The Soviet Union and Cuba seem to be on the point of replacing the United States and Britain as the main outside power brokers in the Rhodesian struggle as it move from the negotiating table to the battlefield.

Competiton between the two Rhodesian black nationalist guerrilla factionfor the diplomatic and military support of the two communist nations is giving the Soviets and Cubans a new strategic importance. They can influence whether there will indeed be a major civil war - as most outside observers now predict - and if so, which of the guerilla factions opposing the biracial government of Prime Minister Ian Smith, is more likely to be the ultimate winner.

The two guerrilla components of the Patriotic Front have failed to unite despite black African efforts to hold them together. Completion between the two each with about 10,000 troops is now focused on their efforts to win the Soviet-Cuban favor.

Robert Mugabe, who leads the Mozambique-based Zimbabwe African National Union (ZANU), has been mounting an intensive campaign to win over Cuban President Fidel Castro, apparently in the hope Castro will intencede with Moscow on Mugabe's behalf - as he did for Ethiopia's military leaders early last year.

Zanu historically has been backed by China, while its rival, the Zimbabwe African People's Union (ZAPU), has long been the chief recipient of Soviet and Cuban military assistance.

The five "front-line" African states, whose united position in pressuring the Patriotic Front was once regarded as crucial to the success of the Anglo-American initiative for Rhodesia peace have become progressively more divided in their loyalties to the two guerrilla factions.

Mozambique and Tanzania have shifted toward open support for Mugabe's Zanu in the past few months and Zambia and Botswana have shifted toward Zapu. This is hardly surprising given the presence of tens of thousands of pro-ZANU refugees and guerrillas in the former two countries and as many pro-ZANU ones in the latter two.

But two secret meetings Zambia has hosted over Rhodesia in the past year, first between Rhodesian Prime Minister Smith and Zambian President Kenneth Kaunda and then between Smith and Zapu leader Joshua Nkomo, have deepened the rift running through the five front-line states.

Angola, the fifth in the group, is ideologically closer to ZANU, which openly espouses a Marxist-Leninist line, but it is allowing the Cubans, Soviets and possibly East Germans to train and arm ZAPU guerrillas at several camps in eastern Angola.

Amidst this disarray among the front-line states, Mugabe is pressing to consolidate his support from Tanzania and Mozambique. Last weekend he flew to the Angolan capital of Luanda, almost certainly in a bid to win over President Agostinho Neto.

Meanwhile, Nkomo has just returned to Zambia from another quiet visit to Moscow. Mugabe reportedly has visited there once or twice in the past few months for secret talks with Soviet officials.

Mugabe's obvious main concern is sources of arms. His rivals, ZAPU, is heavily equipped with Soviet arms while ZANU continues to suffer a chronic shortage of even AK-47s - rifles of Soviet design that also are manufactured by ZANU's patron, China.

Mugabe said in an interview recently that he thought ZANU would have been much further along in its armed struggle if it had sufficient arms.

His bid for more arms during a visit to China last spring reportedly ended in failure and there are widespread reports in several African capitals of a partial rift between the two over this issue. Chinese diplomats, meanwhile, hint that Mugabe's turn toward the Soviet camp for arms may explain Peking's current attitude.

The bespectacled ZANU leader seems to have used a new-found friendship with the military leader of Ethiopia, Col. Mengistu Hail Mariam, to improve his ties with Cuba.

In the past year, he has made at least three trips to Addis Ababa, partly to arrange for the training of some ZANU guerrillas in Ethiopia with Cuban assistance, according to unconfirmed reports here.

In mid-September, Mugabe briefed Castro in Addis Ababa on the Rhodesian situation. Mugabe had by then already traveled at least once to Havana to meet with the Cuban leader.

Asked about his developing relations with Cuba, Mugabe said he was still working to overcome a historical "gap" between them. While he seemed encouraged, he gave no indication that Castro was ready yet to provide him with arms.

Noting that Zapu had established close ties with the Cubans years ago, Mugabe said: "We've started developing similar relations with them and we hope that we will count them amongst our friends.

"They have never been our enemies, nor even our opponents. It was just an historical phenomenon that there was this gap between us, just as there has been an historical gap between Mozambique and Cuba until relations started developing a few years ago."

Cuban diplomats in severals southern Africa capitals are telling Western journalists there will be few Cuban arms directly for Zanu - and only indirectly when and if the combined ZANU-ZAPU military command which exists on paper begins to function.

Mindful of the cost to Cuba of the Angolan civil war among three nationalist factions, Havana seems to be as worried as Western capitals about a similar bloody struggle between ZANU and ZAPU if their guerrillas succeed in toppling the present transitional government.

Still, with ZANU now clearly the main guerrilla force operating inside Rhodesia, it seems unlikely that Cuba or the Soviet Union can afford to ignore totally a movement professing Marxism-Leninism so vehemently and seeking their support so persistently.

Reports here and elsewhere in Africa say Moscow and Havana have been pressing both factions for some time now to form a united party, choose a single leader and revive their joint military command before they take power in Rhodesia.

There are no signs that the Soviets or Cubans are succeeding any more than the front-line states in this endeavor. But they now seem to be the only outside powers in a position to head off an eventual struggle for power between the two guerrilla groups and also anxious to do so.