STROM THURMOND enrolling his child in an integrated public school in South Carolina? The Strom Thurmond? Thirty years ago the thought would have been unthinkable. In 1968, Mr. Thurmond was the Dixiecrat candidate for President, the opponent for every civil-rights bill that came down the track, and as avid a supporter of segregation as, let us say, George Wallace, Orval Faubus or Ross Barnett. A child of his going to school with black children? Even 15 years ago, you would have had to be kidding.

But there the story was - on page 3 of Friday's newspaper. And there was the picture of the senator escorting his six-year-old daughter to her first-grade class in a Columbia, S.C., public school that is 50 per cent black. There were, so far as we know, no demonstrators, no recriminations, no shouting and no boycott. At most, the commentary on the decision of the senator and his wife to enroll their daughter in that particular school was limited to a few snide suggestions about the potential political gain he might reap from it.

We don't know whether the decision was made in some part for political reasons, as that commentary suggests, or whether it was made wholly in the best interests of Nancy Moore Thurmond, as her mother says it was. In historic terms, it doesn't matter; either way, it is a striking demonstration of how much has changed in a relatively few years. Black votes meant nothing to Mr. Thurmond 30 or even 15 years ago; they may now mean the difference between victory and defeat next year when he seeks his fifth term in the Senate. Similarly, no white politician in South Carolina would have dared, 20 years ago, to send a child to an integrated school whether or not he believed it was in the child's best interests.

This, of course, is not the first move Sen. Thurmond has made to narrow the gap that once existed between him and black citizens everywhere. He was the first member of the South Carolina congressional delegation to hire a black staff member, and he once recommended a black civil-rights lawyer for a federal judgeship. But his particular decision seems to us to be richly revealing. Public schools were at the heart of the fight over segregation. They were the issue on which the arch-segregations made their stand - in South Carolina, as well as in all the other states of the South. When one of the chieftains of that ill-fated band walks with his child into a school environment he fought to prevent at almost any cost, it tells you something quite wonderful about how much the principles - and the politics - of this nation have changed in less than a third of a century.