In his resignation this week as chief of the ruling National Party in South Africa, Pieter W. Botha surprised a great many people in his own government. Even though he remains as president, his departure as political party chief is sure to touch off a power struggle. And it may not do much, if anything, toward ending the practice of apartheid.

For all his faults, Botha at least had a long-range goal of granting some political power to the country's 27-million blacks, without taking power from the 4.5 million whites.Botha's goal is an unrealistic dream that doesn't go nearly far enough. But his successor as party leader, Education Minister Frederik W. de Klerk, is considered even more conservative than Botha, and much less of a reformer. If that turns out to be true - and there is some question about de Klerk's real philosophy - it cannot be good news for blacks seeking a share of power.

Traditionally, the party leader and president have been the same office in South Africa. This is the first time the two have been split. The 73-year-old Botha suffered a stroke Jan. 18 that left him partially paralyzed. He apparently wants to stay on as president until the National Party is guaranteed another five years in office.

Technically, Botha's term of office as president expires next September, but he could legally postpone elections until March 1990. However, he is expected to call for a vote long before then.

Racially divided South Africa clearly cannot rush overnight into a one-man, one-vote system where apartheid vanishes and racism ceases to exist. It took the U.S. a long time to deal with its race problems; the situation in South Africa is vastly more complicated.

But if peace is to prevail in South Africa instead of a future civil war bloodbath, then more significant progress must be made toward eliminating the evils of apartheid. That means that de Klerk, or whoever becomes the next president, must take aggressive steps to bring the nation closer to real democracy.