"Challenges will always be there but we must engage," Cosatu president Sdumo Dlamini told ANC supporters at the party's election manifesto launch in Nelspruit .
"We call on those who run away to come back. If you run away, you can't engage."
Dlamini said the Congress of South African Trade Unions (Cosatu) would always support the ANC.
Last month, the National Union of Metalworkers of South Africa (Numsa) decided at its special congress not to support the ANC during this year's election. Numsa is Cosatu's largest affiliate.
Earlier this week there were rumours that Numsa was in talks with Julius Malema's Economic Freedom Fighters (EFF) but the union's general secretary Irvin Jim and the EFF have both denied this.
Cosatu has been dealing with internal battles after suspending general secretary Zwelinzima Vavi for having an affair with a junior employee, among other issues. Vavi and Numsa have lodged court applications against Cosatu on the matter.
Not for sale
Dlamini warned on Saturday that Cosatu's unity was not for sale.
"The unity of Cosatu is not a playground … That unity we will defend with the last drop of our blood," he said. "Even when leaders go astray, it is workers' responsibility [to call] them to order."
Dlamini said Cosatu had endorsed the ANC's election manifesto and participated in putting it together.
"We have carried a mandate from Cosatu … that we influence debate. We have engaged, we have fought, we have harassed each other to make sure [we do this]."
He said the trade union federation would help the ANC implement its manifesto.
'Do not be fooled'
South African Communist Party (SACP) general secretary Blade Nzimande told workers to be careful and not allow themselves to be separated from the ANC.
"The only people who will benefit from that is the enemies of our cause and not the working class," he said.
"The ANC needs Cosatu, just as Cosatu needs the ANC … the SACP wants the ANC to be a united force."
The party appreciated advances made by the ANC since 1994. "Education has been expanded and improved, over nine million children get fed at least one meal a day at school … these are the children of the working class," he said.
"Do not be fooled."
ANC colours fly at stadium
The Mbombela Stadium was filled to its 41 000-person capacity for the launch of the ANC's manifesto.
The stadium was a sea of yellow, green and black as thousands of ANC supporters danced and waved flags, and party members sang songs as they waited for the event to start.
Those who were not able to get into the stadium watched the day's events from an overflow area.
A group of ANC supporters on the field carried a coffin made out of cardboard with "RIP Juju" spray-painted on it in red, in reference to EFF leader Julius Malema.
Malema was expelled as ANC Youth League president in 2012.
The road to the stadium was lined with food vendors and others selling ANC regalia.
Police kept an eye on supporters while traffic remained gridlocked on roads surrounding the stadium as minibus taxis, buses and cars ferrying supporters tried to make their way to the event.
"I love the ANC and I will vote for it again. ANC lives … we are going to make Nelson Mandela very happy," said Benny Mthombeni (45), who was draped in ANC colours.
Pensioners and the disabled queued patiently at least 2km from the stadium for transport to ferry them into the venue.
Inside the stadium, crowds went wild as popular dance music blasted through the speakers.
President Jacob Zuma will launch the ANC's election manifesto at the stadium, giving a detailed description of what the party plans to do in the next five years.
At the manifesto gala dinner on Friday night, Zuma said it is a "wonderful manifesto".
It looks at 20 years of democracy, but also at what the party has done in the past five years. The manifesto looks at policies and plans that have helped the country. And it deals with the National Development Plan, rural development, job creation and the economy, as well as food security, health, education, crime and corruption. – Sapa