President Carter asked visiting Tanzanian President Julius K. Nyerere yesterday for his crucial support of a new diplomatic attempt to resolve the conflict over majority rule in white-dominated Rhodesia.

The two leaders were said to have agreed there is still "a possibility" of heading off "massive bloodshed and civil war" throughout Rhodesia, where black guerrilla spokesmen say the time for diplomacy is gone.

However, "It cannot be overemphasized White House press secretary Jody Powell told reporters after the first Carter-Nyerere meeting, "that hope for a realization of that [Diplomatic] possibility involves an extremely difficult and complex process."

Nyerere, the first black African leader to be welcomed at the Carter White House for a state visit, is a pivotal figure in the Carter administration's African diplomacy in Rhodesia and in South African-ruled Namioia (Southwest Africa).

The United States and Britain are now drafting proposals to try to produce a peaceful transition to majority rule in Rhodesia, where 270,000 whites rule 6 million blacks.

Secretary of State Cyrus R. Vance, at the end of his present Arab-Israeli travels, is scheduled to met in London with British Foreign Secretary David Owen, to conclude the latest Rhodesian peace plan. Owen, perhaps joined by Andrew Young. U.S. ambassador to the United Nations, will then seek support for the plan from presidents of the black nations backing the guerrilla war against Rhodesia.

Nyerere is chairman of the frontline presidents of Rhodesia's neighboring states of Tanzania, Zambia, Mozambique, Angola and Botswana. He was publicly asked by Carter yesterday for his advice and his counsel and his friendship and his guidance" for achieving "peace with justice" in Africa.

Welcoming Nyerere on the South Lawn of the White House with full military honors. Carter reached back to the memories of the Kennedy administration, when Nyerere last visited the White House, to rekindle and strengthen the U.S. link with Tanzania. During the Nixon-Ford years, the relationship was frigid, until 1976 when Secretary of State Henry A. Kissinger suddenly reversed U.S. policy toward black African nationalism, and launched an aborted attempt at a Rhodesian settlement and "majority rule" in all Africa.

Carter lauded Nyerere yesterday as "a senior statesman . . . a scholar, a philosopher, a great writer," who "has forgone material wealth and ease in a sacrificial way for his own people."

The gray-haired Nyerere, whose mild humble appearance conceals the intellect and political skill to bargain shrewedly with the world's Communist and non-Communist leaders responded warmly to the welcome.

There was no allusion on either side yesterday to Nyerere's long-standing complaint about U.S. policy in Africa, that for years it looked at Africa "through anti-Communist spectacles," ignoring Africa's basic needs.

Nyerere, who has relied heavily on Chinese and other Communist aid to develop his poor nation, has maintained that Africa's nationalists must "accept help from wherever they can get it" to achieve their own objectives.

To achieve black majority rule in Rhodesia and Namibia and South Africa, the stronghold of white-minority rule. Nyerere has said the United States must join in efforts to assure that South Africa is "isolated economically, politically and socially, by the rest of the world.

Nyerere spent 15 minutes in private with Carter yesterday, and then they were joined by other advisers for 90 minutes of talks centered on the Rhodesian conflict. Nyerere will hold a press conference at Blair House this morning. He added to his schedule a meeting with members of the Congressional Black Caucus, before his concluding meeting with Carter today.