PRETORIA (Reuters) - The ANC in South Africa's richest province backed Deputy President Kgalema Mothlanthe on Friday as its choice for leader of the ruling party, keeping him in the hunt for a position that would tee him up to be president of Africa's biggest economy.

However, Motlanthe refused to immediately seize on the support of African National Congress (ANC) members in Gauteng, the province that includes Johannesburg and Pretoria, as the chance to launch a formal challenge to President Jacob Zuma.

Zuma is firm favorite to win re-election to the helm of the ANC at a party leadership conference next month, despite a slew of scandals and sluggish economic growth in the three years since he won the national presidency in elections.

He has also been criticized for his handling this year of three months of violent labor unrest in the mining sector that included the police killing of 34 striking platinum miners on August 16. The labor strife dented South Africa's image with investors and led to downgrades by credit ratings agencies.

Speaking to the foreign media in Pretoria, Motlanthe said he felt "neutral" about the Gauteng backing, adding that he was yet to make up his mind whether or not to run against Zuma.

"I'm still agonizing over it," he said.

Besides Gauteng, KwaZulu Natal - the province with the biggest voting clout in the December ANC election - has come out in support of Zuma, as has the smaller Free State.

South Africa's remaining six provinces are due to reveal their decisions later on Friday and - barring any last minute surprises - all the signs point to Zuma securing enough backing at the Dec 16-20 conference to win re-election.

Zuma himself became leader of Nelson Mandela's 100-year-old liberation movement in 2007 after spearheading a grassroots internal campaign to unseat then president Thabo Mbeki, who was seen by many ANC members as aloof and too pro-business.

(Reporting by Ed Cropley; Editing by Pascal Fletcher)