Sen. Marco Rubio (R-Fla.) is standing by his contention that by not killing Osama Bin Laden when he had the chance, former President Bill Clinton is more responsible for letting the Sept. 11 attacks happen than former President George W. Bush.

But Rubio omits key facts that would undermine his argument. Clinton faced difficult choices that make his decision more understandable, and Bush did not act on opportunities to prevent the terror attacks.

Rubio argued during Saturday's GOP debate that because Clinton decided not to try to kill Bin Laden, he effectively permitted 9/11 to occur.

He struggled to qualify his argument on NBC’s “Meet the Press” Sunday, claiming that he had only meant to say that Clinton was more culpable than Bush.

“My argument is, if you're going to ascribe blame, don't blame George W. Bush, blame a decision that was made years earlier, not to take out Bin Laden when the opportunity presented itself,” Rubio told host Chuck Todd.

Regardless, Rubio ignored the challenges Clinton faced as he considered whether to try to kill bin Laden with an airstrike in 1998 and 1999 -- and the lack of certainty that Clinton would have succeeded, even if he had tried.

The 9/11 Commission Report details those challenges.

“First, in December 1998, the principals’ wariness about ordering a strike appears to have be vindicated: Bin Laden left his room unexpectedly, and if a strike had been ordered, he would not have been hit,” the report states. “Second, the administration, and the CIA in particular, was in the midst of intense scrutiny and criticism in May 1999 because faulty intelligence had just led the United States to mistakenly bomb the Chinese embassy in Belgrade during the NATO war against Serbia.”

One can still blame Clinton for not at least trying to kill Bin Laden, of course.

But it is not clear that Bush was any better than Clinton, and there is reason to think he holds an even greater share of the blame for allowing the Sept. 11 attacks to occur.

Rubio fails to acknowledge evidence that the George W. Bush administration did not take the threat of Al Qaeda seriously enough in the runup to 9/11, despite intelligence suggesting the terrorist group was a growing menace. The White House received 36 warnings from the CIA prior to Sept. 11 about the overall security threat posed by Al Qaeda, according to the 9/11 Commission Report.

The 36th and final warning, in August 2001, famously titled “Bin Laden Determined To Strike In U.S.,” was also the first to present the possibility of an Al Qaeda strike inside the country.

Richard Clarke, who was counterterrorism chief under presidents Bill Clinton and George W. Bush, has been especially critical of the Bush administration’s lack of vigilance toward Al Qaeda. Clarke’s frustration in the summer of 2001 prompted him to consider seeking a transfer from his post.

“My view was that this administration, while it listened to me, didn't either believe me that there was an urgent problem or was unprepared to act as though there were an urgent problem,” Clarke testified to the 9/11 Commission.

“And I thought, if the administration doesn't believe its national coordinator for counterterrorism when he says there's an urgent problem and if it's unprepared to act as though there's an urgent problem, then probably I should get another job.”

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