"In the advertising business we call it an "S" curve, the black Rhodesian account executive said today, drawing a sideways letter S on a piece of paper. "The question is whether Mugabe will be at the top or the bottom of the curve in comparison to Muzorewa on Wednesday."

Princeton-educated Atherton Mpsiunga, using the language of Madison Avenue, thus described the main battleground in this week's Rhodesian election the fight between guerilla leader Robert Mugabe and former prime minister Abel Muzorewa for votes from their Shona tribesmen who comprise more than 80 percent of the 7 million black population.

"Like any new product," Mugabe started out strongly, but then he dipped, Mpsiunga said. The question is whether he has bounced back yet.

Although scientific polling is not used in Rhodesia, most election analyists seem to agree that Bishop Muzorewa, in contrst, has made a comeback after a difficult month following the triumphant return from exile of guerilla leaders Joshua Nkomo and Mugabe.[In a potentially controversial development, Rhodesian security forces, starting with police, were being gradually moved into guerrilla assembly camps to replace Commonwealth monitoring forces when they withdraw after the election, the Reuter news agency quoted British military sources as saying. The move could inflame tensions on the guerrilla side, especially among Mugabe's troops, observers said.]

During the bishop's low point, an African servant told his white employer, "I can't tell you who is winning but I know who is losing -- Muzorewa." That sentiment is very possibly still true on the eve of the three days of voting for a black-majority government starting Wednesday, but crystal ball-gazers here are somewhat less certain.

The uncertain impact of intimidatin of voters carried out mainly by supporters of Mugabe and Muzorewa tends to cloud predictions. One factor helping Muzorewa's resurgence is his superior campaign organization and financing since he fought and won an election last year while his rivals were still fighting the guerrilla war.

Because of the competition between the three political leaders, Rhodesia is witnessing its first election where the outcome cannot be readily predicted. The most likely result is that Mugabe will finish first but far short of a majority, leading to much horsetrading to form a coalition after the results are announced March 4.

Ironically, in such a situation, who finishes second may well turn out to be more important than who is first if the margin is not great.

If Nkomo, who has solid support from his minority Ndebele tribe, is second, he could gain the premiership in alliance with Muzorewa and the 20 seats held by former prime minister Ian Smith's Rhodesian Front party. A third-place finish, however, would probably end his chance to be premier and thus propel him back into an alliance with Mugabe, his guerrilla ally.

Mugabe's main problem is that he has not been able to campaign effectively for more than two weeks since he narrowly escaped assassination when a 90-pound bomb exploded on a road just after his car had passed following a political rally in central Rhodesia.

Since than he has canceled all his campaign rallies and has not left Salisbury. He has even taken to sleeping in a different bedroom every night in his surburban home.

He is now campaigning by press conference and statements issued through his spokesman. One cynic, in a takeoff on his party's penchant for Marxist rhetoric, commented: "He seems to prefer the media to the masses."

Mugabe's lack of public appearances and constant battles with Britain's temporary colonial governor, Lord Soames, over cease-fire violations and election intimidation have blunted his campaign. In addition, some of the euphoria of his return as the conquering war hero whose troops forced the election has worn off.

Muzorewa has centered much of his campaign on attacking Mugabe for his avowed Marxist beliefs, but the guerrilla leader hardly looks the part of the feared, fire-breathing Marxist. Instead he comes over more as a scholarly, conservatively dressed schoolteacher.

He is known for talking over the heads of his listners, unlike Nkomo, who seems to charm his audiences with his infectious laughter and folksy conversation.

Also, in contrast to Nkomo, who runs his Patriotic Front party with an iron hand, Mugabe often takes a long time to make a decision, partly because of the many rifts in his Zimbabwe African National Union (ZANU) party.

At last year's London peace talks that led to the elections, observers used to joke that "it would take ZNAU three days to decide on a breakfast menu." The indecision led some to wonder just how effectively Mugabe could govern.

Muzorewa's problem has been to shed the image of being-controlled by the whites. Many Africans felt that in his six months in power after a landslide election victory, he concentrated on reassuring the 200,000-member white minority instead of improving conditons for blacks.

Muzorewa has, had a hard time trying to cast off the image of being Smith's creation.

During his current campaign, Muzorewa has been quick to flare at criticism.

Of all the major candidates, the bishop has the worst relations with the media. His press conference frequently descend to bickering with reporters. He frequently rebuffs an unwelcome question, especially about the source and amount of his lavish campaign funds, with the reply: "None of your business."

Muzorewa's virtually unlimited financial backing, apparently from South Africa, has given him a distinct advantage in the campaign. He has the use of four helicopters and a propjet plane, all of which create an image of power with his African constituency. The British colonial government has not placed any limit on campaign funding or its sources.

Despite frequent predictions that he is headed for defeat, the bishop remains outwardly confident.

He has just had 80,000 leaflets printed saying, "Thank you for reelecting me."