In nominating her to the ambassador's post, Obama called Rice "a close and trusted adviser" and said she "shares my belief that the U.N. is an indispensable -- and imperfect -- forum."

At the same time, she has drawn some attention for the way she operates.

Insiders say Rice is ambitious and aggressive. Colum Lynch of the Washington Post and Foreign Policy told CNN that one of her nicknames at the U.N. Security Council is "The Bulldozer."

"I think that everyone has complicated feelings about her," Lynch said.

He characterized her as "very personable, likeable, charming, smart, funny, down to earth" but also someone with sharp elbows.

"You don't want to get in the way of her, and whether that's a point to her advantage or a point against her, well, I guess it depends on who you are asking," Lynch said.

Benghazi controversy

In the days following the September 11 attacks on the Benghazi consulate that killed Ambassador Christopher Stevens and three other Americans, Rice became the administration's point person on the matter. In multiple TV appearances after that attack, Rice cited a hateful video that fueled a spontaneous mob attack as the reason for the deaths.

Senior U.S. officials have said that Rice's comments were based on an intelligence assessment that was later updated to reflect a preliminary view that demonstrators were not the culprits.

The most strident Republicans suggested the characterization of the attack as a mob gone awry might have been the basis for a cover-up during a ferocious political campaign.

Criticism intensified as the explanation of events slowly shifted, with the administration eventually raising the possibility that the attack was planned by al Qaeda.

Some leading Senate Republicans said they could not support Rice if Obama nominated her to replace Hillary Clinton as secretary of state. While Obama has not indicated whom he might appoint, White House sources had said that Rice was a top candidate.

Rice did little to quell criticism of her when she visited the Hill to meet with her toughest critics over Benghazi -- Republican Sens. John McCain of Arizona, Lindsey Graham of South Carolina and Kelly Ayotte of New Hampshire.

The session, called by Rice to answer questions about her comments, ended up with the lawmakers saying they were "significantly troubled" by many of her answers.

Graham said Rice's comments amounted to a "statement disconnected from reality" and that his concerns about Rice were "greater today than they were before."

McCain said the "information that she gave the American people was incorrect."

In a statement after the meeting, which also included Acting CIA Director Michael Morrell, Rice said the two stressed that "neither I nor anyone else in the administration intended to mislead the American people at any stage in this process, and the administration updated Congress and the American people as our assessments evolved."

After Rice had withdrawn, an administration official voiced frustration that Rice had been unfairly targeted over Benghazi.

"It's absolutely a fact that she had nothing to do with the security presence in Benghazi and the intelligence collection or assessment. All she did was some interviews using cleared talking points that reflected our best understanding of the situation as we knew it at that time," the official said. "The fact that four people died in Libya is a tragedy, but Susan Rice had nothing to do with that tragedy. The fact that they focused on her talking points is a disservice to everyone who cares about this issue."

After Rice's announcement, Graham released a statement saying, "I respect Ambassador Rice's decision. President Obama has many talented people to choose from to serve as our next secretary of state."

Graham added that he would continue to try to get the bottom of the Benghazi matter.

Obama had fiercely defended Rice since Republicans first began to question her nomination -- first in the second presidential debate and later in his first news conference after his re-election.

He continued his support in a statement responding to Rice's decision.

"I have every confidence that Susan has limitless capability to serve our country now and in the years to come, and know that I will continue to rely on her as an adviser and friend," the president said. "While I deeply regret the unfair and misleading attacks on Susan Rice in recent weeks, her decision demonstrates the strength of her character, and an admirable commitment to rise above the politics of the moment to put our national interests first."