Vice-President Atiku Abubakar goes to the Supreme Court shortly in what is seen as his last chance to take part in Saturday’s presidential poll.
He is challenging his disqualification over corruption allegations, which he says are politically motivated.
The strongest remaining opposition candidate has told the BBC he is not confident the election will be fair.
The ruling party is doing well in last Saturday’s state governorship contests, winning 11 of the 14 declared.
Opposition supporters took to the streets in parts of the country on Sunday, claiming the results had been rigged.
Opposition presidential candidate Muhammadu Buhari said election officials and police had been biased against his All Nigeria People’s Party (ANPP) in seven states.
“To get whole states where the elections were not fair, and where there was violence, is quite disturbing,” he told the BBC.
“The PDP stalwarts are organising thugs, carrying away boxes, diverting other election materials.”
The presidential contest should see the first time one civilian administration hands power to another since Nigeria’s independence from Britain in 1960.
President Olusegun Obasanjo is standing down after two terms in office.