ABUJA (Reuters) - Nigeria's Senate debated for a second day on Wednesday whether to call for President Umaru Yar'Adua to formally transfer executive powers to his deputy after an absence of more than two months for medical treatment.

The 58-year-old leader has been in Saudi Arabia since November receiving treatment for a heart condition but has not formally transferred power to Vice President Goodluck Jonathan, triggering debate over the legality of government decisions.

Although the Senate does not have the authority to force Yar'Adua to hand over power, if it calls for him to do so and he ignores its advice, parliament could in theory move to impeach him for misconduct, according to the constitution.

"The Senate is heavily divided between pro-Yar'Adua, pro-Jonathan and the undecided who want to see which way the pendulum swings before they opportunistically jump on board," said Abubakar Momoh, politics professor at Lagos university.

"The demonstrable political behaviour of the members of the Senate so far ... suggests the Yar'Adua lobby is likely to have the upper hand, this is my sense" he told Reuters.

Senators were locked in a five-hour closed-door debate on Tuesday without resolution. Ayogu Eze, chairman of the Senate's media committee, said every member who wanted to do so was being given the opportunity to speak in a "painstaking" session.

"We want to avoid a situation where you belong to one camp in the night and belong to another in the day. Everyone must speak on the record," said one senator, who declined to be named.

Whatever the upper house of parliament decides, the very fact it has had to debate the issue piles further pressure on Nigeria's leaders to resolve the uncertainty over who is actually running Africa's most populous nation.