"It is clear that this is not based on the merits of the candidates but is a political exercise," he said.
His comments referred to the likelihood that an American would be chosen given the US has the most votes at the Bank.
His withdrawal leaves just two candidates - Nigeria's Ngozi
Okonjo-Iweala and Korean-American Jim Yong Kim, who is the favourite for
the job.
Mr Ocampo, who is a professor at Columbia University, also
said he was at a disadvantage given the lack of support from his own
government.
He said he would now support Mrs Okonjo-Iweala "to facilitate
the desired unity of the emerging and developing economies around one
candidate".
Mrs Okonjo-Iweala thanked Mr Ocampo for his "unselfish decision" to withdraw.
The World Bank is expected to name its new president on
Monday. The successful candidate will replace Robert Zoellick who has
been in the role since 2007.
The US has always held the top job at the World Bank while a
European has always led its sister organisation, the International
Monetary Fund (IMF).
Source: BBC News