The pair lost their jobs over corruption allegations but recently formed separate parties ahead of voting.
Zuma, 81, and 64-year-old Ace Magashule, a close ally and former ANC secretary-general, said on Friday that they would soon unveil plans for a joint political future.
The pair, who both lost their jobs over corruption allegations, have recently formed separate parties in the run-up to a general election due to take place between May and August.
The “Magashule Zuma United Front” will mark a “departure from traditional politics towards a more inclusive, people-centric approach”, Magashule’s party, the African Congress for Transformation (ACT), said in a statement.
The move could further dilute support for the ANC, which is struggling in the polls and could see its share of the vote drop below 50 percent for the first time since the advent of democracy in 1994.
“Based on the current political climate, the announcement could be detrimental for the ANC because they don’t have much time left until elections,” Hlengiwe Ndlovu, a lecturer at the Wits School of Governance in Johannesburg, told the Agence France-Presse news agency.
“Despite the two leaders being scandal-tainted, we know that the voters in poor communities don’t really care.”
Last month, Zuma drove a new split in the ANC, vowing to campaign and vote for the new Umkhonto We Sizwe party, or Spear of the Nation, named after the ANC’s old military wing, which he was part of in the apartheid era.
The former head of state, who has never hidden his bitterness at the way he was pushed out of office, pointedly told a press conference that “it would be a betrayal” to campaign for the ANC under his successor, President Cyril Ramaphosa.
Magashule, a former premier of the Free State province, was kicked out of the ANC last year over graft accusations but remains popular with parts of the left-leaning electorate. He formed the ACT in August.
Author and political analyst Leslie Dikeni said the new alliance was further evidence of deep rifts within the ruling party. But doubts remain over whether it would pose “any serious threat to the ANC”, he added.
In power for three decades, the ANC has had its once-stellar standing mauled by allegations of corruption and mismanagement amid a weak economy hampered by power cuts, high unemployment and rampant crime.
“This event promises to be a crucial moment in shaping the political landscape, heralding a new era of collaboration and change,” the ACT said of its new alliance.