The main female candidate in Nigeria's upcoming presidential election said she is withdrawing her candidacy to help build a coalition to defeat the ruling All Progressive Congress.

Oby Ezekwesili of the Allied Congress Party of Nigeria (ACPN) announced her decision on Thursday in a series of posts on Twitter.

"I have decided to step down from the presidential race and focus on helping to build a Coalition for a viable alternative to the #APCPDP in the 2019 general elections," she said.

"This coalition for a viable alternative has now more than ever before become an urgent mission for and on behalf of the citizenry," she added.

Over the past three months, I have been in private extended talks with other candidates to make a coalition possible that would allow Nigerians to exercise choice without feeling helplessly saddled with the #APCPDP. — Oby Ezekwesili (@obyezeks) 24 gennaio 2019

Her decision to step down surprised some of her supporters.

"I feel sad that she has stepped down, I am however happy that the national good is what she is thinking about. Her decision to step down and promote a coalition of all the small parties, as we prepare for the 2019 presidential elections, shows true leadership," gender advocate Josephine Effah-Chukwuma told Al Jazeera.

"We need a formidable coalition that can face these two major parties - PDP and APC," Effah-Chukwuma said.

Only the ruling All Progressives Congress (APC) and the Peoples Democratic Party (PDP) have won the country's presidential elections since the restoration of democratic rule in 1999.

Ezekwesili's withdrawal leaves incumbent Muhammadu Buhari and former Vice President Atiku Abubakar as the main contenders for the election, slated for February 16.

Political commentator Gbenga Soloki told Al Jazeera the decision to form a coalition against the main parties won't change the political landscape.

"I don't really see the alleged force staging any upset. The electorate are largely illiterate, who are either having sympathy for the ruling All Progressives Congress or the major opposition PDP," Soloki said.

"The timing is wrong and will not produce any appreciable result. Then comes the issue of sincerity of those in the third force. Nigeria is not ripe for such," he added.

Ezekwesili, a two-time minister and former vice president for Africa at the World Bank, cofounded the #BringBackOurGirls campaign to raise awareness about the more than 200 schoolgirls kidnapped by the armed group Boko Haram in 2014.