President Obama, assuming a tough-talking tone, warned Republicans Wednesday to lay off U.N. Ambassador Susan Rice over her controversial Sept. 16 comments on the Libya consulate attack -- and at the same time opened his administration up to questions about why, if she "had nothing to do with Benghazi," the White House chose her to explain it to the public.

At issue is Rice's claim, on five Sunday shows five days after the attack, that the assault was "spontaneous."

Calling Rice an "easy target," Obama challenged Republican lawmakers during his press conference to "go after me" instead -- a challenge quickly picked up by Sen. Lindsey Graham. The South Carolina Republican fired back: "Mr President, don't think for one minute I don't hold you ultimately responsible for Benghazi. I think you failed as commander in chief before, during and after the attack."

The president's full-throated defense of Rice follows weeks of criticism from Republicans about Rice's TV appearances. In those interviews, she insisted the Benghazi attack was sparked by outrage over an anti-Islam film -- despite evidence that it was a coordinated terror attack.

Amid speculation that Rice could be tapped to succeed Secretary of State Hillary Clinton, Sen. John McCain, R-Ariz., and others vowed to do "whatever is necessary" to block such a nomination.

But Obama said Wednesday that Rice has done "exemplary work" -- while at the same time, defending her by stressing how removed she was from the situation.

"For them to go after a U.N. ambassador who had nothing to do with Benghazi and was simply making a presentation based on intelligence that she had received, and to besmirch her reputation, is outrageous," he said.

The comment that she had "nothing" to do with the case could bolster suspicions that the administration intentionally chose somebody who was out of the loop for those Sept. 16 interviews.

Obama said Rice made the appearance "at the request of the White House" and "gave her best understanding of the intelligence that had been provided to her."

"If Sen. McCain and Sen. Graham and others want to go after somebody, they should go after me," Obama said, adding: "When they go after the U.N. ambassador apparently because they think she's an easy target, then they've got a problem with me."

The president said he would nominate Rice if he thinks she is the best person for the job, but he added, "that's not a determination that I've made yet."

The appearance marked the first scheduled solo news conference for Obama in eight months.

On the heels of Election Day, the administration is dealing with a host of controversies and legislative challenges. Aside from Benghazi, the administration is reeling from the abrupt resignation of former CIA Director David Petraeus over an extramarital affair. According to official accounts, the White House was not informed about the situation until last Wednesday -- however, the investigation had been ongoing for months, raising questions about why the Justice Department and FBI did not notify others in the administration or on the Hill.

Obama said Wednesday he was withholding judgment on the process. He also said he had "no evidence" that classified information was disclosed that in any way would negatively impact security.

Obama also renewed his call for House Republicans to let tax rates rise for America's top earners, while urging them to pass a bill "right now" that preserves Bush-era tax rates for everyone else. And the president, at the opening of the press conference, returned to a campaign-trail habit of invoking the economic problems he inherited from the George W. Bush administration -- reminding Americans that the economy "is still recovering from a very deep and damaging crisis."

The comments on Rice drew retorts from all three of the senators who originally raised concerns -- McCain, Graham and Sen. Kelly Ayotte, R-N.H. Those three senators also put forward a proposal Wednesday to empanel a "select committee" to investigate the Libya attack.

McCain, on the Senate floor, defended his concerns about Rice and echoed Graham's comments about Obama's culpability.

"This president and this administration has either been guilty of colossal incompetence or engaged in a cover-up," McCain said. He added, "This attack clearly could have been prevented."

McCain closed by saying, "we're not picking on anybody," just trying to "find out the facts."

Other potential candidates for secretary of state include Sen. John Kerry. The chatter in Washington that Obama might instead go with Rice raised alarm among Republicans, who have repeatedly hammered her Sunday show appearances on Libya. Graham said Wednesday "I don't trust her."

Obama also said Wednesday that it's "important" to find out exactly what happened in Benghazi and vowed to cooperate with Congress.