Both houses of Nigeria's National Assembly have passed a motion for sick President Umaru Yar'Adua to hand power to his deputy until he recovers.
The BBC's Ahmed Idris in Abuja says the move among the political elite to back the vice-president is unprecedented.
Mr Yar'Adua has been in hospital in Saudi Arabia since last November.
His allies have been loath to see Vice-President Goodluck Jonathan sworn in as acting president because of Nigeria's delicate regional balance of power.
Mr Yar'Adua is a northerner while Mr Jonathan is from the south.
Since democracy returned to Nigeria in 1999, power has alternated between north and south and some northern power-brokers do not want to see their "turn" cut short.
Mr Yar'Adua's absence has sparked legal challenges, cabinet splits and mass protests.
It has led to a freeze in government business and threatens progress made in halting unrest in the oil-rich Niger Delta.
Cabinet challenge
Our correspondent says members of the Senate and House of Representatives and state governors are usually loyal to the president, and the office of vice-president carries little weight.
But the governors decided last week to back Mr Jonathan to become acting president and have been lobbying senators ever since.
The Senate carried the motion declaring that Mr Jonathan "shall henceforth discharge the functions of the office of president, commander-in-chief of the armed forces of the federation, as acting president".
But it is unclear what the next step will be.
Some analysts have suggested the assembly's motion is not legally binding and could face court challenges.
Others say the move has laid down a challenge to the cabinet - due to meet on Wednesday - and presidency to accept the handover, or risk inflaming public opinion.
The cabinet has previously insisted that the president is able to continue in office, although splits subsequently emerged within ministerial ranks.
Elusive leader
Separately, the Senate is also expected to consider a move to impeach the president.
Two weeks ago, the assembly asked Mr Yar'Adua to provide a letter confirming he was sick, which would pave the way for a formal transfer of power.
But there has been no letter, and anti-Yar'Adua factions are expected to give a Thursday deadline for the letter, or threaten impeachment.
A presidential aide last week told the BBC such a letter would be written soon.
Mr Yar'Adua has not been seen in public since flying to Saudi Arabia for medical treatment in November.
He gave a radio interview to the BBC's Hausa service in early January in which he pledged to return to Nigeria as soon as the doctors would allow.
But nothing more has been heard from the leader, who is suffering from an inflammation of the lining around the heart and has long suffered from kidney problems.


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