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 longtime proponents of a larger American role in preventing humanitarian crises and protecting human rights.

The ideological shift signaled by the choices highlights a central quandary 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 in the face of stiff challenges on the domestic front has remained a question well into his second term.

Susan E. Rice, whom Obama named 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 answer as the administration reviews its policy on 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 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 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 members of both parties greeted Power’s nomination warmly. In a statement, Sen. John McCain (R-Ariz.) said that Power is “well-qualified for this important position” and that he hopes the Senate “will move forward on her nomination as soon as possible.”

Obama’s second-term foreign policy has 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 policymaking 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, although some thought 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. Obama eventually split the difference in setting the path toward concluding the United States’ 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 regarding the Iraq invasion and its aftermath.

But Donilon has 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 a National Security Council staff member for Africa during the 1994 Rwanda genocide. President Bill Clinton decided against acting to prevent the killings, and he apologized for the inaction after leaving office.

Years later, Rice spoke out in favor of enforcing a no-fly zone in Sudan to protect civilians from attacks organized by the government of Khartoum.

Rice was also one of the chief advocates in 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 Libyan leader Moammar Gaddafi.

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

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

Power also advocated for intervention in Libya 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 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 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 worldview to Syria because of the complexities involved on the ground. There’s a lot I think they need to work through on that policy.”