Both are veterans of his 2008 campaign and have strong personal relationships with Mr. Obama. But they will be working for a president who has stubbornly resisted intervening in the most dire human-rights calamity of the day, the civil war in Syria. Given Mr. Obama’s fixed views, it is not clear whether even Ms. Rice and Ms. Power could prod him into action.
Ms. Rice, who is becoming national security adviser, and Ms. Power, who is replacing Ms. Rice as American ambassador to the United Nations, teamed up in 2011, along with Secretary of State Hillary Rodham Clinton, to persuade Mr. Obama to back a NATO-led intervention in Libya that was designed to head off a slaughter of the rebels in Benghazi.
In her new role, Ms. Rice in particular will be able to exert even more influence, occupying a West Wing office down the hall from a president who has already concentrated foreign policy decision-making in the White House.
But as Mr. Obama and his aides have long argued, Libya is no Syria. The first was a clear-cut case in which air power could prevent Col. Muammar el-Qaddafi from killing thousands of rebels in their stronghold; the second, a sectarian struggle, pits a regime with sophisticated air defenses against rebels scattered throughout the country.
Neither Ms. Rice nor Ms. Power has spoken out publicly in favor of a more aggressive American response to the blood bath in Syria, which is perhaps not surprising, given Mr. Obama’s well-known views and their own roles as rising stars in his administration.
Administration officials said that in the debate last summer about whether to supply the rebels with arms – a proposal pushed by the then-director of the Central Intelligence Agency, David H. Petraeus – Ms. Rice sided with those who opposed it. Over time, however, officials said, she has become more open to lethal aid, given the stalemate in the civil war.
Gary Bass, a professor of politics and international affairs at Princeton University, said that in formulating its Syria policy, the administration would have to answer a basic question.
“Do you think of Syria as being a Rwanda or a Bosnia, where human rights concerns trumped everything?” he said. “Or do you see it as more like Iraq, where it’s not clear there’s a good side to get behind?”
There are other voices for stronger action, including Secretary of State John Kerry. He may find common cause with Ms. Rice on Syria even as he struggles to carve out an influential role in an administration where decision-making resides at the White House.
Ms. Power and Ms. Rice, who are friends, each bring their long, sometimes painful histories to this issue.
For Ms. Power, who made her name as a journalist covering the wars in the former Yugoslavia, Bosnia was a formative experience. In her book “A Problem from Hell,” she presented a history of genocide in the 20th century and a withering critique of the failure of the United States and other countries to respond to them.
For Ms. Rice, who began her career in the National Security Council during the Clinton administration, Rwanda was a crucible. President Bill Clinton’s inaction in the face of genocide there fueled many of the people who worked for him, including Ms. Rice, not to allow a repeat.
Years later, she told Ms. Power, who was then a journalist writing about the episode, that “I swore to myself that if I ever faced such a crisis again, I would come down on the side of dramatic action, going down in flames if that was required.”
With the death toll in Syria surpassing 80,000, little sign that the violence is ebbing, and the growing threat of a regional proxy war, Ms. Rice and Ms. Power may face that reckoning again soon.