There are a couple of things wrong with this, either of which is fatal to the argument.
First, and most obviously, we have not been kept safe since September 11, 2001. Although it had been swallowed by the memory hole until just recently, everyone should remember the Anthrax Attacks - attacks that only targeted "liberals." So, even on its own terms this argument is incorrect.
More significantly, however, is the fallacy of timing. Conservatives employing this argument will always list the terrorist attacks that predated 9/11 in order to drill home the impressiveness of us "not being attacked again since." The list will look something like this:
It was one of series of terrorist attacks that included the blackhawk down incident, the first World Trade Attack, Kobar Towers Attack, the Twin Embassy Attack, the USS Cole Attack..."Wow," you think, "that was a lot of attacks from 1993 to 2001." Of course, only one of those attacks is a good analogue for the 9/11 attacks. Only the first attack on the WTC in 1993 was an attack on a civilian target, inside America. If one were to include attacks like the USS Cole Attack the number of terrorist strikes on "America" would have gone up astronomically thanks to Bush. Given that the first WTC attack was eight years before Al Qaeda's second attack on our soil, it wouldn't be unreasonable to expect something like another 8 years until they try again. That puts it out of Bush's Presidency, and therefore not something to credit him with.
I know this is just the last refuge of a conservative movement that once followed George W. Bush as if he were the physical avatar of the God of Conservatism, but that doesn't mean I should leave it to provide even that illusory comfort, right? President Bush, after what he did to this country, deserves all the blame and derision we can heap on him.