By Randy Fabi

ABUJA, Dec 12 (Reuters) - Nigeria's Supreme Court decides on Friday whether the election of President Umaru Yar'Adua was legitimate, a ruling that could bolster his power or mean political chaos in Africa's most populous nation.

The court is widely expected to uphold Yar'Adua's landslide victory in the April 2007 general election, despite widespread allegations from foreign and local observers that the vote was flawed.

Former military ruler Muhammadu Buhari and ex-vice president Atiku Abubakar, who both ran against Yar'Adua, appealed to the Supreme Court after an election tribunal turned down their demands for a re-run in February.

The court is due to start delivering its ruling at 9 a.m. (0800 GMT) on Friday.

"While the court will likely admit that there were some incidents of rigging during the controversial 2007 poll, the justices will likely argue that the incidents were not widespread enough to overturn Yar'Adua's 75 percent landslide victory," said Sebastian Spio-Garbrah, analyst for Eurasia Group.

The judgment, against which there can be no appeal, should put an end to nearly two years of legal wrangling that has limited Yar'Adua's authority and unnerved foreign investors in sub-Saharan Africa's second biggest economy.

If the court upholds Yar'Adua's victory, the president will face growing pressure to speed up reforms such as fixing the country's shoddy power sector and improving security in the oil-rich Niger Delta.

Yar'Adua's critics believe he has moved too slowly on reforms, pointing to the long delays in drafting a 2009 budget, choosing a new cabinet, and revamping the energy sector.

Should the court rule against Yar'Adua, both he and Vice President Goodluck Jonathan would have to stand down for a new election to be held within three months.

Under the constitution, Senate President David Mark would take over as acting president and be responsible for organising a re-run.

(To see FACTBOX-Nigeria Supreme Court to decide on Yar'Adua election, please click on [ID:nLB738740].)

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