Nigeria's two major political parties hold primary elections this weekend to select their nominees for presidential polls scheduled for February 2019.This time last year, political pundits were hard pressed to name an opposition candidate who could beat President Muhammadu Buhari and his ruling All Progressives Congress (APC). The Peoples Democratic Party (PDP), which enjoyed power since democratic rule was restored in 1999, was in disarray after being ousted from power for the first time in 2015. But since then, Buhari's party has seen a wave of defections in protest at his leadership style and a surge in PDP contenders that could give it momentum as the vote approaches.With Buhari expected to be endorsed unopposed at the APC convention in Abuja, all eyes will be on the southern city of Port Harcourt to see who emerges victorious for the PDP.Barring any surprise, the candidate will come from the Muslim-majority north, following an unwritten rule in Nigeria that the presidency should alternate every two terms between a candidate from the north and south. Who are the main PDP contenders Perennial presidential candidate Atiku Abubakar, a 71-year-old former vice-president from the northeastern state of Adamawa, was the one of the first big names to throw his hat in the ring.Then, after lawmakers crossed the floor in July, former Kano state governor Rabiu Kwankwaso emerged as a leading opposition candidate, promising he can secure the key northern state.Sokoto state governor Aminu Tambuwal joined the PDP defectors and declared his candidacy to pry northern voters away from Buhari, who enjoys mass support across the north. Finally, the ambitious Senate leader Bukola Saraki, a former Kwara state governor who is seen as the shepherd of the defectors, announced he would campaign for the top job. Why is Buhari targeting a second term? Buhari, a 75-year-old former military ruler, is still seen as an anti-corruption crusader in graft-tainted Nigeria and the APC has claimed a PDP victory will mean more state looting.But the opposition maintains the fight against corruption is a political witch-hunt, as most of those arrested and put on trial are PDP members. Questions were raised about Buhari's fitness to govern and run for a second term after he left the country for months in 2017 for treatment for an undisclosed illness. Dubbed "Baba Go Slow" because he took six months to appoint cabinet ministers, he has also faced attacks for his handling of the economy, which plunged into recession in 2016.Meanwhile, with Boko Haram jihadists still launching bloody attacks in the northeast, there are doubts about his repeated claims that the Islamists are close to defeat. What are the main election issues? Personalities, power and alliances are everything in Nigerian patronage politics, as there is little ideological difference between the main parties.Some issues come around every election, not least promises to improve Nigeria's inadequate infrastructure.More specifically, economic growth is a priority with high levels of unemployment, especially among the vast and booming youth population, where joblessness is hovering around 50%. Africa's largest oil producer is tentatively emerging from recession, buoyed only by the rise in global crude prices while the non-oil sector remains stagnant. Buhari's government contends that it has helped diversify the oil-dependent economy but opponents say more liberal policies would have done better to boost growth. Security is also a concern, as tit-for-tat clashes between nomadic herders and sedentary farmers in central states have escalated into a violent crisis with a wider ethnic, political and religious dimension.Rising tensions and divisions have been reflected in hotly contested local party primaries, said Idayat Hassan, director of the Centre for Democracy and Development in Abuja."These primaries have been fractious," she said. "It's one of the most contentious in a long time." What happens next? After the PDP officially chooses its presidential candidate, the party will select a running mate, who will likely come from the largely Christian south. A key question is if the party that enjoyed uninterrupted power for 17 years is able to reinvent itself as a formidable opposition and overcome the power of incumbency at the polls.The APC coalition of parties managed to do so in 2015 to deliver the first opposition victory at the ballot box in Nigeria's history."What we're waiting to see is if the (PDP) will be able to continue as one," said Hassan. "Now they are putting up a united front - but will that continue?"* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.