Candidates Prepare for Debate as Libya Criticism Intensifies LAS VEGAS - President Obama arrived here on Sunday to prepare for his first debate with Mitt Romney, leading in the polls nationally and in most of the critical states, but still fending off Republican criticism over the White House's shifting accounts of the deadly September attack on the American diplomatic post in Benghazi, Libya. Mr. Obama plans to go into virtual seclusion at a lakeside resort hotel outside Las Vegas on Monday and Tuesday, when his staff and outside consultants will drill him for the encounter with Mr. Romney in Denver on Wednesday evening. Mr. Romney was also preparing for the debate but planning to do some campaigning in Denver on Monday. Before Mr. Obama vanished behind the gates of the resort, he appeared at a sunset rally at Desert Pines High School, where he tried, a bit halfheartedly, to play down expectations about how he will match up against his challenger. "Governor Romney, he's a good debater," Mr. Obama said to a skeptical crowd of 11,200. I'm just O.K. "I know folks in the media are speculating already on who's going to have the best zingers," Mr. Obama said, suggesting that he would not be supplying them. He said he hoped for a serious discussion about the nation's problems. As Mr. Obama hunkered down, his aides continued to battle Republican criticism of the White House's handling of the Benghazi attack, which the administration initially characterized as a spontaneous demonstration gone awry, and later described as an organized terrorist act by extremists with possible links to Al Qaeda. Senator Bob Corker, Republican of Tennessee, labeled the administration's changing accounts "bizarre." In a letter to the director of national intelligence, James R. Clapper Jr., Mr. Corker questioned whether the compound was adequately secured before the attack, in which the ambassador to Libya, J. Christopher Stevens, and three other Americans were killed. On Friday, Mr. Clapper's office issued an unusual statement, acknowledging that the intelligence agencies "revised our initial assessment to reflect new information indicating that it was a deliberate and organized terrorist attack carried out by extremists." That statement, however, did little to quell the questions being raised on Capitol Hill. Much of the criticism is directed at the American ambassador to the United Nations, Susan E. Rice, who offered the most detailed preliminary assessment of the attack, describing it a few days later as a "spontaneous reaction" to protests in Cairo that erupted after the circulation of an anti-Islam video on YouTube. On Friday, Representative Peter T. King, a New York Republican who is chairman of the House Homeland Security Committee, called for Ms. Rice's resignation, saying her statements were misleading. White House officials said Ms. Rice based her characterization on the best intelligence available in the hours after the attack. Two prominent Democratic senators - Harry Reid, the majority leader, and John Kerry, chairman of the Foreign Relations Committee - rushed to Ms. Rice's defense, accusing Republicans of politicizing the attack. "Instead of pointing fingers through unfounded partisan attacks, we should focus on bringing to justice those who carried out this attack and ensuring that those who serve at our embassies are safe," Mr. Reid said in a statement. Mr. Kerry, who has called for a State Department investigation of embassy security (and will stand in for Mr. Romney in the president's mock debates), said, "We need to drop the politics and pull together." The administration welcomed scrutiny of the security of American embassies, Joshua Earnest, the White House deputy press secretary, said to reporters on Sunday, insisting that the safety of diplomats was a top priority for Mr. Obama. The administration is reviewing the security at diplomatic facilities around the world. "I recognize, particularly in this political season - and here we are nearing October in an election year - that there are going to be people who are going to be asking politically motivated questions," Mr. Earnest said. I can tell you that the president is not focused on the politics of the situation, he's focused on the safety and security of our diplomats. But, pivoting immediately to politics, the Obama campaign's press secretary, Jen Psaki, asserted that Mr. Romney's proposed budget would cut money for security at American embassies. "This raises into question what their priorities are too," she said. The Romney campaign kept up the pressure as well, accusing the administration of mixed messages. Mr. Romney's running mate, Representative Paul D. Ryan, said on "Fox News Sunday" that the administration's response to the attack was "slow, it was confused, it was inconsistent." Later in the interview, Mr. Ryan complained that the Romney campaign was fighting an uphill battle because the news media was tilted toward Mr. Obama. "I think it goes without saying that there's definitely a media bias," he said to the host, Chris Wallace. "I think most people in the mainstream media are left of center, and therefore they want a very left-of-center president than they want a conservative president like Mitt Romney," Mr. Ryan said. He did, however, acknowledge that the Republican campaign has had its flaws, including what he described as Mr. Romney's "inarticulate" comments about people who pay no taxes and receive government help.