(Adds Yar'Adua delaying trip to China, paragraph 10)

By Estelle Shirbon

ABUJA, Feb 25 (Reuters) - A Nigerian tribunal will rule on Tuesday whether the election of President Umaru Yar'Adua was valid, a decision that could entrench a disputed government or tip Africa's most populous nation into turmoil.

Yar'Adua and his People's Democratic Party (PDP) won a landslide victory in last April's general elections but neutral observers accused the ruling party of widespread vote-rigging.

Yar'Adua's two closest rivals, former army ruler Muhammadu Buhari and then Vice-President Atiku Abubakar, challenged the result before a special tribunal of five judges which is due to start delivering its ruling at 10 a.m. (0900 GMT) on Tuesday.

It will be watched closely across Africa, where elections regularly draw accusations of cheating and the aggrieved often take protests to the streets -- as they have in Kenya -- rather than to court as the Nigerian challengers have done.

The Nigerian judges can uphold the election result or order a re-run, although the legal battle is likely to drag on for months as the losing parties will have the right to appeal.

The tribunal faces a difficult choice between legitimising an election that was labelled "not credible" by international observers, or destabilising a major oil-exporting country just nine years after it emerged from decades of coups and army rule.

"If judgement favours Yar'Adua, we'll all be expected to accept it and move on," said opposition politician Amin Dalhatu.

"It will, however, give people the wrong impression that whatever one gets away with at the polling stations will eventually find legitimacy at the tribunals," he said.

If the judgment goes against Yar'Adua and he appeals, he will remain president until the Supreme Court delivers its verdict. However, he would be weakened and policy-making would likely grind to a halt until the final ruling.

Yar'Adua's spokesman said on Monday that the president would delay his departure for China, originally scheduled for the early hours of Tuesday, until after the ruling is made public.

RISKS

Those who favour a re-run have been encouraged by court rulings annulling the poll victories of six PDP state governors and dozens of legislators who also won their seats last April.

These rulings suggest the judiciary is asserting itself and may not hesitate to get rid of Yar'Adua as well, they say.

But a ruling against Yar'Adua would carry huge risks.

He may choose not to stand again and there are doubts about whether a re-run would deliver a more legitimate president. The ruling party is mired in an internal struggle that has already turned violent.

"If the court does order a re-run, what guarantees are there that the process will be fairer, with the same electoral commission and the ruling PDP in an even stronger position in the states?" said Antony Goldman, an independent Nigeria expert.

Further complicating the situation is the case of Senate President David Mark.

Under the constitution, Mark is supposed to take over if the tribunal or Supreme Court oust Yar'Adua and his deputy. But Mark's own election as a PDP senator was annulled by a court in his home state of Benue on Saturday. He will appeal.

Some commentators believe that because of the dangers the presidential tribunal will favour stability, particularly after the bloodshed that has followed Kenya's disputed Dec. 27 poll.

"The judges will not be oblivious to the violence and chaos in Kenya and the possibility of contagion," said Bismarck Rewane of the company Financial Derivatives in Lagos.

(For views on the tribunal, take a look at the Africa blog: http://blogs.reuters.com/africa/2008/02/14/talking-point-how-will-the-presidential-election-tribunals-verdict-affect-nigeria/)

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