Tom Donilon stepping down as Obama's national security adviser; Susan Rice to replace him

Updated: 9:20 a.m. ET

(CBS News) Tom Donilon is resigning as President Barack Obama's national security adviser and will be replaced by Susan Rice in a significant shakeup to the president's national security team, CBS News has learned. Samantha Power, a former special assistant to the president and senior director for multilateral affairs and human rights, will replace Rice as U.S. ambassador to the United Nations.

Rice, the current U.S. ambassador to the U.N., has received harsh criticism from Republicans for her initial characterization of the Sept. 11 attacks on the U.S. compound in Benghazi, in which Ambassador Chris Stevens and three other Americans were killed. Her account, based on CIA talking points, indicated the attacks stemmed from a spontaneous protest, which proved to be incorrect.

Mr. Obama had considered Rice to succeed former Secretary of State Hillary Clinton as head of the State Department, but Rice withdrew her name from consideration under persistent fire from Senate Republicans. Her new role will not require Senate confirmation.

Donilon had served on the National Security Council for more than four years. The timing of his resignation coincides with President Obama's rare, informal meeting with President Xi Jinping of China, which will take place in California on Friday. The meeting, orchestrated by Donilon as one of his last big projects, is expected to address mounting U.S. concerns over cyber security.

According to a White House official, Donilon is stepping down because he's had killer jobs for over four years: He served as Deputy National Security Adviser until October 2010 and as National Security Adviser since then. His official departure is expected in early July, a White House official said.

Meanwhile, according to CBS News' Major Garrett, "when the calculation was made on largely political grounds that Susan Rice would be too tough a fight in the Senate for Secretary of State," she was told she would get Donilon's job when he stepped down.

"As a longtime Obama loyalist, Susan Rice has wanted the position of National Security Adviser since 2009," Garrett said. "She has always looked at this position as one she would like to have and now she'll get it."

CBS News political director John Dickerson addressed possible explanations for the personnel changes on "CBS This Morning."

"I don't think it's unhappiness [with Donilon's perfomance] ... Rice has a long relationship with the president. When she didn't get the secretary of state job, people in the White House said that this was a real disappointment to the president because they saw the world in the same way," Dickerson said. "They wanted to have [Rice] more in the inner circle."

Powers, meanwhile, faces a challenge in confronting ongoing violence in Syria -- and political debates in the U.S. over how to respond to it.

"Samantha Power has been working in the administration, was a strong advocate for intervention in Libya. She has spent her professional career working against genocide and mass atrocities," Dickerson said. "The question is what then will she do on that question related to the massive refugee flows out of Syria in this new post at the United Nations where they're trying to get the United Nations to move on that question."

President Obama is expected to announce the changes Wednesday afternoon in the Rose Garden, White House officials tell CBS News.

Mark Knoller contributed to this report.