It was not the most gracious or diplomatic thing that Sen. Strom Thurmond (R-S.C.) has ever said. It was, in fact, a classic double standard.

When it comes to President Reagan, the 81- year-old Thurmond told the crowd awaiting Reagan's arrival for a campaign stop here last week, the "age issue" is strictly phony. "He's not too old," Thurmond said. "He's nine years younger than I am."

But when it is the Supreme Court, five of whose nine members are older than Reagan but younger than Thurmond, the age factor is something to count on. "In the next four years," he said, "we're going to have four or five vacancies on the Supreme Court -- if some of 'em get off, as I hope they do."

"Get off" may or may not have been a euphemism, but Thurmond's meaning was unmistakable. After a decade of remarkable stability, in which Gerald Ford and Ronald Reagan each made one appointment to the high court and Jimmy Carter made none, the aging tribunal is likely to undergo major reconstruction.

"Who do you want to appoint them?" was the question Thurmond threw out to the crowd, and it is probably the most important unpublicized issue in this election.

A president may influence the budget of the U.S. government -- at the margins. He may impose his will on the bureaucracy -- a bit. He may, if he is lucky, warm up relations with another country -- by two or three degrees.

But he can profoundly alter the direction of the nation and its government by his appointments to the Supreme Court, and the effects will be felt long after his own time and in ways that go far beyond the scope of his own office.

I cannot think of any area of public policy where the intentions of Reagan and his opponent, Walter Mondale, are clearer or where their differences are more stark and striking.

Mondale as senator was a prime sponsor of the legislation creating and expanding the program of legal services for the poor. Reagan as governor fought the program in California, and as president has tried through budgetary restraints and appointments to curtail its work.

Mondale as vice president recommended the appointment of many of the most liberal judges Carter named to federal district and appellate courts -- including many of the minority and women appointees who changed the face of the judicial branch.

Reagan has used his powers of appointment systematically to fill the courts with conservative jurists -- among them his one Supreme Court appointee, Sandra Day O'Connor.

Reagan has endorsed -- and Mondale has opposed -- constitutional amendments aimed at overturning Supreme Court decisions on abortion rights and school prayer. There can be little question that Mondale would use the power of appointment to protect those decisions in the Supreme Court, while Reagan would use his appointments to secure their reconsideration.

But beyond these specifics, there is a huge gap between the fundamental philosophies of these two men on the role of law and the judiciary in our society.

Except where he would like to roll back previous decisions, Reagan sees the law as a bulwark of existing social, political and economic arrangements. He would make the Supreme Court a forum for asserting states rights against federal standards or controls; a tribunal where property rights were granted at least equal, if not superior, status with claims of human rights; and a place where traditional values and practices are defended against legislation or litigation aimed at changing the status quo.

Few issues stir Reagan as emotionally as those -- such as abortion and school prayer -- where he is a critic of the court. On few matters has he been as consistent in both state and federal office as his insistence on appointing conservative jurists. The battle between his appointees and those of the two liberal Democratic governors who immediately preceded and followed him in Sacramento politicized and polarized the California supreme court. Since O'Connor joined the conservative bloc in the closely divided U.S. Supreme Court, it too has become a real political and personal battleground.

Mondale is the product and exponent of a very different view of the law, but he is equally committed to his ends. From law-school days onward, he has seen the work of lawyers and judges as being part of the ongoing struggle for social justice and individual rights. Some of the most eloquent speeches by this not terribly eloquent man have dealt with the issues of providing legal aid to the indigent and the accused. Some of the most courageous political acts by this inherently cautious politician have come when he was championing legal rights for those seeking social reforms.

As president, either of these men would know exactly what he wants on the Supreme Court -- and how to get it. In choosing between them, the voters are really deciding the kind of laws under which we will live.