Jan 9 (Reuters) - Nigeria's ruling People's Democratic Party (PDP) holds presidential primaries on Thursday which could be the most important political turning point in Africa's most populous nation in more than a decade.
Opposition parties are expected to hold their conventions the following day.
President Goodluck Jonathan is broadly considered the favourite to secure the PDP ticket but his candidacy is controversial because of a party pact that says power should rotate between the north and south every two terms.
Jonathan, a southerner, inherited the presidency last year when his predecessor Umaru Yar'Adua, a northerner, died part way through his first term. Some PDP northerners say the north must be allowed to complete what would have been his second term.
The PDP controls more than two thirds of Nigeria's 36 states and has a majority in both houses of parliament.
Its candidate has won every presidential race since the end of military rule in 1999, meaning victory in the primaries has always been tantamount to winning the presidency. This time, splits in the party mean that may no longer be the case.
Following are details of the key dates and events:
The 27 state governors from the PDP form a powerful caucus in the ruling party and securing their backing could be key to Jonathan's success at the primaries.
Holding the state governorship primaries ahead of the presidential primaries weakens Jonathan's leverage. Had the order been reversed, the governors would have had more of an incentive to back Jonathan to secure their own victories.
Twenty of the 27 governors have publicly said they will back him, but there is no way of telling whether they will ultimately do so at a secret ballot.
A PDP panel will screen all aspirants for the presidential primaries to ensure they comply with party regulations. The two main candidates are expected to be Jonathan and former Vice President Atiku Abubakar, a northerner.
The panel, whose make-up has been kept secret, has the power to disqualify candidates on technicalities. Some analysts have suggested it could try to disqualify Atiku because he only recently rejoined the PDP.
Should other northern politicians such as Kwara state governor Bukola Saraki or former military ruler Ibrahim Babangida be screened, Jonathan could still face a challenger other than Atiku.
The presidential primaries are due to take place in Abuja where delegations from each of the 36 states will meet for a national convention. Each delegation is led by the state governor if he is PDP or by another party official if not.
Each state is called up alphabetically and its delegates vote in what is usually a secret ballot. Polling agents for each of the candidates observe the process.
The count begins immediately after all of the state delegations have voted and the results are expected to be announced immediately, probably in the early hours of Jan. 14.
A simple majority wins.
Some of the main opposition parties, including the Action Congress of Nigeria (ACN), the All Nigeria People's Party (ANPP) and the Labour Party, are expected to hold their presidential primaries the day after the PDP convention.
In theory, this could allow the losing candidate in the PDP primaries to switch to an opposition party and contest the April election against the ruling party.
The Congress for Progressive Change (CPC) has already ratified ex-military ruler Muhammadu Buhari as its presidential candidate at a convention on Jan. 4.
The Independent National Electoral Commission (INEC) has set Jan. 15 as the deadline for the conclusion of the primaries. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Reporting by Nick Tattersall and Camillus Eboh; editing by Myra MacDonald)