Susan Rice, the U.N. ambassador caught up in political controversy over the Benghazi terrorist attack, will replace Tom Donilon as President Barack Obama's national security adviser, a senior administration official said Wednesday.

Donilon had been widely expected to step down in Obama's second term, and the official said he was resigning.

Obama will announce the change Wednesday afternoon at the White House, according to administration officials.

In addition, the president will tap Samantha Power as Rice's successor at the United Nations, the senior administration official also said. Power is a former special assistant to the president and senior director for multilateral affairs and human rights at the National Security Council.

Rice, a longtime Obama confidante, became the focus of Republican criticism after last year's terrorist attack on a U.S. diplomatic post in Benghazi, Libya, that killed four Americans, including Ambassador Chris Stevens.

Five days after the assault on the anniversary of the September 11, 2001, terrorists attacks, Rice appeared on U.S. Sunday news shows to say it was a spontaneous development during a protest, rather than a terrorist strike.

She had been considered a top contender to succeed Secretary of State Hillary Clinton at the start of Obama's second term, but Republican opposition over the erroneous CIA talking points she delivered forced her to withdraw her name from consideration in December.

Obama ultimately nominated former U.S. Sen. John Kerry for the post.

As national security adviser, Rice will play a key role in developing and guiding the administration's foreign policy. Unlike a Cabinet post, the appointment requires no Senate confirmation, allowing Obama to avoid a showdown with Republicans in giving a new job to one of his most public foreign policy voices during the 2008 presidential campaign.

Republicans criticized Obama's choice Wednesday, with conservative GOP Sen. Jason Chaffetz of Utah tweeting: "Judgement is key to national security matters. That alone should disqualify Susan Rice from her appointment."

Sen. John McCain of Arizona, a leading Republican voice on foreign policy, tweeted that he disagreed with Obama's appointment but added that he would "make every effort to work" with Rice on important issues.

Obama picked Donilon -- then deputy national security adviser -- to replace retired Marine Gen. Jim Jones for the influential national security adviser post in October 2010. Donilon was heavily involved in the raid to kill Osama bin Laden in 2011, as well as the administration's strategic shift of foreign policy focus to Asia.

Donilon, who also was chief of staff to Secretary of State Warren Christopher in the Clinton administration, is married to Cathy Russell, whom Obama recently nominated to be the State Department's ambassador at large for global women's issues.

Rice also was considered a possible candidate for the national security adviser job when Jones stepped down in 2010.

CNN's Jim Acosta and Adam Aigner-Treworgy contributed to this report.