Officially it is a time of joy. Nelson Mandela turns 85 today and South Africa will party into the weekend in a blaze of banquets and celebrations attended by pop stars and Hollywood's finest.

The former president will wake up to the air force orchestra playing Happy Birthday outside his Johannesburg home and be serenaded again by the Springbok rugby team.

All week the tributes to an extraordinary life have poured in, and the mix of well-wishers -- township children, left-wingers, conservatives, liberals -- is a showcase of the rainbow nation.

But overshadowing the festivities is the great unspoken question. They call it Wham: what happens after Mandela? What happens after the father of the nation, the architect of its multiracial democracy, the secular saint who averted civil war, dies?

To dwell publicly on the subject is considered morbid but, discreetly, that is what South Africa is doing as Mandela becomes more frail. The man who walked to freedom in 1990 now uses a cane and prefers to sit.

The death in May of his friend and mentor Walter Sisulu, at the age of 90, jolted the realisation that Mandela is the last of a generation of black leaders and that notwithstanding his zest for life he could at any time "go".

Whether the magic can continue after the magician exits the stage is a question looming larger as the political rhetoric heats up in the approach to next year's election.

"When he does go it will put a lot of strain on our fragile young democracy," said Joe Feremane, chairman of the main opposition party, the Democractic Alliance. "It will be like a dark cloud falling."

The Wham issue has been debated since 1994 when apartheid fell and the newly-elected first black president was indispensable in guiding the country towards peace and reconciliation.

He left office in 1999, but despite his self-deprecating comments about being an unemployed pensioner Mandela is still a moral and political force, and he occasionally wades into national debates about HIV/Aids, Zimbabwe and poverty.

As he ages, so the country braces itself for the unknown.

"He is the personification of our unification and reconciliation," said Richard Calland, a political analyst. "He is a sort of safety net, a figure that different groups can turn to when things get difficult."

When he dies political discourse is likely to sharpen, because factions in the African National Congress (ANC) want to redress what they see as its excessive emphasis on reconciliation at the expense of improving the lot of blacks, Calland said.

The ANC has a clear two-thirds majority in parliament, giving it power to rewrite the constitution, and few doubt it will win another big majority in next April's election.

The leader of the Inkatha Freedom party, Mangosuthu Buthelezi, said: "I am very worried ... If we are not careful we are going to have a one-party state." Coming from the leader of a party which fought bloody battles with the ANC in the early 1990s, that could suggest renewed conflict.

But even Buthelezi, who sits in the cabinet, says those days are over, something underlined yesterday when the human rights commission defined the militant black slogan "kill the farmer, kill the boer" as hate speech.

Feremane said Mandela's passing would erode ANC support. "Many people, especially older ones, vote ANC just because of him."

But other opposition figures, such as Trevor Ngwane, whose Anti-Privatisation Forum is fiercely critical of the government's conservative economics, say the post-Mandela South Africa will be a harsher place for the dispossessed.

"He's probably acting as a brake on the government's policy of attacking the poor for the benefit of capital, because with him there they still need to genuflect before the altar of compassion."

Under Mandela's successor, Thabo Mbeki, the ANC has pushed aside its trade union and the Communist party partners in its quest to privatise state assets and attract foreign investment. Their influence is expected to dwindle further in Mandela's absence.

South Africa has a free press and a vibrant civil society, unafraid of criticising the government, but some observers think that his unmistakeably reedy voice will be irreplaceable.

But Allister Sparks, a veteran commentator and the author of Beyond the Miracle, a new book on post-apartheid South Africa, says the notion of a slide into authoritarianism, Zimbabwe-style, is ludicrous.

"To portray Mandela as the last pillar protecting us from catastrophe is absurd. People have been making doomsday scenarios about South Africa all my life and I'm sick of them. The constitution is bedded down and we have a constitutional court not prone to giving political judgments."

Racially tinged rhetoric was returning to politics, but that was less of a worry than the fact that so many remained trapped in poverty, he added. - Guardian Unlimited Â