Yar'Adua's absence has caused a cease-fire with militants in the oil-rich delta to unravel and had left no one formally in charge of the nation of 150 million. The crisis in a powerful country with a long history of coups and military dictatorships has drawn international attention, with US Secretary of State Hillary Clinton and European leaders calling on the nation to follow its constitution.
Newspapers began worrying about possible coup scenarios as Yar'Adua's absence grew longer. However, military leaders said several weeks ago they had no ambitions to take power and would respect the constitution.
It was unclear when Jonathan would be sworn in as president.
The National Assembly's empowering of Jonathan may set the stage for new legal battles and power struggles. The motion would allow Yar'Adua to reassume the presidency if he returns to the country healthy enough to lead the nation.
Yar'Adua, who has been suffering from kidney ailments for long, left Nigeria for Saudi Arabia on Nov. 23. He was admitted to a hospital the next day.
Since Yar'Adua left Nigeria, a major kidnapping and a pipeline attack have occurred in the Niger Delta. Religious violence between Christians and Muslims in central Nigeria also left more than 300 dead and thousands displaced. And a young Nigerian attempted to bring down a trans-Atlantic flight bound for Detroit, sparking tighter security regulations for travelers from the West African nation.
While Nigerian law allows for a smooth transition of power from Yar'Adua to the vice president, the 58-year-old president left without following any of those procedures.
The federal government says Jonathan has been acting in Yar'Adua's place without the constitutional procedures but protesters have taken to the streets, saying the country was essentially leaderless. Several former Nigerian leaders have urged Yar'Adua to step down if he is medically unfit to serve.
Parliament's action on Tuesday could disrupt an unwritten power-sharing agreement between Nigeria's Christian south and the Muslim north. Jonathan, a Christian, would be taking over for Yar'Adua, a Muslim, before his appointed time was up.
Yar'Adua has given one interview since being out of the country. In January, he told the BBC over the phone that he hoped to recover and return to power.
US Assistant Secretary of State for Africa Johnny Carson met Jonathan for talks on Tuesday. "The Secretary of State (Hillary Clinton) wanted me to clearly convey that during this period of uncertainty we believe that it is very important that if indeed there is a transition of political power in Nigeria let it be done democratically," Carson told reporters after the meeting.
"As a fellow democracy, US believes firmly in the values of democracy. We think that its principles are also important in Nigeria, which is Africa's largest democracy and also a friend and a partner of the US," he said.
Carson said he had a "very good meeting" with Jonathan to discuss developments in Nigeria and Africa in general.
Carson, who arrived in Nigeria on Saturday, said President Barack Obama and Clinton expressed their sympathies over the hospitalization of Yar'Adua.
He said the need for electoral reforms in Nigeria after a series of flawed elections in the country featured in his talks with Nigerian authorities. "We talked a little bit about elections in the US, which is also committed to working with Nigeria on helping to improve the electoral process," he said.
