By Nick Tattersall

LAGOS, Sept 18 (Reuters) - Nigerian President GoodluckJonathan on Saturday launched his campaign for elections due inJanuary, but will need to persuade northern factions to back himif he is to win a clear victory in the ruling party primaries.

Although not formally set in writing, there is an agreementamong the political elite in the ruling People's DemocraticParty (PDP) that the presidency should alternate between northand south after every two four-year terms.

Jonathan, a southerner, took over as head of state earlierthis year after the death of President Umaru Yar'Adua, anorthern Muslim who was part way through his first term.

Some northern power brokers say what should have been hissecond term can only be taken by another northerner and thatJonathan should therefore not be standing. Others say it is timefor the "zoning agreement" to be jettisoned.

Here are some questions and answers about the agreement:

WHAT ARE ITS ORIGINS?

Africa's most populous nation is roughly equally dividedbetween Christians and Muslims. More than 200 distinct ethnicgroups generally live peacefully side by side.

But civil war left more than one million people dead between1967 and 1970 and there have been bouts of ostensibly ethnic andreligious violence, particularly in the central Middle Beltwhich lies on the fault-line between north and south.

More than 13,500 people have died in religious or ethnicclashes since the end of military rule in 1999, according toU.S.-based Human Rights Watch.

Clashes between Christian and Muslim gangs in the centralcity of Jos killed hundreds of people earlier this year.

The violence had more to do with rivalry for political andeconomic power than with religious fervour. Some Jos residents-- mostly members of the largely Muslim Hausa ethnic group --are classified as "non-indigenes" and denied opportunities givento those classified as original inhabitants, rights groups say.

The notion of sharing power between north and south aims toprevent such disputes becoming a factor in federal politics.

"It has been an unwritten law since 1979," said AbubakarMomoh, politics professor at Lagos State University.

"This is fostered by circumstances and the tragedy ofhistory and there is nothing you can do about that."

HOW FORMAL IS THIS AGREEMENT?

There is no reference in Nigeria's constitution to the ideaof the presidency alternating between north and south.

But the constitution and manifesto of the ruling People'sDemocratic Party (PDP) both make reference to "geo-politicalbalancing" as a fundamental principle of power sharing.

CAN IT BE ENFORCED?

The PDP's manifesto acknowledges the supremacy of theNigerian constitution, meaning that there is no legal documentrequiring that presidential candidates should come from anyparticular geographic region or religious background.

But any attempt to break the spirit of the agreement couldcause deep divisions within the ruling party and potentiallystoke resentment in parts of the country.

Should Jonathan lose out in 2011, his supporters in his homeregion of the Niger Delta, the restive heartland of Nigeria'smainstay oil and gas industry, could launch protests.

Should he win the PDP primaries, this could also stir upresentment among northern youths, who have already mountedsmall-scale protests to lobby against his candidacy. (For more Reuters Africa coverage and to have your say on thetop issues, visit: http://af.reuters.com/ ) (Editing by Giles Elgood)