The United States appear to be taking over from Britain the main burden of selling their joint peace plan for Rhodesia to the black nationalist guerrillas and may soon become the chief mediator in a last hour search for an agreement acceptable to all the country's rival African leaders.

This stand in striking contrast to the low profile initially adopted by the United States at the time the Anglo-American proposals were first announced last September. It apparently stems from the growing feeling that the ruling Labor Party could abandon its role in the joint plan in what could be an election year in Britain.

At the forefront of the American effort is U.S. Ambassador Andrew Young who spent hours here at the Malta conference on Rhodesia practicing his special style of personal diplomacy on the Patriotic Front, the alliance of black nationalists fighting to overthrow the white minority government in Rhodesia.

Within an hour of his arrival here last Sunday, Young was busy seeking out Front delegates in the bar, lobby and corridors of the seaside Dragonara Hotel to try to convince them of the merits of the plan and the firm American commitment to it.

By the end of the three-day conference, young and his aides, almost all of them black Americans who mixed easily with the black Rhodesians, had personally met practically every important military and political leader of the Front's two factions.

His unorthodox activist role brought praise from Robert Mugabe,coleader of the Front who said at a press conference that Young had made a "tremendous contribution" and described the U.S. role in the search for an internationally acceptable solution to the Rhodesian conflict as "positive."

But Magabe, who heads the Front's radical wing the Mozambique-based Zimbabwe African Nationalist Union (ZANU), also warned against the United States becoming directly involved in the process of "decolonizing" Rhodesia. This task, he said should be left to Britain.

Under the British-American peace plan, the Rhodesian white minority government of Prime Minister lan Smith is asked to hand back power to Britain, the colonial power before the whites declared their unilateral independence in 1965.

Britain would then run an interim administration, backed by a U.N. peace-keeping force, and supervise elections for a black majority government.

The whole British-American plan is being challenged by Smith and three internally based black nationalist leaders who are reportedly close to forming their own multi-racial interim government, shutting out the patriotic Front.

The American fear is that this will serve to polarize the power struggle among rival nationalist factions and lead to an Angola-style civil war that could easily drag the big powers and their respective allies into yet another East-West confrontation in southern Africa.

South Africa is already hinting strongly that it will back Smith's internal settlement a move that is certain to increase East bloc and Chinese support for the Patriotic Front's two factions. The Soviet's and Cubans back the Zimbabwe African People's Union under Joshua Nkomo and the Chinese support Mugabe.

One of the basic problems facing the Western initiative now is the possibility of Britain abandoning its own plan if and when an internal settlement between Smith and three moderate black nationalist leaders is reached in Salisbury.

The governing Labor Party and Foreign Secretary David Owen, the Principal architect of the Anglo-American plan, now seem certain to support Smith's internal settlement. The Labor government feels extremely vulnerable to attacks from the Conservatives, who already have charged that Owens was "sabotaging" Smith's internal settlement effort by dealing with Rhodesian "terrorists."

During the conference here Owen kept his contracts with Front delegates to formal occasions. If he opts for scuttling his own initiative, the would leave Young as the principal Western conduct for getting the plan accepted by both the internal and external black leaders.

In this endeavor, Young has the strong backing of Tanzanian President Julius Nyerere and Mozambican President Samora Machel, who together have exercised considerable pressure on the Front to come to the negotiating table with Britian and the United States.

In addition, Young is not under the political constraints at home that Owen appears to be and has the personal backing of President Carter.

Young is now planning to travel within the next two to three weeks to the frontline states and perhaps even Salibury to promote the Western peace proposals. His initiative may come too late to do much good, however, if Smith now quickly announces his own agreement, and it gains the backing of a significant portion of Rhodesia's 6.3 million black population.

The results of the conference here are basically inconclusive. While a measure of badly needed trust was probably established between the front and the two Western powers, the two sides are still far apart on their views of who should run the interim government and supervise elections for Rhodesia's first black majority government.

In a spirit of compromise, Britain and the United States did accept the Front's proposal for a "governing council" to rule the country during the transition period. But the Western powers want to keep effective power in the hands of their proposed British resident commissioner with representatives of all nationalist factions sitting on the council, while the Front wants a "partnership" limited to itself and the British.

On the issue of building a new black-led national army for Rhodesia, the two sides are also still at considerable odds. The British and Americans have proposed a 10,000 man army, composed of 4,000 guerillas. 3,000 black troops of the present white-dominated Rhodesian army and 3,000 new recruits.

The Front wants all guerillas belonging to its two factions, between 15,000 and 25,000, including in the new army, to which a few thousand black and white troops in the present Rhodesian army would be added to their selection.

Finally, although the Front has agreed to some role for the United Nations in the interim period, it has not given its agreement to the stationing of a peace-keeping force in the country.

Nor does it want Britian to supervise the elections because of its suspicion that London will favor the internal black leaders over those who are responsible for the fighting that has finally forced Smith into negotiating on the basis of universal adult suffrage.

With these sizable differences still to be reconciled, there is a good possibility the Anglo-American plan will be overtaken by Smith's internal settlement plan. Smith is still trying to hold off the day of black majority rule for as long as two years, however, and at least until the end of 1978.

This gives the Western powers still some time and opportunity to find a compromise between their plan and that of Smith.

It is likely that Young and Owen will continue to do just this on the assumption that Smith's plan will be unacceptable to most of Africa and that therefore the internal black leaders will be anxiousto find another internationally acceptable solution.