South Africa's powerful trade union federation, Cosatu, has formally endorsed Jacob Zuma to lead the governing African National Congress.
Cosatu's central committee called on its members to vote for Mr Zuma at the ANC's conference in December.
The move is seen as a blow to Thabo Mbeki who wants to remain ANC leader after he steps down as president.
Jacob Zuma was sacked as vice-president in 2005 after he was named in a corruption trial which later collapsed.
Mr Zuma is also seen as a potential front-runner to succeed Mr Mbeki in the 2009 presidential election.
The ANC responded quickly, with a statement expresing its "unequivocal rejection of this totally unacceptable attempt to tell the ANC how it should constitute its leadership".
The statement continued: "We call on the entirety of our membership to firmly reject this arrogant attempt to usurp their right to choose the leadership of their organisation."
Mr Mbeki has fallen out of favour with both Congress of the South African Trade Unions (Cosatu) and the Communist Party, in effect the ANC's junior partners, over his economic policies.
Mr Zuma was sacked in 2005 after his financial adviser was convicted of fraud.
He was then put on trial on corruption charges - but the case collapsed when the prosecution said it was not ready to proceed.
Mr Zuma also stood trial for an alleged rape - but was acquitted on that charge.
His supporters say all the charges against him were political.
The future ANC leader must first be nominated by ANC branches around the country.
Sources in the party say other candidates may emerge in the coming months.