Nigeria's leading female presidential candidate Oby Ezekwesili on Thursday withdrew from the race, pledging to form an opposition coalition to defeat the ruling party.
Polls to elect a new president and parliament take place on February 16, while governorship and state assembly elections take place two weeks later.
A total of 73 candidates want the country's top job but it is expected to be a contest between President Muhammadu Buhari, of the All Progressives Congress (APC), and Atiku Abubakar, of the Peoples Democratic Party (PDP).
Ezekwesili, from the Allied Congress Party of Nigeria, is a former education minister and ex-World Bank vice-president, who in recent years has campaigned for the release of Boko Haram kidnapping victims, including the Chibok schoolgirls.
The 55-year-old said she decided to pull out after "extensive consultations with leaders from various walks of life across the country over the past few days".
She said she would now "focus on helping to build a veritable coalition to ensure a viable alternative to the #APCPDP in the forthcoming elections".
The PDP was in power from the restoration of civilian rule in 1999 to Buhari's victory in 2015.
But there are few ideological differences between the main parties, candidates for whom have regularly swapped sides between elections.
Both have also faced criticism about their performance in government.
Ezekwesili said a "broad coalition for a viable alternative" was now needed "more than ever before".
Minor parties signed an agreement known as PACT (Presidential Aspirants Coming Together) last year to have a unified candidate to challenge the main two parties.
Political commentators believe only a strong coalition of minor parties can challenge the dominance of the APC and PDP.
In 2015, Buhari's APC was able to defeat then ruling PDP after forming a coalition of fringe parties with the support of some leading PDP defectors.
Entrepreneur and motivational speaker Fela Durotoye, 47, of the Alliance for New Nigeria, has been tipped to lead the coalition.
Others include Kingsley Moghalu, 56, a former deputy governor of Nigeria's central bank, from the Young Progressives Party, and the publisher of the online news site Sahara Reporters, Omoyele Sowore.
Sowore, 47, is a former pro-democracy activist and student.
At an election debate on Saturday, Durotoye said the APC and PDP were "two sides of the same bad coin".
Moghalu agreed: "One is a kettle and one is a pot. And they call each other black."
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