In 20 years of service with Rhodesia's security forces, Allan Savory became known as an innovative expert on the tactics of combatting the black guerrillas fighting against white minority rule of that southern African country.

Now, Savory is playing a different role. At present, he's in Washington trying to warn the American public not to believe Prime Minister Ian Smith's argument that his plan for a transition to black minority rule is the only way to save Rhodesia from Marxism.

"Smith's plan is a sham." Savory said in an interview yesterday. "It's an unrealistic attempt to preserve white control behind a facade of black rule: and as such, it can only lead to increased civil war and bloolshed in Rhodesia."

In an effort to get that message across, Savory and two associates - all of them leaders of the moderate white opposition to Smith within Rhodesia - have been dogging the prime minister's footsteps during his controversial visit to seek U.S. support for his transition government.

"Mr. Smith has stressed endlessly that the American people should give him a fair hearing," Savory said. "It's ironic to hear him appeal for a fair hearing in America when, in his own country, opponents of his policies have been denied a hearing for their views for more than 10 years."

"We would implore American interviewers to ask Smith the questions which he avoids answering in Rhodesia," Savory added.

The questions he then enumerates all involve the contention that Smith is a white supermacist who has used dictatorial methods to stifle any meaningful opposition in Rhodesia, who has watered down promises to end racial discrimination to the point where they are meaningless and who is maneuvering through docile black leaders to keep the real power in Rhodesia in White hands.

These charges are made by a man who formerly was a major architect of the campaign to beat back Rhodesia's black nationalists through the use of military force and who served for several years as a member of the coalition of deputies supporting Smith in the Rhodesian parliament.

"It was precisely because of that experience that I became convinced Smith was wrong," Savory said.

"I came to realize that we couldn't solve Rhodesia's problems militarily," he said. "The tide of black nationalism is too strong to be beaten with arms. I saw that if we kept following reactionary, politically incompetent ideas, we would lose in the end. So I crossed over."

For Savory, that meant becoming president of the National Unifying Force, a coalition of moderate and left-of-center white political parties opposing Smith. He says that his coalition has the sympathy of about 50,000 of Rhodesia's whites, while most of the remaining 200,000 support Smith in his hard-line policies.

But Savory insisted, "The things for which we stand - genuine free elections for all Rhodesians without white manipulations - is the position supported by the overwhelming majority of the country's 5 million blacks."

That's the message Savory and his two colleagues - Nick McNally, a civil rights lawyer, and Lance Reynolds, a businessman - are trying to deliver on their U.S. trip. Specifically, they're trying to warn American public opinion "not to be seduced by Smith's siren song" and to keep on the policy track the Carter administradition has adopted toward Rhodesia.

The administration, in partnership with Britain, has been calling for a peace conference that would bring the Smith government into a power-sharing arrangement with the Patriotic Front forces waging guerilla warfare against Rhodesia from bases outside the country.

However, Smith, charging that the front's leaders, Joshua Nkomo and Robert Mugabe, are allied with Marxist forces, has refused to deal with them. Instead, he has moved, in conjunction with three internal black leaders, to implement his own plan for moving Rhodesia to what in claims will be majority black rule.

"It's folly bordering on madness to believe that you can have a successful transition without the cooperation ofthe Patriotic Front," Savor said. "Whether you like them or not, they have too much support among the blacks to be ignored."

Smith, he asserted, "believes that if he gets U.S backing, the support of the guerrillas will somewhat fade away."

"That," Savory said, "Is why the United States must stand fast. If America continues to insist on an all-parties conference, we believe Smith finally will have to concede that his go-it-alone plan has no choice; and he will have to accept the inevitability of such a conference."

"There is a real danger that even that approach won't work," he conceded, "but it's the only hope there is for a peaceful solution in Rhodesia. Time is running out for us; and as it does, the biggest danger is that there will be less and less chance to turn back from a bloodbath."