By Ed Stoddard and Joe Brock
JOHANNESBURG (Reuters) - South Africa's deputy President Cyril Ramaphosa saw his chances of becoming the country's next leader increase on Monday when the powerful mining union he helped found before he made a fortune in business backed him to succeed President Jacob Zuma.
The debate over who should follow Zuma, either when his term ends in 2019 or before, has heated up since the ruling African National Congress (ANC) suffered its worst local election results last month, exposing party divisions.
No one has declared an ambition to take over but informal positioning is well under way, with the party split between backers of Ramaphosa, a former anti-apartheid leader popular with investors, and those who feel he would be too pro-business.
A decision has been taken "to support the candidacy of Cyril Ramaphosa for president" (of the ANC), the National Union of Mineworkers (NUM), one of South Africa's biggest unions, said.
With just under 200,000 members, the NUM plays an important role in galvanizing public support for the ANC at elections and its endorsement could encourage other unions to follow suit.
But Ramaphosa's position as a director on the board of platinum producer Lonmin when South African police shot dead 34 wildcat strikers in 2012 undermines his popularity.
An articulate though often wooden speaker, he is likely to face strong competition if he does compete, including from Zuma's ex-wife, Nkosazana Dlamini-Zuma, head of the African Union.
Zuma is expected to stand down as ANC president at a party conference late next year, ahead of national elections in 2019 when his tenure as the country's president will end. He has faced calls to quit early from several members of the ANC and prominent business leaders following a string of graft scandals.
The ANC's dominance of South Africa since apartheid ended in 1994 means it is widely expected to win the 2019 election, making its next leader almost certain to become president.
In ANC tradition, the deputy usually ascends to the top post, but some in the party and its allied trade unions think Ramaphosa would take the party too far from its left-wing roots.
A lawyer by training, Ramaphosa, 63, was a founding member of the NUM, forged in the crucible of the struggle against apartheid in the 1980s.
He went on to become the ANC's chief negotiator in talks that led to the end of white rule and Nelson Mandela's election as South Africa's first black president in 1994. His business interests have ranged from mining to McDonald's outlets.
NUM's current General Secretary David Sipunzi said: "It is not only NUM that is gunning for Cyril for president, "without giving details. He said, however, that NUM wanted Zuma to see out his second term as the country's leader.
Frans Baleni, a former head of NUM who worked with Ramaphosa in the 1980s, described Ramaphosa as a good listener with a sharp attention to detail who did not tolerate shoddy work.
"He has a high level of empathy," he said.
The ANC rules in an alliance with the South African Communist Party and trade union group, COSATU, both of which will be influential in lobbying for Zuma's successor.
The NUM is one of a string of unions in COSATU, which played a key role in opposing white-minority rule and says it represents 2 million workers. It has yet to publicly back any candidate.
"COSATU has not come up with an official position. We must speak to all the unions and come with a clear mandate," Matthew Parks, COSATU's Parliamentary Coordinator, told Reuters.
Gary van Staden, a political analyst at NKC African Economics, said the NUM endorsement would bolster Ramaphosa's position. "Other COSATU members should line up soon behind Cyril. His prospects are good."
But while Ramaphosa comes from the minority Venda tribe, Dlamini-Zuma, 67, is a Zulu, the largest ethnic group in South Africa, and would likely have the support of Zuma's powerful voting block within the ANC were she to run.
Around one in five South Africans are Zulu and politicians from Zuma's home Kwa-Zulu Natal province, a key ANC stronghold, have influence over top party decisions.
Despite their separation, Zuma backed her for the AU job and gave her a position in his cabinet. Analysts say she would be unlikely to follow up on several high-profile corruption cases that have plagued his tenure.
Zweli Mkhize, a Zulu and the current ANC Treasurer-General, has also been mentioned as a potential party leader.
($1 = 13.6435 rand)
(Editing by Keith Weir and Philippa Fletcher)