JULIUS NYERERE, president of Tanzania, has personal prestige, is a relative moderate and exercises considerable political influence. All this has made it essential for President Carter to gain his sympathy for the budding British-American plan to bring majority rule to Rhodesia with as little additional violence as possible. But it is unclear, after his Washington visit, whether this subtle African leader gave his support to a British-American initiative or whether Mr. Carter was persuaded to give his support to the policy seemingly being followed by Mr. Nyerere.

The premise of that policy is that the current white leadership of Rhodesia can be induced to surrender only by the use of force on the part of the guerrillas, several thousand of whom have been trained and are based in Tanzania. And surrender is pretty plainly what Mr. Nyerere has in mind. In earlier years, he was ready to advise the nationalists of Zimbabwe (Rhodesia's African name) to accept something less than the one-man/one-vote franchise they now demand. But Ian Smith's refusal to compromise, when compromise was still available, undercut moderates like Mr. Nyerere, and it left him no choice but to become the sponsor of guerrilla warfare, which he is today. That is why he declared in Washington that not only the governmental structure headed by Ian Smith must go, but also the white-led army must go - to be replaced by the guerrilla army of the Popular Front.

Perhaps it will come to this. Perhaps the British-American plan, whose finishing trouches are to be worked out shortly in London, will come down to making arrangements not so much for the orderly sharing of power among whites and blacks as for the transfer of power to blacks. Perhaps the principal contribution of the plan will be to reduce prospects for a post-victory civil war among blacks - a conflict that many observers think is coming in independent Zimbabwe and one that Mr. Nyerere, scarred by Angola, devoutly hopes to avoid.

That would not be an ignoble result, though it would be quite different from the stated purpose of the British and American diplomacy, which is to help usher in a one-man/one-vote political system in a way that leaves room for continuing white participation. Mr. Carter, however, has put a heavy stress on including the guerrillas in a settlement. To the contention that this gives them a blackmail power out of proportion to the political power they could expect to win in elections, officials respond that a settlement not accepted by the guerrillas won't be stable and lasting. So Mr. Nyerere's Washington interlocutors were, to a rather considerable extent, ready to accept his message.

Mr. Nyerere dismissed the possibility that Ian Smith might offer an honorable proposal for an "internal" settlement to those nationalist politicians in Rhodesia who do not have guerrilla forces at their command. On the record, his skepticism is fully warranted. Even at this very late state of the deterioration of his country, Mr. Smith has not made a concrete one-man/one-vote proposal that a nationalist could accept. The South Africans, we gather, would like him to, notwithstanding South Africa's refusal to consider any such scheme for itself. Pretoria wants the new government of Zimbabwe to be as stable and friendly, or at least as tolerant, as possible. But before Ian Smith can turn honestly to blacks, he must defeat, in the elections scheduled for Aug. 31, a challenge from diehard white. Will he then turn to them?

The chances are smaller, by the day, that the guerrillas and their supporters outside the country would heed him if he did.