The APC elected a veteran politician from the Christian south as its inaugural chairman on Saturday, as it looks to win voters on both sides of a religious divide and unseat President Goodluck Jonathan's next year.
At its first national convention since being formed by the merger of four opposition parties last year, the All Progressives Congress (APC) elected 74-year-old John Odigie Oyegun to its top post in the early hours of Saturday, party officials said.
The APC is expected to give the ruling People's Democratic Party (PDP) its sternest test since it swept to power at the end of military rule in 1998 and, like its rival, has been keen to portray itself as a national party, rather than a regional one.
Oyegun is the former governor of Edo State in the mainly Christian south. The selection of a southern chairman increases the likelihood the APC will field a candidate from the north in next year's presidential election, analysts have said.
Under pressure over allegations of corruption and its inability to quell the insurgency of militant Islamist group Boko Haram, the PDP has sought to portray the opposition as beholden to northern interests.
Public criticism of Jonathan, a southern Christian, has increased since Boko Haram abducted more than 200 girls in April from a school in the village of Chibok.
The APC has also been bolstered by dozens of lawmakers defecting to it from the ruling party.
Jonathan angered many in his PDP when he ran in the 2011 presidential election, thereby breaking an unwritten party pact that the presidency switches between the north and south every two terms.
He has not confirmed he will run again, but campaign-style posters and banners bearing his image have sprung up around the capital.