Former prime minister Ian Smith moved today to break the white minority's political partnership with Bishop Abel Muzorewa by urging Rhodesian whites to vote for Patriotic Front guerrilla leader Joshua Nkomo if that would prevent a Marxist government from coming to power here.

Smith's move caused an immediate uproar among the leading black parties here. A spokesman for Muzorewa bitterly denounced the former political leader of the whites, saying Smith was being "absolutely dangerous and mischievous."

The wily leader of the Rhodesian whites who led the country to illegal independence in 1965 made it clear that his support for Nkomo would be only to block Robert Mugabe, a Marxist and coleader of the guerrillas, from getting power.

Smith told white voters that he would like "to avoid them both but if you have to choose between them then . . . Nkomo is the better bet." Elections are set for Feb. 27 to 29. Many of the 200,000 white would find it difficult to support Nkomo, since his forces shot down two Rhodesian airliners in the last 18 months, killing about 100 whites.

Some whites have threatened to kill the veteran nationalist leader, who has been surrounded by heavy security since his return from exile earlier this month.

There have been indications in recent weeks that Smith was planning to jerk the rug out from under Muzorewa, believing that Muzorewa would not be able to defeat Mugabe or Nkomo.

Until the campaign speech yesterday and further clarification today, however, Smith had refrained from any public shift of stance -- limiting his remarks to criticism of the bishop for being a weak leader.

But yesterday Smith told an audience of about 75 whites, "we must do all we can to stop" Mugabe from winning. "The important thing is: Keep the people away from the Marxist," Smith said.

"The choice could even be between Mugabe and Nkomo. This may sound distasteful to some people who have been on the receiving end, such as the Viscount [airliner] disasters . . . but the best choice could be Nkomo."

Citing how West Germany had reconciled with its enemies in the West after World War II, Smith said, "we must be realistic."

Smith has considerable power to affect the outcome of the election, because of the complex voting system established as part of the London peace settlement signed last month.

His Rhodesia Front Party is certain to win all 20 seats set aside for whites in the 100-member Parliament. Thus, if the whites join with a coalition of black parties having a total of 31 seats, they could guarantee their black partners an absolute majority even if Mugabe alone won a majority of the black seats.

With nine black parties running under a proportional representation system, it appears unlikely that any one party will gain an overall majority. Shifting of alliances is likely before formation of a government.

Nkomo, who has fought for black majority rule for almost three decades, would also be a key figure in any bargaining. Smith and Nkomo frequently flirted with an alliance in the past before the white leader reached an "internal" settlement with Muzorewa almost two years ago that eventually led to the bishop being elected prime minister.

Muzorewa's party, the United African National Council, was quick to condemn Smith. Ernest Bulle, the party's vice president, called Smith a "master of political intrigue, both dangerous and mischievious," adding that "he has no part to play" in the future of Rhodesia.

A spokesman for Nkomo's Patriotic Front described the move as "unwelcome support. He's a nothing -- people know his time has passed."

Mugabe, speaking at a press conference, brought home the danger to Nkomo saying: "The more one accepts to be sponsored by Mr. Smith and the Rhodesia Front, then the greater the danger of his destruction as a political leader.

"Mr. Smith has politically slain quite a number of leaders in this country. [Ndabaningi] Sithole is gone, slain by him; [James] Chikerema is gone, slain by him; Muzorewa is gone, slain by him. Who is next?"

Mugabe had his first meeting this morning with the British governor, Lord Soames. The governor's office issued a statement saying Soames "expressed grave concern at the level of political intimidation, especially in the eastern province," a Mugabe stronghold, "and urged Mr. Mugabe to do all in his power to see that it ceased forthwith."

No such sharp statements had been issued after Soames' earlier meetings with Muzorewa and Nkomo.

At his press conference, Mugabe reiterated criticism of Soames for deploying Rhodesian security forces and troops loyal to Muzorewa against the guerrillas and repeated charges that 6,000 South African troops remain in the country.

Mugabe said he was "disillusioned about the lack of honesty on the part of British officials."

"Rhodesian troops are closing in" on the 22,000 guerrillas assembled in camps, he said, adding that he feared that "they will be annihilated."

Mugabe also complained of the governor's plans to call up thousands of reservists before the election.

"We wonder what they are trying to do. Are they preparing for an encounter?" he asked.