Nigerian Finance Minister Ngozi Okonjo-Iweala and former Colombian Finance Minister Jose Antonio Ocampo are set to be nominated to lead the World Bank, sources with knowledge of emerging market efforts to find candidates said on Tuesday.The candidacies of Okonjo-Iweala and Ocampo, who have credentials as both economists and diplomats and according to sources the respective backing of Brazil and South Africa, pose a challenge to the United States, whose hold on the top post has never been contested.But with its majority of votes and the expected support of European countries, the United States is still likely to ensure that another American will succeed Robert Zoellick, who plans to step down when his term expires at the end of June. Washington has held the presidency since the Bank’s founding after World War Two, while a European has always led the International Monetary Fund. It has yet to publicly identify a nominee to succeed Zoellick.The deadline for submitting nominations is Friday, and the Obama administration has said it will name a candidate by then.