South Africa's most powerful labor group endorsed Jacob Zuma as the next leader of the ruling African National Congress on Thursday, aiming to nudge him into the presidency and the country towards socialism.

The move by the Congress of South African Trade Unions to back Zuma, a charismatic left-leaning politician who has seen his star tarnished by corruption and sex scandals, could help push President Thabo Mbeki out of the ANC leadership race.

Any new ANC leader would likely go on to succeed Mbeki as the nation's president when he steps down in 2009, due to the electoral stranglehold that the ANC has enjoyed since the end of white minority rule in 1994.

Zuma, with strong backing from the 1.8 million-member COSATU and the smaller but equally influential South African Communist Party, both formal coalition partners of the ANC, has vowed to seize both offices.

"These are the people we have identified (as candidates) - Jacob Zuma for president," COSATU General Secretary Zwelinzima Vavi told a COSATU conference in Johannesburg. "What we have done is identify ... people who stand for our views."

The labor federation cannot formally nominate candidates for ANC posts and does not have voting rights at the party's leadership congress in December. But it is expected to wield influence due to overlap of ANC and COSATU membership lists.

Its effort to do so, however, sparked an angry response from the ruling party.

"The ANC expresses its unequivocal rejection of this totally unacceptable attempt to tell the ANC how it should constitute its leadership," it said in a statement late on Thursday.

"We call on the entirety of our membership to firmly reject this arrogant attempt to usurp their right to choose the leadership of their organization."

Mbeki, who took over the reins of the ANC from Nelson Mandela in 1997, has said he would run for a third term as ANC chief if asked, but he cannot serve a third term as president.

In addition to backing Zuma, COSATU also endorsed Kgamela Motlanthe, the ANC's secretary general, as its choice to replace Zuma as ANC deputy president. Motlanthe is a soft-spoken intellectual with ties to both the Zuma and Mbeki camps.

He has, as a result, been mentioned as a possible compromise candidate if the bitterly divided party decides that neither Mbeki nor Zuma are suitable for the leadership.

COSATU and the SACP hope the ANC will discard the centrist pro-business policies they say have left millions of poor South Africans on the margins of the country's booming economy.

"We will spare none of our energies to build the people's movement, the ANC," COSATU President Willie Madisha said in closing remarks to the trade union federation's central committee meeting on Thursday.

CLASH OF PERSONALITIES

Once the clear frontrunner to succeed Mbeki and still popular with the ANC rank-and-file, Zuma has accused political enemies of engaging in a conspiracy to deny him power.

Zuma, an ethnic Zulu who is credited for healing the divide that once existed between the tribe and the ANC, was sacked by Mbeki as South Africa's deputy president in 2005 after he was tied to an arms-related bribery and fraud investigation.

The graft case collapsed last year. He was also acquitted of raping an HIV-positive family friend in a separate trial earlier in 2006, but not before making a number of statements on the witness stand that raised serious questions about his judgment.

The most damaging may have been when he said he had taken a shower to protect himself from HIV after having sex with his accuser. That perpetuated one of many myths about the virus that has helped make South Africa the epicenter of Africa's AIDS/HIV epidemic, with 12 percent of its population infected.

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