(Bloomberg) -- Nigerian state governors are divided over whether to uphold a convention of rotating the presidency between the north and south of the country.

President Muhammadu Buhari, a Muslim from the north of Africa’s most populous nation, is due to step down after elections in early 2023. Before selecting their candidates next year, Nigeria’s two main political parties have to decide where they should come from.

With 18 months to go, the governors of Nigeria’s 36 states are adopting public positions along regional rather than party political lines: the southern leaders say it’s their turn to field the next president, while their northern counterparts argue they shouldn’t be excluded from the contest.

The next head of state “must come from the southern part of Nigeria in line with policies of equity, justice and fairness,” Rotimi Akeredolu, chairman of the Southern Governors’ Forum, which represents the heads of 17 states, said earlier this month.

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That position was rejected by the Northern Governors’ Forum, which speaks for the remaining 19 states. Insisting on a president from the largely Christian south is “quite contradictory with the provisions of the constitution,” Simon Lalong, the forum’s chairman, said on Monday.

Akeredolu and Lalong are both members of the Buhari’s ruling All Progressives Congress, while the associations they head both include governors from the main opposition Peoples Democratic Party.

Candidates should not be prevented from running for president because of their origins, according to the Northern Elders Forum, an influential political organization. “We have the majority of the votes,” NEF spokesman, Hakeem Baba-Ahmed, said on Sept. 19. “Why do we need to accept a second-class position when we can fight for and get a first-class position?”

The PDP governed the West African nation from the restoration of democracy in 1999 until Buhari’s first election victory 16 years later. The party’s unofficial rule of switching the presidency between north and south was upended by Umaru Musa Yar’Adua’s death in office in 2010 and Goodluck Jonathan’s defeat in 2015.

The APC is yet to adopt a stance on whether it will only consider southerners as potential presidential candidates. The PDP has established a committee to distribute its top posts among Nigeria’s six geopolitical zones and it’s anticipated that if the post of party chairman goes to a southerner, its presidential nominee will come from the north.

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