He has no army pedigree, lived in the West for decades and has criticized successive rulers of Egypt, from Hosni Mubarak and the generals who replaced him to the now ousted Islamist president, Mohamed Mursi.
Now, more than two years after the anti-Mubarak rebellion, Mohamed ElBaradei, 70, is the designated negotiator for the opposition forces that clamored for the army to remove Mursi and his Muslim Brotherhood-dominated government.
Military, political and diplomatic sources tipped him as favorite to head an interim government under Adly Mansour, a senior judge sworn in on Thursday as Egypt's acting president.
ElBaradei met General Abdel Fattah al-Sissi on Wednesday and was present when the armed forces commander announced that Mursi was no longer president and suspended the constitution.
“ElBaradei is our first choice,” a source close to the army said. “He's an international figure, popular with young people and believes in a democracy that would include all political forces. He is also popular among some Islamist groups.”
The source was referring to some members of the Nour party, an ultra-orthodox Salafi group that has been both an ally and a rival of Mursi's Muslim Brotherhood.
Political sources said ElBaradei, who won the Nobel peace prize for his work as head of the International Atomic Energy Agency (IAEA), would also be acceptable to Western powers that have studiously avoided calling Mursi's removal a military coup.
Other contenders include former Prime Minister Kamal Ganzouri and former Central Bank governor Farouk el-Okdah.
ElBaradei said on Thursday that the army's roadmap, under which a panel for national reconciliation will be set up and the constitution will be reviewed before parliamentary and presidential elections, would “continue the revolution” of 2011.
He entered Egypt's political stage in 2010, stirring up decades of calcified politics under Mubarak by saying he might run for president in 2011 if a fair vote was guaranteed.
At the time, many of his countrymen admired his pluck in challenging Mubarak, who had rigged elections for decades, but otherwise found it hard to identify with the be spectacled, soft-spoken United Nations diplomat who had spent so long abroad.
Mubarak's allies painted ElBaradei, a defender of human rights and social freedoms, as a “foreign agent”. The opposing Islamist camp dismissed him as “too liberal” for Egypt.
A hate-figure for some hardline Islamists, ElBaradei was named in February, along with a leftist politician, by a Muslim cleric as meriting the death penalty under Islamic law for seeking to topple Mursi.
ElBaradei has long pressed for Egypt to sign a $4.8 billion loan deal with the International Monetary Fund, which Mursi's government initial led in November but never ratified.
The deal could help kick-start an economy battered by the collapse of tourism and investment due to political turmoil since the 2011 uprising. But it would also entail politically risky tax increases and cuts in fuel and food subsidies.
ElBaradei, a supporter of private enterprise, says austerity is the price Egypt must pay for the IMF loan.
“I think there is no other option. Nobody disagrees that the economy is ailing, and nobody disagrees that you have to take certain measures,” he said in an April 30 interview.
Egypt needed private investment, but to attract that it needed political consensus, something he said was lacking under the Muslim Brotherhood government.
“Private investment obviously needs 100 percent political consensus. People need to come here and see there is law and order, institutions, a minister who can sign a piece of paper. It's not there right now,” ElBaradei argued.
He disappointed many of his supporters in January 2012 when he pulled out of the presidential race while the interim army council was in power, saying the election would be a farce without a constitution defining the president's role.
Cynics said the liberal contender, who was still spending much of his time overseas, withdrew because he stood no chance against the Brotherhood's well-oiled voting machine.
ElBaradei's critics have often called him out of touch with ordinary Egyptians, communicating mostly via Twitter, although he has sought to counter this lately with more public speeches.
The latest tide of protest that has swept Mursi away has also brought ElBaradei, who founded the liberal Dustour(Constitution) party in 2011, back into the limelight.
Steadfast in his rejection of one-man rule and in his advocacy of “peaceful disobedience” to fight oppression, he says building bridges is vital in a polarized Egypt.
“Success or failure is now on the altar of political consensus, because if you don't have consensus, you don't have stability,” ElBaradei said in the April interview with Reuters and the Associated Press in April at his Cairo villa.
“Without stability, you don't have an economy going on, and without an economy going on, you will end up with hungry, angry people.”
In his time at the IAEA, ElBaradei is credited with helping transform a little-known U.N. technical body into a high-profile agency prepared to take a stand on political issues relating to non-proliferation - which critics said went beyond its mandate.
Before the 2003 U.S.-led war in Iraq, ElBaradei highlighted the lack of evidence that Saddam Hussein was developing nuclear arms, angering many in the U.S. administration, but winning admirers among President George W. Bush's critics.
Soon after leaving the IAEA he became more politically involved and returned to Egypt in February 2010 to an exuberant welcome from supporters who hoped he would run for president.
He sees the National Salvation Front, the umbrella that groups non-Islamist parties, including his own Dustour, as eventually splitting into liberal and leftist blocs.
“You only need two big parties that can stand up to the right, or whatever you call them. I think everyone agrees that in the long run that is where the NSF should go, that we have a big liberal party and a big leftist party. It's probably easier with regards to the liberal party. We have been talking.”