Echoes of the new constitution introduced in South Africa last year continue to reverberate. By offering a political voice to the Asian and mixed-race minorities but not to the black majority, it produced a wave of black unrest. The government responded evasively with a vague promise of black "political participation." Black agitation surged, the white business community started declaring that apartheid is bad for business, and anti- apartheid pressures, popular and official, began building in the United States. Result: President P. W. Botha, insisting that South Africa will not bend, is bending -- on the key issue of black political rights.

His latest step is an unprecedented hint of a political role for urban blacks, who work in the white economy and live outside the artificial "homelands" to which apartheid consigns them for the purpose of denying them political rights in South Africa. It is only a hint: this is the typical crab-like performance of a leadership that, while anxious to disarm foreign and domestic criticism, is no less anxious to avoid further erosion of its core white constituency. But it is a major event.

Among whites, liberals reacted to it with cautious approval. The Afrikaner element that has broken with him found confirmation of its certainty that he is pursuing racial integration. It appears, however, that Mr. Botha designed his words to preserve his political base -- whites who believe in controlled change to survive.

Can their sort of change meet the sort of change blacks demand? Mr. Botha pleads with "responsible black leaders to take the hand that is being extended to them." But a huge burden rests on him to offer enough promise of equality and dignity to let them take his hand.

Here it is fascinating to ask what Mr. Botha had in mind by allowing South Africa's best-known nationalist, the long-imprisoned Nelson Mandela, to give a rare interview to a (British) newspaper just now. Last month Mr. Botha said he would not negotiate with any group favoring change through violence. Mr. Mandela now says his African National Congress would halt violence if the authorities "legalize us, treat us like a political party and negotiate with us." This, like Mr. Botha's remarks, is, in effect, bargaining.

In such bargaining Americans have only a secondary role, but an important one. It is to press, unremittingly, for change. The administration encourages change quietly, others push more forcefully. It seems to be having an effect.