The British plan for lasting peace in Zimbabwe-Rhodesia, like the Kissinger plan for peace in Vietnam, is designed with the utmost good intentions.

Under the plan, the coalition of blacks and whites now running the country will compete in elections against the parties of the Patriotic Front, who now make war on the Salisbury government.

The all-parties election, presumably, will let the people decide who is to rule, end the seven-year war and bring an era of peace, prosperity and racial harmony.

That is the British aim.

Catastrophe and chaos are other possibilites. A tribal civil war, a military coup, South African intervention or a major East-West confrontation are other potential outcomes.

Once an isolated island of white supremacy on a black continent, unrecognized by the government of the world, Rhodesia held elections in April that gave 72 of the 100 Parliamentary seats to black parties and made a black, Abel Muzorewa, prime minister. The government put the turnout at 65 percent of the potential electorate.

The country's name was changed to Zimbabwe-Rhodesia, but recognition of the new government was still denied. The Patriotic Front boycotted the election and continued their guerrilla war on the new regime.

With the consent of all parties and the African "front line" states, the British evolved the plan for new elections that will include the Patriotic Front and will be held once a ceasefire is arranged.

The problems are these:

The country's 7 million blacks are politically fragmented. In any new election, there are likely to be a half-dozen contending parties including Muzorewa's United National African Congress and the two Patriotic Front parties, the Zimbabwe African National Union led by Robert Mugabe, and the Zimbabwe African People's Union led by Joshua Nkomo. There will also be a white party that is guaranteed 20 of the 100 Parliamentary seats under the British plan.

There is virtually no possibility that Muzorewa's party can win 51 of the 80 black seats and form a government. He could, however, put together a coalition with minor black parties and the whites and emerge as prime minister. c

That result would force the Patriotic Front to decide to accept the results or continue the war.

Another scenario is a victory by Nkomo and Mugabe, running on a unified Patriotic Front ticket. That outcome would have several ramifications.

The first is the distinct possibility of a Nkomo-Mugabe power struggle that could lead to tribal war. Nkomo's Matebele tribal groupings of the western provinces and Mugabe's Shona tribal groups of the east have fought for centuries. The Matebele, outnumbered 4-to-1 by the Shona, dominated the country until the white colonizers moved in 90 years ago. The enmities persist and crop up even now in armed clashes between Nkomo's guerrillas and those loyal to Mugabe.

If such a tribal war developed, how would the present security forces of the Salisbury government -- led by white officers -- respond? Would they take sides or simply take over the government through a military coup?

Even unified, the Nkomo-Mugabe forces could not prevent a coup. They are no match for the largely black but white-led armies of Salisbury.

A successful effort by Nkomo and Mugabe to create a Marxist state aligned with the Soviet Union, East Europe and Cuba would also raise troubling questions for South Africa, Prime Minister P. W. Botha has said:

"We cannot idly stand by and allow a neighboring country to be forced at the point of a gun to accept a form of government that the people don't like. It is not in the interests of Southern Africa for there to be chaos in Rhodesia. . .We support all people who are well-disposed toward us and Bishop Muzorewa is an independent leader who is well-disposed toward South Africa."

Mugabe has warned South Africa that in the event of an attack on a Patriotic Front government in Zimbabwe-Rhodesia he would not hesitate to "call on. . .friends" for help.

"Let not South Africa forget," he said, "that we are not without friends. And if anyone is under the wrong impression, our friends are all inside Africa."

His friends also include Cuba and the Soviet-East European bloc, which is his main arms supplier.

None of these grim scenarios necessarily will be acted out. The British vision may be realized. But there is no guarantee of that. CAPTION: Picture, ROBERT MUGABE . . .would nationalize key industries; Picture 2, JOSHUA NKOMO . . .coming with an open mind; Picture 3, ABEL MUZOREWA incumbents's advantages