"Had I known that the enemy was going to use airplanes to strike America, to attack us, I would have used every resource, every asset, every power of this government to protect the American people," he said Thursday, during an appearance in Nashua, N.H.
Mr. Bush made no mention of the man who made the charges, Richard Clarke, his onetime counterterrorism chief, but he was clearly trying to undermine his credibility.
The president also said actions his administration has taken since Sept. 11, including the formation of the Department of Homeland Security, have made America safer.
"We're trying to do our solemn duty to protect America," he said.
Meanwhile, White House spokesman Scott McClellan said the president's offer to meet with the independent Sept. 11 commission remains open, although no date has been set. He also suggested Mr. Bush would be prepared to meet with the leaders of the panel for more than the one hour the White House first suggested.
The White House statements came a day after Clarke, a counterterrorism adviser to the past three presidents, testified on Capitol Hill that Mr. Bush hadn't done enough to protect the country from terrorists.
The administration, intensifying its effort to discredit Clarke, took the unusual step Wednesday of revealing he was the anonymous official who had defended President Bush's anti-terrorism strategy in August 2002.
National security adviser Condoleezza Rice, who was Clarke's boss, accused him of offering differing versions of his role in providing a plan to combat terrorism.
"This story has so many twists and turns, he needs to get his story straight," said Rice, who has refused to testify in public before the Sept. 11 panel.
In nationally televised testimony, Clarke said the administration scaled back the struggle against al Qaeda after taking office in 2001, making it "an important issue but not an urgent issue" until the Sept. 11 attacks. But in the 2002 discussion with reporters, Clarke outlined a multi-pronged approach for confronting al Qaeda that he said the White House had developed over several months leading up to the attacks.
Rice, in a meeting with reporters, released a Sept. 15, 2001, e-mail Clarke sent to her that said, "When the era of national unity begins to crack in the near future, it is possible that some will start asking questions like did the White House do a good job of making sure that intelligence about terrorist threats got to the FAA (Federal Aviation Administration) and other domestic law enforcement authorities."
He attached an earlier memo from before Sept. 11 in which Clarke warned such agencies that "a spectacular al Qaeda terrorist attack was coming in the near future."
"Thus, the White House did insure that domestic law enforcement ... knew that (his office) believed that a major al Qaeda attack was coming and it could be in the U.S.," Clarke's e-mail said.
She suggested that e-mail was self-serving, and conflicted with other more recent, "Against All Enemies: Inside America's War on Terror," and during his testimony Wednesday before the commission investigating the Sept. 11 attacks. (Clarke's book is published by a company owned by Viacom, which also owns CBSNews.com.)
Earlier, McClellan, the White House press secretary, said that Clarke "in his own words, provides a point-by-point rebuttal of what he now asserts. This shatters the cornerstone of Mr. Clarke's assertions."
Asked about his briefing comments, Clarke said he had chosen to "put the best face" on Mr. Bush's policies while working for him in 2002.
"I think that is what most people in the White House in any administration do when they're asked to explain something that is embarrassing to the administration," Clarke told the commission investigating the terrorist attacks.
Just before Clarke began testifying Wednesday afternoon, McClellan read lengthy excerpts of the Aug. 4, 2002, briefing that Clarke gave reporters.
McClellan quoted Clarke criticizing the Clinton administration. "There was no plan on al Qaeda that was passed from the Clinton administration to the Bush administration," Clarke said.
At the time of the original briefing, the White House had insisted that journalists refer to Clarke only as a "senior administration official." But on Wednesday, the administration changed the terms. Fox News asked the White House for permission to reveal Clarke as the source, and the White House agreed, McClellan said.
After Fox aired its report, White House officials told other members of the news media they, too, could identify Clarke as the source.
The decision to reveal Clarke as the source in the August 2002 illustrated the White House's determination to blunt Clarke's attacks on Mr. Bush in an election year.
"Let's remember why we are having this conversation, because Mr. Clarke made assertions that we have said are flat-out wrong," McClellan said. Moreover, in his book, "Mr. Clarke certainly decided on his own to go ahead and reveal conversations that were considered private previously," the spokesman said.
Asked at the commission hearing Wednesday whether he intended to mislead journalists and their readers in 2002, Clarke said no.
"When you are special assistant to the president and you're asked to explain something that is potentially embarrassing to the administration, because the administration didn't do enough or didn't do it in a timely manner and is taking political heat for it, as was the case there, you have a choice," he said.
One "choice that one has is to put the best face you can for the administration on the facts as they were, and that is what I did."