U.S. and British diplomats have agreed on a new set of tactics aimed at ending the guerrilla war and achieving black majority rule in Rhodesia, according to British and American diplomatic sources.

The key element calls for the close involvement of the United States in every negotiating step, thus exploiting what one British diplomat called Washington's "political clout" with black and white leaders in southern Africa.

The plan came under strong attack this week from the radical black Rhodesian leaders, Joshua Nkomoand Robert Mugabe. "There is no need for us to sit in on a conference involving the intrusion of the big powers," they said in a joint statement Tuesday in Lusaka, Zambia. We totally object to any suggestion that we involve the big powers to meddle in the Rhodesian affair." Their statement is taken here as a serious objection to the projected U.S. role.

Other points of the plan, the work of British Foreign Secretary David Owen, are:

Skipping a transition government and moving directly to a conference that will write a constitution providing black majority rule. It was the disagreement between Rhodesia's white leader, Ian Smith, and black nationalists over control of a proposed transition government that wrecked the Geneva conference on the country's future earlier this year.

Seeking agreement on central points of the constitution through separate, bilateral talks with rival black and white leaders before any conference is held. The conference would be the final stage to ratify the powers of a new state, including determining who could vote and what positions electors will vote for.

According to both British and American diplomats, many details remain to be worked out Importantly, there is no great optimism that the new tactics will work any better than those offered by former U.S. Secretary of State Henry Kissinger.

Anglo-American officials fear, however that letting events drift will ultimately lead to the creation of a black Marxist state ready for Soviet or Cuban penetration.

Owen heard Nkomo's and Mugabe's objections in private during his eight day visit to southern Africa earlier this month, it is understood. He is said to have noted that Mugabe was more vehemently opposed to the new plan than Nkomo, a difference that could foreshadow a split.

If the pair continue their opposition, however, U.S. diplomats expect that Washington will have to take a more modest role.

Owen has suggested that the United State cosponsor the eventual constitutional conference that he would probably chair. But that is unlikely if Mugabe and Nkomo both oppose it.

Both Washington and London hope that one or both of the Rhodesian black leaders will change their view that "Independence" will come only by military victory over Smith.

The Anglo-American assessment is that Mugabe and Nkomo are running an "inefficient" guerrilla war. There is no doubt, it is said, that the guerrillas would eventually triumph, but only after a bloody engagement lasting up to five years. By then, Mugabe and Nkomo, who are political figures, may be supplanted by guerrilla leaders. Both, it is thought, could be enticed to see their vested interest inparticipating in constitutional process.

At the other extreme, Owen is said to have emerged from his talks with Smith without knowing whether the white leader accepts the inevitability of majority rule. The belief here is that Smith has suffered two setbacks in recent months and could also be amenable to a constitutional solution.

Smith first failed to gain white control of Kissinger's constitution-writing plan. He also failed to reach an "interal solution" by dealing with the moderate black leader, Bishop Abel Muzorewa, and bypassing Nkomo and Mugabe. Muzorewa refused to enter into the proposed deal encouraged by Angrew Young, the U.S. ambassador to the United Nations Muzorewa met in Washington Wednesday with Secretary of State Cyrus Vance.

The hope is that Smith will conclude he can protect white Rhodesians better by accepting majority rule. The diplomats envision a constitution that would reserve some seats in a new Rhodesian legislature for whites and perhaps give them a vetwo over such matters as nationalization schemes. The alternative, the diplomats believe is a guerrilla triumph that will lead to mass emigration and many deaths among the country's 270,000 whites.

The Anglo-American diplomats believe there is no need for an agreement embracing all the present leaders.They think a deal could work that shaved off an extreme at either end.

Washington and London are to decide next week whether to announce that they will cosponsor a constitutional conference. Some diplomats are urging delay until progress is made in the separate, bilateral talks with rival black and white leaders. Others think the conference should be announced as an ultimate objective.

Diplomats in both capitals envision an outside force maintaining law and order during a vote. A mixed-race force of Commonwealth troops is under discussion. It would contain British officers and might be commanded by a Briton, representing the lawful colonial power.

The United States has barred its military intervention, the diplomats say, but itwould offer money and supplies to Commonwealth force.