PRESIDENT REAGAN had needed to review his policy in South Africa, and the speech he is to make tonight is one of the most important and difficult of his presidency. His past words on South Africa are part of the difficulty. He has left many people believing that he lacks conviction in the cause of equal rights. This speech could at least make a start at repairing the damaging suspicions that burden his policy.

The president had hoped to make a splash by naming a black as his new ambassador. One choice fell by the wayside. The notion of a black envoy has been derided as a sham, as a substitute for a new policy. But, assuming a nominee of competence and integrity, there is no sham in personifying the American ideal and policy goal of multiracialism. It is not a bad statement to make to South Africa. It is a good one.

The Reagan policy of ''constructive engagement'' has seen South Africa make reforms that in other days would have been met with respect. Rising expectations both inside and outside South Africa, however, have swamped these gains, discredited the concept of white-controlled incrementalism and created a demand for swift negotiated progress to equality. This is the large, new and -- for the administration -- awkward political fact. It bears directly on the two leading policy issues, negotiations and sanctions.

*The South African government pronounces itself ready for negotiations but has yet to offer acceptable terms to the African National Congress, the leading black nationalist organization. Mr. Reagan himself has hung back from the ANC. He should come forward in order to influence Pretoria and to blunt communist inroads into the ANC and to deflect it from indulgence of black terrorism.

In the past the administration saw the choice -- soundly, we thought -- as persuasion or pressure. But popular demands and official repression have shriveled persuasion as an option. The choice is no longer constructive engagement or sanctions, we have come to think, but sanctions or escalating civil war. Sanctions will be of uncertain effect, psychologically and economically, against a powerful, determined white minority. South Africa's black citizens and neighbors will pay a cost for them. But intelligently crafted sanctions can remove any lingering expectation of Western relief for apartheid. They are more than a feel-good exercise.

It is a prudent judgment, no mere pipe dream, that majority rule is coming. Idealism and the national interest both demand that Americans be seen not as begrudging neutrals but as sympathetic participants. In this way Americans can best exert a benign if modest influence on South Africa's passage and attempt to ensure that the coming order will be healthy, democratic and friendly to the West.