By Senan Murray
BBC News website, Abuja
With the PDP's money and power, Yar'Adua is favourite to win
Despite assurances from the frontrunner in Nigeria's presidential race that he is well enough to stand in next month's elections, there is growing speculation that Umaru Musa Yar'Adua might be replaced as the ruling party's candidate.
It follows his evacuation to a German hospital where he is being treated for an undisclosed kidney condition.
But the country's electoral law does not allow such changes with less than 40 days to the poll.
The Independent National Electoral Commission (Inec) says the law only allows it to postpone the elections if a candidate dies.
"At this stage, there is no room under the Electoral Act 2006 for any political party to change candidates," says journalist and political analyst Habeeb Pindiga in the capital, Abuja.
"The People's Democratic Party must field Yar'Adua for the election as long as he is alive. That is the position of the law. But if a candidate should die, then Inec will have to postpone the election," he says.
Some critics of President Olusegun Obasanjo accuse him of knowingly backing a sick man so he could hang on to power if Mr Yar'Adua died.
But Mr Obasanjo's spokesman, Uba Sani, dismisses the claim as "absolutely ludicrous" and says President Obasanjo will stand down in May.
"It's unbelievable that anyone would say that. The president has said he is already packing out of the (presidential) villa and people are still coming up with such wicked speculations."
The PDP also says it has no plans to replace Mr Yar'Adua.
"There is no cause for alarm," the PDP said in a statement. "The PDP, for the purpose of clarity, does not in any way feel burdened by the candidature of Yar'Adua."
Analysts say even if Mr Yar'Adua returns from his German hospital hale and hearty, his health will dominate the remainder of the campaigns leading up to next month's elections.
"It means a lot for the elections. It could change the entire election. His ill-health will raise questions about how long he'd last as president should he win the election," says political analyst Usman Mohammed.
He thinks it would be unwise for the PDP to replace Mr Yar'Adua even if it could at this stage.
"Changing the candidate a month to the election will negatively affect the electoral fortunes of the party.
"After all we all know that it is not popular support that determines election results in Nigeria."
Mr Yar'Adua went for his medical check-up after feeling breathless during campaigning.
Describing his poor health as "nothing serious", Mr Yar'Adua told the BBC's Hausa Service that he was "only human and like any other person can fall ill".
"It's not just me. It's the same with every human being," he said.
"People fall ill and if it is the will of God, they get well again. Whoever we are, when it's time for us to die, we cannot live for one more day."
A tall, thin, quietly-spoken man, Mr Yar'Adua was a relatively unknown northern Muslim governor before he won the governing party primaries with the backing of President Obasanjo.
As the PDP's candidate, with access to its money and power, the former chemistry teacher is still considered to be the favourite to become Nigeria's next president.