President Obama named Susan Rice to replace Tom Donilon as his national security adviser and Samantha Power to replace Rice at the United Nations in the Rose Garden Wednesday. (The Washington Post)

President Obama announced a major shuffle of his national security team on Wednesday, ushering out a cautious Washington insider and elevating two long-time proponents of a larger American role in preventing humanitarian crises and protecting human rights.

The ideological shift signaled by the choices highlights a central dilemma for Obama as he seeks to make a mark on the world at a time of austerity — and war weariness — at home. How ambitious Obama intends to be abroad at a time of stiff challenges on the domestic front has remained an open question well into his second term.

Susan E. Rice, named by Obama Wednesday to succeed Thomas E. Donilon as national security adviser, and Samantha Power, nominated to follow Rice as U.N. ambassador, will have the opportunity to provide an answer as the administration reviews its policy in Syria, winds down the war in Afghanistan and seeks to stop Iran’s nuclear-enrichment program.

In a Rose Garden announcement, Obama called Rice, who does not need Senate confirmation, a “a fierce champion for justice and human dignity.”

“But she’s also mindful that we have to exercise our power wisely and deliberately,” Obama said. He also praised Power as “a relentless advocate for American interests and values,” urging the Senate to approve her nomination as quickly as possible.

The changes come as Obama struggles for political momentum in his second term, a period when U.S. presidents have traditionally focused first on domestic issues before turning more of their attention overseas.

At home, some of Obama’s most important domestic policy initiatives have faced obstacles in a divided Congress, where Power’s nomination was greeted warmly by members of both parties. In a statement, Sen. John McCain (R-Ariz.) said Power is “well-qualified for this important position” and hoped “the Senate will move forward on her nomination as soon as possible.”

Obama’s second-term foreign policy has so far been more reactive than ambitious, as he manages the U.S. endgame in Afghanistan and considers whether to do more on behalf of Syria’s armed opposition amid a worsening civil war.

The elevation of Rice and Power could quickly change the tenor of the administration’s foreign policy debate, much of it centered in the White House, where Donilon and his deputies have concentrated policy-making authority.

Jon B. Alterman, director of the Middle East program at the Center for Strategic and International Studies, said the shuffling “brings in two women the president has worked closely with and seems to like personally, even if their temperaments clash with the mantra of ‘no drama Obama.’”

He added, “I would think the challenge for both of them is to figure out how much room for maneuver they have.”

The departure of Donilon, a lawyer by training who has played a key role in shaping Obama’s pragmatic approach to foreign policy, has been openly discussed within the White House for months, though some believed he would stay until fall.

Donilon has been the administration’s biggest champion for reorienting U.S. foreign policy toward the fast-growing economies of Asia, an initiative typified by this weekend’s summit meeting between Obama and the new Chinese president, Xi Jinping.

Supported by one of his closest friends, Vice President Biden, Donilon has also argued for a quick end to the U.S. war in Afghanistan.

Both positions have put him at odds with other agencies, particularly the Pentagon’s military commanders, who favored a slower drawdown in Afghanistan than Donilon wanted. Obama eventually split the difference in setting the path toward concluding America’s longest war at the end of 2014.

Donilon’s advocates say he deserves credit for largely keeping the peace within the national security team during sometimes difficult policy debates over Afghanistan, the Iraq war and the approach to the dormant Middle East peace process. The same was not true during George W. Bush’s presidency, particularly over the Iraq invasion and its aftermath.

But Donilon has also been criticized, inside and outside the White House, for running a ponderous policy review process that has angered outside agencies and exasperated some national security officials.

Rice, Obama’s third national security adviser, will lean more toward policy advocacy than mediating differences, according to those who know her.

She has been a key adviser to Obama since before his election in 2008, and one of her seminal foreign policy experiences came during her time as National Security Council staff member for Africa during the 1994 Rwanda genocide. Then-president Bill Clinton decided against acting to prevent the killings, and apologized for the inaction after leaving office.

Years later, Rice spoke out in favor of a strong American military response in Sudan to enforce a no-fly zone over Darfur to protect civilians from attacks organized by the government of Khartoum.

Rice was also one of the chief advocates inside the Obama administration for the United States to intervene militarily in Libya two years ago. At the time, Obama pushed her to secure a broader Security Council resolution to authorize the use of force beyond a no-fly zone. The intervention, supported by the Arab League, eventually helped topple Libya leader Moammar Gaddafi.

But months later, Rice watched as the aftermath of the Libyan intervention upset her career. Republicans accused Rice of being misleading when she presented administration-approved talking points about the deadly attacks on a U.S. compound in Benghazi last September, derailing her chances of her being nominated as secretary of state.

On Wednesday, McCain, one of her most vocal critics, and other Senate Republicans signaled a grudging willingness to work with Rice in her new role.

“They have deep moral commitments and they believe they are activists — both of them,” said Michael Doyle, a former assistant secretary general of the United Nations who is now a professor at Columbia University, referring to Rice and Power.

“But they also have equally deep experience operating in the Washington bureaucracy — and know what’s possible, what’s not, what lever can be moved and which one can’t,” he continued. “That’s just as important.”

Power advocated for intervention in Libya, too, as the senior director for multilateral affairs and human rights on the National Security Council staff.

A Pulitzer Prize winner for her critical writing on the American response to genocide, Power has also been influenced by the Rwanda genocide and by the wars in the former Yugoslavia, which she covered as a journalist.

But neither Power nor Rice has been nearly as vocal in advocating for a larger U.S. role in Syria’s civil war, which has killed more than 80,000 over the past two years.

“There’s no doubt Susan and Samantha have been shaped by their experiences,” said Tommy Vietor, who served as the National Security Council spokesman for much of Obama’s first term. “There is no direct application of that world view to Syria because of the complexities involved on the ground. There’s a lot I think they need to work through on that policy.”