As Nigerians await a tribunal ruling on the validity of last April's discredited presidential ballot, political analysts fear the continent's most populous nation could slide into a political crisis. Gilbert da Costa has details from Abuja.

The cancellation of a presidential election result by an election tribunal is unprecedented in Nigeria. Despite this, opposition party leaders say they are confident ahead of Tuesday's verdict.

The Action Congress, one of two opposition parties challenging the election, says it has done enough to secure a favorable ruling. Lai Mohammed speaks for the party.

"I believe our lawyers have done the best they can do," he said. "We have no doubt in our minds that given the efforts of our lawyers and the compelling evidence that we have adduced, that we have given our best."

"If you look at what happened in Benue state where three out of three senatorial seats have been nullified, then one begins to wonder whether there was any election at all. We are quite confident our case for nullification should be upheld," he added.

The elections in 2007 offered Nigeria the opportunity to achieve a constitutional succession from one civilian administration to another for the first time since independence in 1960.

Analysts say former president Olusegun Obasanjo, in his desperation to ensure the victory of his hand-picked successor, undermined the integrity of the electoral process in favor of candidates of the ruling party.

Local and international monitors described the elections as deeply flawed. Several elected officials have seen their election victories annulled by courts because of irregularities.

Everyone is now talking about what would happen if President Yar'Adua's victory were overturned.

Abdullahi Jalo, a supporter of the president and a ranking member of the ruling People's Democratic Party, says the election tribunal should be guided by the reality of plunging Africa's most populous nation into turmoil in making its decision.

"The judges are Nigerians too," said Jalo. "The election in Nigeria, the result had been issued and the president had started working long ago. Now the president is heading to one year. The better thing for the country is to leave the status quo to continue for betterment, peace and stability of the country."

Mr. Yar'Adua can appeal against the ruling if the tribunal annuls the result. If he losses the appeal or decides not to challenge the decision, he would relinquish power to the senate president, who is expected to conduct fresh elections within 90 days.

The current senate president is contesting the cancellation of his won election; setting the stage for what some say is a recipe for chaos in a country with a history of political instability.