* Agang's Ramphele throws in lot with Democratic Alliance
* Opposition present united front against ANC
* Ruling party's majority not seen under threat
By Wendell Roelf
CAPE TOWN, Jan 28 (Reuters) - Anti-apartheid activist Mamphela Ramphele will run for president for the Democratic Alliance (DA) in this year's South African election, giving the main opposition party a prominent black leader to challenge the ruling ANC.
But the choice of Ramphele, which some political analysts believe was aimed at shaking off a perception of the DA as the guardian of white privilege, is unlikely to turn popular support against the African National Congress, which led a decades-long struggle against the apartheid system.
The move might even backfire on Ramphele, a medical doctor and former World Bank managing director who commands respect among the black majority as the partner of Black Consciousness leader Steve Biko, beaten to death in apartheid police custody in 1977.
Ramphele's year-old Agang political party has struggled to gain traction despite growing disaffection among voters with President Jacob Zuma's ANC, in power since the end of white-minority rule in 1994.
"I can think of no better person to be our presidential candidate in this crucial election," DA leader Helen Zille told a news conference in Cape Town, before embracing her new political ally.
Ramphele - a successful businesswoman who made millions as a mining industry executive - alluded to the death in December of Nelson Mandela, South Africa's first black president, as a symbol of the changing political landscape.
"I believe this decision is in the best interests of South Africa as we head into turbulent waters," she said. "The death of Nelson Mandela has changed many things for South Africa."
The ANC is expected to romp to victory in this year's polls, despite seeing its support wane over the years amid charges it has failed to lift millions of blacks out of grinding poverty while some officials have been implicated in corruption.
The party won nearly two thirds of the vote in the last elections in 2009 and its overall majority this year is not in question.
Ramphele has described the ANC as "authoritarian, intolerant of criticism and unaccountable", and in 2012 accused Zuma of leading an assault on the post-apartheid constitution.
But her supporters will likely be worried that her voice might be drowned out in the DA, which remains largely white-dominated.
"She will be seen as a 'weak' joiner, more likely to be shaped by where she is going than to be the shaper," said independent political analyst Nic Borain.
However, the immediate public reaction suggested some voters see the move as a tactic by the DA to win black support.
"The DA doesn't represent black aspirations, the hierarchy of the DA is white male," said independent film maker Sindile Mnguni, smoking a cigarette next to a six-metre bronze statue of Mandela outside the Sandton City mall.
"(Ramphele) is opportunistic, that's my take on it. Why did she even bother starting a party in the first place? It's a cop out," Mnguni added.
According to Papi Thomas, a 25-year-old financial adviser in Johannesburg, the move appeared to be a bid by Agang to get into parliament through the back door, since it might not make it alone.
"That changes my opinion on who I might have voted for. Who knows? The ANC might get my vote now," Thomas said. (Additional reporting by Tiisetso Motsoeneng and Helen Nyambura-Mwaura; Writing by Ed Cropley and Stella Mapenzauswa; Editing by Mike Collett-White and Robin Pomeroy)