The announcement yesterday is a blow to president Jacob Zuma whose support within the working class is fast eroding.
The 330,000-member National Union of Metalworkers of South Africa (Numsa) also called on Mr Zuma, who was booed at a memorial for Nelson Mandelalast week, to resign.
The loss of the union’s support is another sign that the broad labour alliance forged with ANC in the struggle against apartheid is falling apart.
“Numsa will neither endorse nor support the ANC or other political party in 2014,” general secretary Irvin Jim told a news conference yesterday at the end of a meeting of its members.
Numa is the biggest block in the Congress of South African Trade Unions (Cosatu), which is itself part of a formal three-way governing alliance with the ANC and South African Communist Party (SACP).
However, Numsa has been increasingly at odds with all three over the past year, accusing them of neglecting workers in favour of “neo-liberal” pro-business policies.
Mr Jim said all members could campaign for the ANC but would have to do this “in their own time and using their own resources”.
“It is clear the working class cannot any longer see the ANC or the SACP as its class allies in any meaningful sense,” he said.
The ANC is expected to win next year’s election, although there will be serious questions about Mr Zuma’s leadership if it falls short of 60 per cent of the vote after being in power for two decades. In recent elections the ANC has exceeded 60 per cent by comfortable margins.
Numsa’s decision could hurt the ANC by depriving it of a formidable campaigning machine for the 101-year-old former liberation movement, which has struggled to adapt to being in political power.
Union support helped propel Mr Zuma to power in 2009 but the relationship has soured since police shot dead 34 striking miners at Lonmin’s Marikana platinum mine last year, the bloodiest incident since the end of apartheid.
There has also been a public outcry in the past few weeks over a $21 million (£12m) state-funded security upgrade to Mr Zuma’s private home.
“This is a very serious blow to the ANC. Numsa represents the core of the urban, African working class in sectors such as manufacturing and this is the defection of that historic ally,” said political analyst Nic Borain.
Mr Zuma, a polygamous Zulu traditionalist, has built a political base for himself in his rural KwaZulu-Natal home province and risks alienating urban voters.
Numsa also said it planned to seek new members in other industrial sectors, overturning a central anti-poaching principle in the union movement, and posing a threat to the Cosatu-aligned National Union of Mineworkers (NUM).
Until recently South Africa’s biggest union, NUM has lost tens of thousands of members in the platinum belt in the last two years to the upstart Association of Mineworkers and Construction Union (AMCU) in a bloody turf war in which dozens have died.
Numsa’s encroachment on its rivals’ territory could destabilise already fractious labour relations.
“If Numsa competes with the NUM and AMCU for members, it significantly raises the risks for instability in the mining sector,” Mr Borain said.