Johannesburg: South Africa’s largest trade union withdrew its support for the African National Congress (ANC) on Friday, a move that is likely to erode the party’s dominance ahead of national elections next year and reorder the politics of a country that the party has governed with huge majorities since the end of white rule two decades ago.

The National Union of Metalworkers of South Africa, which calls itself “the biggest union in the history of the African continent,” with 338,000 members, announced on Friday after a special congress that it would seek to start a socialist party aimed at protecting the interests of the working class. It was a direct rebuke to the ANC, which since its days as an underground movement resisting apartheid rule has portrayed itself as the champion of South Africa’s downtrodden.

“It is clear that the working class cannot any longer see the ANC or the SACP as its class allies in any meaningful sense,” Irvin Jim, the union’s secretary general, said at a news conference, referring to the governing party and its partner in government, the South African Communist Party (SACP).

The announcement came at the end of 10 days of mourning for Nelson Mandela, the man who led the ANC to victory in the nation’s first fully democratic elections in 1994 and was to many people here the moral compass of the party. His successors, especially the current president, Jacob Zuma, have come under increasing fire for allegations of corruption and cronyism.

Several government agencies are investigating $20 million (Dh73.46 million) in improvements to Zuma’s private home that were billed as security upgrades but included luxuries like a swimming pool and amphitheatre at government expense, according to a preliminary government report.

More broadly, some in the left wing of the labour movement have decried the government’s coziness with big business, exemplified by its handling of a wildcat strike by miners in Marikana last year. The government ordered the police to break up the strike, and the police fired upon a group of miners, killing 34 people.

ANC’s deputy president, Cyril Ramaphosa, had been on the board of platinum mining company whose workers were striking.

“The congress called on President Jacob Zuma to resign with immediate effect because of his administration’s pursuit of neo-liberal policies,” Jim said, describing Zuma’s government as “steeped in corruption, patronage and nepotism.”

The call by the union to abandon the ANC has turned what had been fissures in the broad alliance that governs South Africa into gaping chasms that could, in time, end the ANC’s grip on national power.

The nation’s broader labour union alliance Cosatu and the South African Communist Party form two legs of the stool that has kept the ANC firmly in government since the end of white rule in 1994. The metalworkers’ union is part of Cosatu.

“It is a massive development,” said Mondli Makhanya, a newspaper columnist and political analyst.

“People have long been predicting the breakup of the alliance, and it never came to pass; they always managed to rescue it at a certain point. But not this time.”

ANC and its alliance partners have always represented a big tent in South African politics, including elements of the old Stalinist left, wealthy business leaders, labour unions and deeply conservative traditional leaders. Holding that mix together has always been tricky, but as inequality has deepened and more voters come to think that a small elite with links to the party has enriched itself at the expense of the majority of voters, unity has frayed.

Zuma has become a lightning rod for such criticism. His reputation has plummeted so deeply that he was booed by thousands of people at a national memorial held on December 10 for Mandela, enduring humiliation as he stood on stage beside world leaders.

Elections scheduled for April 2014 were already expected to be a crucial test for the ANC, which has seen its popularity eroded by corruption scandals, poor government services and the killings in Marikana.

“The ANC monopoly is being dented in all sorts of way by internal factionalism, disenchantment of voters, and now this,” said Steven Friedman, a political analyst at the University of Johannesburg.

One upstart party, the Economic Freedom Fighters, is seeking to siphon off disgruntled young people, many of whom are jobless. Agang, a party led by the businesswoman and anti-apartheid activist Mamphela Ramphele, has aimed its message at middle-class voters fed up with Zuma, who was charged with raping the daughter of a friend, although he was later acquitted.

The Democratic Alliance, once perceived as a largely white party, has made inroads in black areas where voters are angry about poor schools, roads and medical facilities.