The father of the nation, 94, is in hospital again. He transcends party politics but many fear that without him the country faces instability as memories of the apartheid struggle fade

It has become an ominously frequent ritual. Officials announce that Nelson Mandela, 94, is in hospital; their statements are hungrily dissected for subtexts and hints between the lines; the president's spokesman is bombarded with calls and fails to give satisfaction; TV crews gather outside a hospital on a best guess of where Mandela is being treated; editors polish obituaries and supplements; Twitter fills with prayers and unfounded rumours; and millions of South Africans are on edge, pondering: what happens after Mandela?

South Africa's first black president, who is the closest thing the republic has to a king, tested the nation's nerves again last week when he went into hospital with recurring pneumonia. Not for the first time, there was a general rush to expect the worst, confounded by the old trouper's signature resilience. By yesterday, Mandela was said to be breathing without difficulty. As journalist Phillip de Wet tweeted: "Worth repeating: odds don't favour old people hospitalised with breathing trouble. But so far Nelson Mandela has beat those every time."

Death has to get lucky only once, however. When it does, expect mourning on an unprecedented scale, including a funeral likely to be attended by Prince Charles, Oprah Winfrey and every living US president. The occasion and the grief will draw many South Africans together, irrespective of race, class or allegiance. But what then for the country and its dominant political force, the African National Congress?

One small but stubborn body of opinion refuses to go away: that Mandela's death will herald the unravelling of South Africa. Some believe the anti-apartheid hero and paragon of racial reconciliation is the glue that holds this diverse, sometimes discordant nation together. David Blair of the Daily Telegraph blogged recently: "For as long as he lives, South Africans breathe a little easier and believe in their country a little more. When the day after Mandela dawns, that belief will be shaken, not dramatically or immediately, but slowly and perhaps imperceptibly. South Africa will, quite simply, be a different country."

With less nuance, a persistent myth holds that black people have been waiting for his passing before unleashing a "night of the long knives", a "genocide of whites" to "cleanse" South Africa.

This has reputedly spurred a tiny number of whites to stockpile food in bunkers or prepare to flee the country. They point to murders of white farmers and a perceived threat of Zimbabwe-style chaos.

AfriForum, a racial minority rights group, claimed it was aware of leaflets warning of a "killing spree" and messages on social media such as: "You guys must just wait until the day Mandela dies and then we'll come for you."

Ernst Roets, its deputy chief executive, said: "We get a lot of fear. We do get calls from people saying they're scared about the day Mandela dies and what they should do. There are fringe organisations that say flee the country. We are encouraging people to be aware and look after their own safety. This is a dangerous country and crime is a problem, but if we want to make a prediction, there's not going to be an all-out race war. There might be isolated incidents, but I think most people, white or black, want to live in peaceful coexistence."

The discourse of potential instablity in a South Africa without Mandela is not unique to white people. Mbali Ntuli, a young black politician, said: "Mandela is always going to be symbolically powerful because of how he heralded our transition to democracy. It's powerful because we always feel that we could revert to chaos. His legacy still acts as a curb on young people today who might otherwise turn to violent means."

That the Nobel peace laureate's death will trigger carnage is a "simplistic view", said Ntuli, 25, "but I think there's some truth in it and the youth in his party seem to believe this could happen. I don't think it will be blacks turning on whites. The 'night of the long knives' would be the have-nots turning on the haves in a class war. But I'm an optimist and I like to believe that we're not so basic as to have a person die and suddenly erupt into civil war."

Even Ntuli's willingness to discuss the death of the father of the nation – a taboo subject among many older, traditional South Africans, some of whom insist on almost North Korean-style secrecy and idolatry – is indicative of a changing of the guard. Indeed, she embodies a challenge to the ANC, the party to which Mandela devoted his life and unswerving loyalty.

Ntuli chairs the youth wing of the main opposition party, the Democratic Alliance (DA). When South Africa goes to the polls next year, she will be courting nearly two million people born after apartheid ended in 1994 and able to vote for the first time. Unlike their elders, this "born-free" generation is reaching the voting age of 18 with no party allegiances. The ANC's ability to trade on past glories is fading amid frustrations over joblessness and inequality.

"Nelson Mandela is the last real symbol of the era of struggle, heroes who had real credibility," Ntuli said. "Others had credibility, but it is now shot because of their various loyalties. You now see a lot of interrogation of politics among young people; there's very little nostalgia left. Young people think about why they vote, whereas before they wanted to vote for a certain party because of its legacy."

Her own story is a wakeup call to the ANC. The daughter of a taxi driver and teacher, Ntuli grew up in both the townships and suburbs of KwaZulu-Natal province, but her middle-class accent led to her being branded a "coconut" (white on the inside). She recalled: "When I entered politics I was quite interested in the ANC, but found the way I speak and the friends I have made me feel an outsider there."

The Democratic Alliance was quick to send goodwill wishes to Mandela last week and has never been shy of quoting him, while arguing that the ANC is losing sight of his original vision. Allister Sparks, a veteran journalist and political commentator, likens him to Abraham Lincoln – a Republican who now transcends party lines and is revered by Republicans and Democrats alike.

Mandela is long retired from politics, but the ANC brought him out on stage during its final pre-election rally in 2009, a coup de théâtre that elicited cheers and struggle-era songs from a packed stadium in Johannesburg. "They exploited him a bit, which I thought was disgraceful," Sparks recalled. By next year's election he will be so frail that the governing party will be unable to play its trump card on what will be the 20th anniversary of his election as president.

"The ANC is in trouble whether he is alive or dead," Sparks predicted. "I think it will lose more ground. I hope it does, because it has become arrogant. Its strength has been diminished in each election since 1994 and this time it may go under 60%. It is riven with factions and eventually it will disintegrate. That will not be bad for the country, but good for the country. The decline of the ANC is in the interests of democracy. We need to reach the point where we can have periodic regime change at the ballot box."

He said: "The idea that South Africa is hanging on to one man and one party is a failure to understand the processes we have to go through. The predictions of South Africa falling apart have been going on all my life and I've been a journalist for 62 years."

Along with the Democratic Alliance, the ANC also faces a threat from a new party, Agang, founded by Mamphela Ramphele, a businesswoman, academic and activist who was politically and romantically involved with the Black Consciousness founder Steve Biko. Some analysts believe Ramphele can appeal not only to the disaffected black middle class, but also the unemployed masses who regard the DA with suspicion, perceiving it as still reflecting white interests.

Obbey Mabena, an ANC member and former exile, said: "I think a point has been reached where something has to give now. This is the first election where a significant slice of the electorate don't know what apartheid was about. They will judge what's out there on its own merits and it will certainly count against the ANC."

Nevertheless, while the margin is open to speculation, the ANC machine is certain of another victory next year. The big question is what will happen at the following election in 2019 as the struggle fades further into the past. About 40% of the South African population was born after 1994. It is estimated that the born-free generation will make up about a third of voters by 2019. By then Mandela would be 101.