(Reuters) - Nigerian President Umaru Yar'Adua's two-week absence from Africa's most populous nation has fuelled concern over the state of his health and raised questions about what would happen if he was forced from office early.
His victory in April 2007 polls has also been challenged at the Supreme Court by his two main rivals. The court is expected to uphold his win but should the ruling, due later this year, go against him, he could be forced to step down.
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Below are answers to some questions on what could happen:
HOW WELL IS YAR'ADUA?
Yar'Adua, known to have a chronic kidney problem, travelled to Saudi Arabia last month for a Muslim pilgrimage. A source in the presidency said he had received medical treatment during the trip but the government says he is healthy and will return soon.
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A medical source in Saudi Arabia said Yar'Adua had undergone an operation but gave no further details.
His absence from the country for more than two weeks, despite indications that he would be home by now, has added to concerns for his health and drawn opposition calls for more transparency on his condition.
WHAT DOES THE CONSTITUTION SAY?
In the event of temporary incapacity, the vice-president takes over as acting president. But if the president becomes so indisposed that he is permanently incapable of running the government, the vice-president is sworn in as president.
He then in turn appoints a new vice-president, subject to approval by the National Assembly. The new team completes the unexpired period of the four-year presidential term.
COULD THAT BE A SMOOTH PROCESS?
The political climate in Nigeria is, however, not as straightforward as stated by the constitution.
There is an unwritten agreement among the political elite that the office of the president rotates between the north and south of the country.
President Umaru Yar'Adua, a northern Muslim, assumed office in May, 2007 after eight years of rule by former President Olusegun Obasanjo, a southern Christian.
Vice-President Goodluck Jonathan is also a southern Christian.
Should he take over in the event of Yar'Adua's departure from office, there would be an expectation that the presidency should be handed over to a northerner. This has raised speculation that Jonathan could be forced to resign.
Were he to do so, the constitution states that the president of the Senate would take over as acting president for no more than 3 months, during which new elections would be held.
WHAT ABOUT THE SUPREME COURT RULING? If the Supreme Court annuls the April 2007 election, both Yar'Adua and Jonathan would leave office.
The president of the Senate would take over as acting president and organise fresh elections within 3 months.
WHERE MIGHT THIS LEAVE POLICY?
Yar'Adua's critics believe he has already moved too slowly on reforms such as restructuring the energy sector and have questioned the seriousness of his efforts to curb corruption.
The doubts over his health and Nigeria's political direction, whether or not he is forced out early, risk holding up the process even further.
If Yar'Adua were unable to continue and controversy arose over whether Jonathan could take his place, that might also stoke anger in the oil-producing Niger Delta since the vice-president is from the region.
Attacks by Niger Delta militants seeking a greater share of the region's wealth have already heavily cut oil output in Nigeria, the world's eighth-biggest exporter.googletag.cmd.push(function() { googletag.display('comments'); });