South Africa's deputy president is being urged to challenge President Jacob Zuma for the leadership of the ruling African National Congress — a position that would all but guarantee him the presidency of the nation.

Kgalema Motlanthe, who fought against white rule and was imprisoned on notorious Robben Island, is not a name that's immediately recognized around the world, even though he once served as president of South Africa for eight months. But in December, Motlanthe will have a chance to run for the top spot in the ANC when the party holds its leadership conference.

The ANC leader will be the party's candidate for president in the 2014 national election. Such is the strength of Nelson Mandela's party that the ANC candidate is virtually assured victory. Dubbed Zuma's "silent opponent" in the South African press, Motlanthe has not announced his candidacy for the ANC's top spot but some have been pushing the self-effacing politician to make a stand and challenge Zuma.

"We have to restore the dignity of the ANC. You can't fight the struggle in a white suit ... Get out of the white suit and into overalls and get your hands dirty," ANC Youth League Deputy President Ronald Lamola said at a rally Motlanthe attended Saturday, according to The Sunday Independent, a local newspaper.

Motlanthe has not publicly hinted at his plans, even as Zuma faces persistent criticism of his leadership of the ANC. But a new authorized biography of Motlanthe has sparked conjecture that he may challenge Zuma next month. "Kgalema Motlanthe" by Ebrahim Harvey appeared barely two months before ANC members gather for the crucial conference and as some ANC branches openly declared their support for Motlanthe as ANC president.

"The timing would suggest that he meant to try and say, 'Here I am,'" said Shadrack Gutto, a professor of African renaissance studies at the University of South Africa. Gutto said Motlanthe would offer a "totally different type of leadership" and that "he would be a credible leader" in a way Zuma is not.

The biography portrays Motlanthe as someone who could rescue the ANC's credibility in the eyes of those who are disappointed with the party's failure to stem social and economic inequality. But the book also suggests that he may lack the aggressive political drive needed to battle the 70-year-old Zuma.

Still, some analysts believe Motlanthe, 63, has a fair chance following what many see as Zuma's poor handling of labor unrest in South Africa's crucial mining sector and criticism for spending a purported $23 million and more in government funds on improving his rural private residence.

Harvey writes that Kgalema is "acutely aware of the ills in the ANC and that, unless they are dealt with sooner rather than later, the future for the ruling party will be bleak."

Motlanthe himself is being cagey. One of his spokesmen, Thabo Masebe, said Motlanthe "doesn't want to think about a particular position."

"It is up to the will of the (ANC) branches," Motlanthe is quoted as saying in the biography. "My position is that nobody must try to canvass for themselves in the run-up to elections ... But if I am nominated for such a position when the electoral commission approaches me and says I have been nominated for such a position, I will then either accept or decline."

Motlanthe was president of South Africa from September 2008 to May 2009 after then President Thabo Mbeki resigned. Mbeki had been ousted as the leader of the ANC and the divided party agreed on Motlanthe as a safe, non-controversial caretaker president until the national election, which Zuma won. Motlanthe was widely credited with offering measured, sober leadership as president, according to his biographer.

"Most of those I spoke with say it is gravitas that strikes them most when they think of Kgalema as a leader," Harvey writes. "Kgalema has a dignified seriousness to him. In mass meetings he would probably not be as spontaneous with ululations as Zuma."

Motlanthe was an altar boy and as a young man had hoped to become a Roman Catholic priest, but the apartheid government denied him permission to train outside South Africa. Motlanthe was later influenced by Steve Biko's Black Consciousness Movement and was a voracious reader of general literature as well as Marxist texts. He became well educated despite not having a university degree. He favors wire-rim spectacles and wears a goatee, which has gone gray.

Motlanthe has solid anti-apartheid credentials. In 1977, he was convicted of sabotage and other charges and sentenced to 10 years in prison, most of which he served on Robben Island at the same time as Mandela and Zuma. On his release he joined the National Union of Mineworkers, a powerful labor group, where he grew in stature until 1997, when he was elected secretary-general of the ANC. Since then he has been an influential member of the ruling party, praised as a unifier who shies away from populism.

But Harvey notes that many of those who have worked with Motlanthe describe him as indecisive and averse to taking risks. Those are traits Motlanthe would have to overcome to take on Zuma, a veteran politician who still retains a good deal of charisma.