POLITICS : South Africa’s Mr. Fix-It Looks More Like Mr. Fluff-It to Some : President Nelson Mandela’s heir apparent is drawing fire.

President Nelson Mandela issued a surprising order this week to his top aide and heir apparent, Deputy President Thabo Mbeki: Take a two-week vacation.

“He works from early morning to very late at night,” Mandela told the Sunday Times of Johannesburg. “Sometimes he (goes to bed) at 2 o’clock, sometimes 3 o’clock, and that is impossible. I have told his wife . . . they must go away for a fortnight because he is overworking.”

Ironically, Mbeki did not hear the news for several days: He was on a 16-day working trip to Australia, New Zealand, Malaysia, Britain and France. When he returned Tuesday, the clearly jet-lagged Mbeki told reporters that he had not had time to meet with the president and, anyway, was far too busy to take a holiday.

“Certainly, I would be very happy to have this day off,” he said with a laugh. As for the vacation, he added: “No, I don’t think so.”

After one year in office, Mbeki, 52, seems in perpetual motion. If Mandela is South Africa’s indispensable leader, his versatile deputy appears at the crux of every crisis and in the most foreign capitals. He is Mandela’s chief speech writer and most influential adviser. But questions are growing about how well Mbeki is handling the pressure, and whether his star is dimming in the glare of several highly publicized mistakes. The answers are critical since, by all accounts, Mbeki is the leading candidate to succeed Mandela when South Africans vote in their next national election in 1999.

“Thabo Mbeki was Mr. Fix-It--now he’s becoming Mr. Fluff-It,” blared a recent front-page headline in Johannesburg’s liberal Weekly Mail & Guardian. The story was highly critical of Mbeki, who is second only to Mandela in the African National Congress, the dominant party in the government coalition.

Earlier this year, for example, Mbeki was assigned the thankless task of either reining in Winnie Mandela, the president’s turbulent estranged wife and defiant deputy minister, or arranging her quick dismissal.

He failed at the former. And he bungled the latter. The president fired Mrs. Mandela on March 27, but then was publicly embarrassed when he felt compelled to reinstate her after she sued, saying he hadn’t followed the law. She was refired two days later, but then quit before the sacking took effect.

More ominously, Mbeki has been widely slammed for announcing on April 22 that an investigation by his office had found no evidence for allegations that disgraced anti-apartheid leader Allan Boesak had misappropriated hundreds of thousands of dollars intended for children victimized by apartheid. Indeed, Mbeki said, Boesak was owed $65,000 by his nonprofit foundation.

Lawyers for the Scandinavian donors who had made the charges immediately denounced Mbeki’s three-page report as “ludicrous and preposterous,” saying it ignored reams of “compelling evidence” in a 600-page presentation they had completed.

Editorials, analysts and radio talk show hosts ridiculed Mbeki’s exoneration of Boesak as a whitewash intended to preempt a separate police probe.

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Sporting a graying goatee and crisply tailored suits, Mbeki has played a key role since his return in 1990--after nearly three decades in exile--in calming the fears of white South Africans and charming foreign investors.

No one suggests that Mbeki’s blunders have derailed his political future. But even Mandela is voicing doubts. Mbeki must delegate more to lighten his load, he said this week, and “must confine himself to policy matters” rather than “day-to-day problems.”

“He is sometimes criticized by our own people, who say he is indecisive when he is faced with a situation that requires firmness,” Mandela added. “But the man is an asset, not only to the ANC but to the country as a whole.”