Despite its emergence as an issue in the presidential contest and despite the source of the claim against George W. Bush, it is useful to look at the facts in the case. Bush had been president for almost nine months on the day of the attacks. His administration had been warned repeatedly by the intelligence community and by outside experts to take al Qaeda more seriously. Peter Beinart lists the many warnings and responses by Bush administration officials in a new piece.
I would add the warnings by the bipartisan Hart-Rudman Commission, which as early as 1999 warned that there would be a terrorist attack on the U.S. homeland and which early in 2001 recommended creation of a National Homeland Security Agency. [Their recommendations later became the template for creation of DHS.] Another early proponent of these ideas was Mac Thornberry of Texas, now the chairman of the House Armed Services Committee.
Of course President Bush responded admirably after the attacks. So did the Congress, in a remarkable display of bipartisan cooperation -- until the administration wanted to expand the war to Iraq and targeted even hawkish Democrats as somehow weak on terrorism. Remember how hard the White House worked to hide the evidence of its negligence and tried to shift the blame to the FBI and CIA. In fact, the buck always sits on the president's desk.