|Northern factions within the PDP chose Abubakar to run in the primaries above three other northern contenders [EPA]
Atiku Abubakar, Nigeria's former vice president, has been chosen to challenge Goodluck Jonathan, the incumbent president, for the ruling People's Democratic Party (PDP) nomination in next year's presidential election.
Jonathan is from the south and his decision to also seek the party nomination is controversial as it goes against an arrangement within the PDP that rotates the presidency between the predominately Christian south and the Muslim north.
According to convention, Nigeria's next president, who will be elected in April, should hail from the north.
Northern factions within the PDP chose Abubakar above three other northern contenders - Ibrahim Babangida, a former military ruler, Bukola Saraki, the state governor of Kwara and Aliyu Gusau, a former national security adviser.
"He has been chosen as the consensus candidate that will run against the president in the People's Democratic Party primaries," Garba Shehu, Abubakar's campaign spokesman, said.
Jonathan inherited the presidency this year after the death of Umaru Yar'Adua, a northerner who died part way through his first term.
The president's supporters say he was elected on a joint ticket with Yar'Adua and can complete what would have been the second term.
His opponents say only a northerner can succeed him, leaving the ruling party divided.
Yvonne Ndege, Al Jazeera's correspondent in Abuja, the Nigerian capital, said: "What he [Jonathan] had hoped for was that he would be the nominee for the ruling party, People's Democratic Party, without any challenges.
"But he's being challenged by members of his own party, including Atiku Abubakar.
"There is a core within the party who believe that because the president only became president because the former president died in office, he should step aside and allow a northener, from Nigeria's predominantly Muslim north, to continue the mandate of ... Yar'Adua, which should have ended in 2015.
"Jonathan does have a colossal battle on his hands, Atiku Abubakar was the vice president between 1999 and 2007, he has vast experience, he was one of the architects of the ruling party and he has a massive warchest."