But an outright assassination would be a dangerous new turn in Egypt's turbulent transition since the toppling of autocrat Hosni Mubarak nearly two years ago.
There are certainly precedents.
The Muslim Brotherhood, which was banned until Mubarak's fall, renounced violence in the 1970, but it killed a string of top politicians in the 1940s and 1950s and was accused of trying to assassinate former leader Gamal Abdel-Nasser.
In 1981, then-President Anwar Sadat was gunned down by Islamic extremists on live TV. The Islamic militant insurgency against Mubarak in the 1990s saw a number of assassinations. One radical tried to stab Nobel Laureate novelist Naguib Mahfouz, but he narrowly survived. Writer Farag Fouda, who was a scathing critic of Islamists, was gunned down as he left his office, days after a fatwa called for his death as an apostate.
Fouda's daughter, Samar, warned ElBaradei this week about the new fatwas.
"They killed my father after a fatwa permitting the shedding of his blood. Don't take lightly what might happen and what they say. They are sick," she wrote on her Twitter account.
Some activists worry that there is already a pattern of their members being targeted — whether for intimidation or worse.
Hamdi el-Fakharani, a lawyer with the Salvation Front, says he has been beaten up several times by suspected Brotherhood members in his hometown, the industrial city of Mahallah el-Kubra in the Nile Delta. Brotherhood members have raised lawsuits against him, accusing him of inciting riots against Morsi in the city the past two weeks.
"It is very possible that one day while the Front leaders are meeting all together, a man with machine gun kills us all," said el-Fakharani. "I am really worried about my family and myself. I change cars all the time."
The opposition is also alarmed about the killing of several young activists since November, most of whom worked on anti-Brotherhood pages on Facebook. Three were shot to death amid crowds of protesters during police crackdowns on rallies in which others died as well. Still, their comrades are convinced that they were specifically targeted.
The first was 16-year-old Gaber Salah, known by the nickname Jeka, who died after being hit by a spray of birdshot at his head and chest from close range during a protest near Tahrir Square on Nov. 15. He was a founder of a Facebook page called "Together Against the Muslim Brotherhood."
Others involved in the page have faced troubles. One founder, a Christian, was summoned by the National Security Agency, the main internal security apparatus, and told not to work on the page anymore, said another administrator of the page, who spoke on condition he be identified only by his first name, Ahmed, for fear of reprisals by police. Yet another founder was snatched from his home on the same day that Jeka was shot, Ahmed said.
"What do you think? I believe this is all planned," Ahmed said.
Mohammed Hussein, also known as Kristi, the founder of a Facebook page called "Brotherhood Liars," was shot to death outside the palace on Feb. 1.
Another prominent activist, Mohammed el-Gindi, disappeared from a Jan. 25 protest in Tahrir, was later brought to a hospital in a coma and died last week. Medical reports say he had burns from electrical shocks on his tongue, wire marks around his neck, smashed ribs and a broken skull. The Interior Ministry denied he was ever held by police.
Justice Minister Ahmed Mekki told the state-run Al-Ahram newspaper Saturday that the coroner concluded el-Gindi died in a car accident.
Omar Morsi, founder of an anti-Brotherhood page called MolotovCola, disappeared from a Jan. 27 protest and his whereabouts was not known for days. He is now comatose in a Cairo hospital with a shotgun pellet in his head.
His father, Ahmed Morsi — who said he voted for the president in last year's election — accused the security forces.