive of the white minority's repression of the black majority.

Think about it. How would you feel if solely because of your race you could not live where you wanted, could not vote, had to live separated from your family just so you could make a living? How would you like it if you almost never saw your spouse or your children just because the government said that no one of your race can live in a certain area?

Now that you are thinking of that, think of what you would feel about another government, the United States, that says, "Be patient. Take the long view. Bear in mind the strategic importance of southern Africa. Remember communism and the Russians and how complicated things always are." For constructive engagement, it appears there's not enough spit in the mouth.

This is the lesson of the Kennedy trip, and he ought to be praised for bringing it home. Of course things have changed since 1966. Of course there are blacks who now hate the United States and the capitalism with which we are, rightly, identified. Once again, we are willing to trade the future for just a bit more of the present -- a mostly white nation identifying with fellow whites, obsessed with anti- communism, thinking the laugh's on Kennedy when surely it is on us.

Edward Kennedy is a politician, and he has to be measured as one. So it is natural that the pros fault his advance work and say he muddled his message. But there is yet another way to measure apolitician, and that is by his willingness to go to the truth and tell us something we did not know before. Kennedy did that. He told us that American policy in South Africa is a failure. Any trip that teaches us that is a success.