Such is the electoral dominance of the African National Congress in South Africa that all the politics that matters takes place in the former liberation movement. The complacency that such dominance breeds is what lies behind the humiliating defeat of Thabo Mbeki in the election for the party leadership by his rival, Jacob Zuma, who is now in prime position to succeed Mbeki as president in 2009 if he is not found guilty of corruption.

But the result, and the likely fallout, may hasten an outcome that is essential for the long-term health of democracy in South Africa: the break-up of the ANC tripartite alliance with the South African Communist Party (SACP) and the Cosatu trade union federation. What was a broad-based liberation movement will have to become at least two political parties creating its own government and opposition.

The Achilles heel of South Africa's democracy is its weak opposition parties. Coupled with the ANC's overwhelming majority, the result has been diminished trust in government, politicians, democratic institutions, and the ANC's leadership. Despite presiding over the longest economic boom since 1981, Mbeki became increasingly unpopular, not least because so many poor black South Africans, the alliance's core membership, were seeing so little benefit.

The president's remoteness, and the spectacular complacency of his ruling circle, were reinforced by the lack of opposition. This meant that Mbeki could question the existence of Aids while millions were dying of the disease; could dismiss rampant crime as a "figment of white imagination", as his police minister described it; and could, days before the conference that ousted him, bitterly attack an independent survey showing that the poorest in South Africa have become poorer since 1996. Voters have reacted the only way they could, by staying away from the polls.

In the last general elections, in 2004, only 58 per cent of those entitled to vote did so, compared with 64 per cent in 1999 and 85 per cent in 1994. The lack of electoral competition is compounded by a proportional system that gives voters no say in who goes to parliament, only what parties they represent. Since parties select candidates, MPs are more accountable to the party leadership than the electorate.

Tensions within the ANC grew with Mbeki's attempts to make it an even broader church by incorporating forces more in tune with his market-friendly, centrist policies. Thus the party of apartheid, the National Party, merged with the ANC, and members of the Zulu nationalist Inkatha Freedom Party joined in droves. For many ANC members these changes have been too quick and too much.

Mbeki himself has said publicly, in the early 1980s and again in the early 1990s, that in a "normalised" South Africa the ANC would split from its alliance partners, becoming a centre-left party, the other two forming a new party further to the left. One suspects not without reason that this lay behind his efforts to bring new elements into the ANC.

The succession battle has opened divisions between the left, rallied by Zuma, and the centrist wing that will be difficult to heal. If Zuma is prosecuted, his supporters will call it a witch-hunt, adding to the pressure for a political reconfiguration.

Last week's leadership contest was the ANC's first competitive election in 58 years, and the democratic space it has opened will be difficult to close, no matter who leads the ANC or the country.

W M Gumede is the author of 'Thabo Mbeki and the Battle for the Soul of the ANC' (Zed Books)