Libreville - Two opposition heavyweights in Gabon are to back former African Union chief Jean Ping in his bid to oust President Ali Bongo at the ballot box in August, one of them said on Tuesday.Former parliamentary speaker Guy Nzouba Ndama, 70, and former prime minister Casimir Oye Mba, 74, plan to make their decision official at a public meeting with Ping on Tuesday, Nzouba Ndama said."In the public interest, I bow to the decision of parties and civil society," Nzouba Ndama said after withdrawing from the August 27 presidential race in favour of Ping, 73.The three men have been seen as the main contenders to beat Bongo in the equatorial African country, a former French colony, that exports oil and tropical hardwoods.Members of Ping's entourage are presenting him as the "single candidate" to beat Bongo, although more than a dozen people have entered the race.Ping, who served as chair of the AU commission from 2008 to 2012, was previously foreign minister for almost a decade. He was considered close to 57-year-old Ali Bongo's father, Omar Bongo, who ruled Gabon from 1967 until his death in 2009.'Ridiculous things'The electoral commission has approved a total of 14 candidates to contest the election, but none of the others have the political weight of the incumbent and Ping.Opposition activists had called for Bongo to be blocked from running, claiming that he was ineligible as a Nigerian who had been adopted by the former president, which would make his candidacy unconstitutional."I'm in the situation of being an outgoing president with a track record and they prefer to come and get me on ridiculous things," he said in an interview on Friday, referring to opposition claims he falsified his birth certificate to prove his eligibility for Gabon's top job.On July 25, the constitutional court rejected an appeal against the eligibility of Bongo, who came to power in a disputed election following his father's death.