A Nigerian court ruling is expected to rule on whether Vice-president Atiku Abubakar can run in the presidential election on 21 April.
Many see Atiku Abubakar as the ruling party's main challenger
Mr Abubakar went to court after his name was omitted from a list of candidates cleared to run in the poll.
He became a leading opposition candidate after falling out with former ally President Olusegun Obasanjo.
Electoral officials say corruption charges against him would have to be dropped for him to be eligible to run.
Despite an earlier court ruling that the electoral commission could not disqualify candidates, officials say Nigeria's constitution bars Mr Abubakar from running for office.
"Commissioners have no choice but to apply the constitution," Independent National Electoral Commission (Inec) Chairman Maurice Iwu said.
Mr Abubakar was indicted for graft by a government ministerial panel which investigated corruption in an oil fund.
"Once you have been indicted, Inec cannot do anything about it," Mr Iwu said.
President Obasanjo is standing down after two terms and Umaru Yar'Adua is standing on behalf of his People's Democratic Party (PDP).
Mr Abubakar, a founding PDP member defected to the opposition Action Congress (AC) after he fell out with President Obasanjo.
AC is one of the two main opposition parties contesting the election, and continues to hold mass rallies around the country, despite the block to its candidates' presidential ambitions.