WASHINGTON >> Samantha Power took the podium at Columbia University on Monday night sounding hoarse and looking uncomfortable. In two hours President Barack Obama would address the nation on Libya, and Power, the fiery human rights crusader who now advises Obama on foreign policy, did not want to get out in front of the boss.

“I’m not going to talk much about Libya,” she began, although when it came time for questions she could not help herself. “Our best judgment,” she said, defending the decision to establish a no-fly zone to prevent mass atrocities, was that failure to do so would have been “extremely chilling, deadly and indeed a stain on our collective conscience.”

That the president used almost precisely the same language was hardly a surprise. For nearly 20 years, since her days as a war correspondent in Bosnia, Power has championed the idea that nations have a moral obligation to prevent genocide. Now, from her perch on the White House National Security Council, she is in a position to make that case to the commander-in-chief — and to watch him translate her ideas into action.

“She is clearly the foremost voice for human rights within the White House,” said Ken Roth, executive director of the advocacy group Human Rights Watch, “and she has Obama’s ear.”

The Irish-born Power, now 40, functions as kind of an institutional memory bank on genocide; her 2002 book on the topic, “A Problem from Hell,” won the Pulitzer Prize. While she was by no means alone in advocating military intervention in Libya — Hillary Rodham Clinton, the secretary of state, was a pivotal voice — the president’s decision to pursue that course represents something of a personal triumph for her.

It is also a public relations headache. Critics say Power is pushing the United States into another Iraq. (Power, like Obama, was a vocal opponent of that war.) The American Thinker, a conservative website, complains that Obama has “outsourced foreign policy” to Power.

Power, who declined an interview, is trying to maintain a low profile, still seared, perhaps, by the memory of how she flamed out as an Obama campaign adviser by calling Clinton “a monster.” The two women have since patched it up — the late diplomat Richard Holbrooke, friend to Clinton and mentor to Power, arranged a rapprochement — and Power arrived at the White House determined to “stay in her lane,” in the words of one friend, and avoid any encounters with headlines.

Yet for all Power’s efforts to shun the spotlight, there has long been a whiff of celebrity about her. In addition to her Pulitzer, she possesses two Ivy League degrees (Yale undergraduate, Harvard Law); an arresting mane of red hair and a personal life that captivates magazine writers.

When she married constitutional law scholar Cass Sunstein — they met on the Obama campaign trail and he now runs the White House Office of Regulatory Affairs — Esquire dubbed them “The Fun Couple of the 21st Century” and photographed them on the squash court, in tennis whites. (Yes, Power is also an athlete; she played on the Yale squash team.)

She arrived in Bosnia as a freelance journalist at age 22, “a flame-haired, freckled girl with guts,” in the words of one reporter who knew her. Diplomats admired her intellect and passion. She was fun to be around, people who knew her said, and not shy about haranguing U.S. officials for what she saw as the United States’ failure to act.

“She would argue that the failure of the Clinton administration to engage in airstrikes against the Serbs and to take military action to stop the genocide was immoral,” said Peter Galbraith, ambassador to Croatia at the time.

He recently turned the tables on Power, sending her an email in which he warned her not to let Libya become “Obama’s Rwanda,” a reference to former President Bill Clinton, who has expressed regret over failing to intervene to prevent atrocities there. Galbraith said Power, having learned the lesson that “when you’re inside government, you live with constraints,” did not reply.

Friends say Power is sensitive to any notion that she had outsized influence with the president.

The United States did not go to war in Libya because “there was some dramatic meeting in the Oval Office where everybody tried to persuade the president not to do this, and Samantha rolled in with her flowing red hair and said, ‘Mr. President, I stand here alone in telling you that history calls upon you to perform this act,” said Tom Malinowski, who runs the Washington office of the advocacy group Human Rights Watch and is a friend of Power. “That’s not how it happened.”

Obama sought out Power in early 2005, when he was a new senator and had just read her book. After a four-hour dinner, they found themselves so much in sync that she volunteered to take a leave from her Harvard professorship to work for him.

The book argues that genocides — in Armenia, Nazi Germany, Cambodia, Rwanda, the Darfur region of Sudan — have occurred because governments averted their eyes, and individuals made conscious choices not to intervene.

“The most common response,” Power wrote, “is, ‘We didn’t know.’ This is not true.”

As a journalist, she was one of the first to chronicle the bloody ethnic cleansing in Sudan. In 2004, on assignment for The New Yorker, she traipsed through refugee camps at Chad and slipped into rebel-held areas in Darfur, to interview survivors and witness villages that were burned to the ground. Some experts say her work influenced former President George W. Bush to apply the label genocide to the situation in Darfur.

But if Power was able to prick the collective conscience of elected officials as an outsider, on the inside she has confronted the difficulties of making policy in a complex environment with competing demands.

She has been successful in urging the Obama administration to embrace congressional legislation calling for the arrest of the leader of The Lord’s Resistance Army, which enslaves children as guerilla fighters. As of last year, the White House has a full-time staffer devoted to monitoring war crimes and mass atrocities — a position that Power pushed for. But in Darfur, violence has escalated as the administration has shifted its attention to south Sudan.

On Libya, Power’s critics — and even some admirers — suggest she may be helping to set a precedent that will invariably fail.

“I think what she is doing is good,” said Bill Nash, a retired Army general who commanded forces in Bosnia and knows Power from their time at Harvard, “but I suspect it is more black and white to her than the real world portrays.”

In her long-scheduled speech at Columbia on Monday night, Power did not dwell on such questions. Rather, in a dry monotone, she gave a bland recitation of Obama’s human rights policy. When it was over, she was mobbed by book-toting autograph-seekers.

When she spied a gaggle of reporters, she stiffened and lowered her head.