By my lights, Donald Trump actually pulled his punches in the South Carolina debate last night, with regard to 9/11. While he noted that it happened on the GW Bush Administration's watch, he only made one very oblique reference to the extraordinary negligence that Bush and his colleagues displayed in ignoring vehement, repeated warnings from the intelligence community.
A fuller description is available here, for example.
As the above-linked article makes clear, the Bush Administration's record of astonishing negligence goes well beyond the infamous August 6 briefing, entitled "Bin Laden Determined to Strike in U.S." that prompted Bush to say dismissively to the briefer, "All right, you've covered your ass now." There were actually weeks of warnings that the Administration obdurately ignored.
Officials in the U.S. intelligence community were waving their arms and screaming; the Bush Administration deliberately ignored them because it had ideological priors about non-state actors. Also, top people in the Bush Administration were determined to reject and alter Clinton Administration priorities right and left. Since Bin Laden had been a high-level Clinton priority, for them he was almost automatically a non-priority.
As Trump said at the debate, people make mistakes, so how to evaluate all this (and what relevance it has 15 years down the road) is open for discussion. There's no "truther" aspect here - although the Bush Administration benefited enormously from its own negligence, in that 9/11 proved a political godsend for them, they were neither smart enough nor evil enough to let it happen on purpose.
But even leaving aside the question of how a Democratic administration would have been judged in the political process for comparable negligence (impeachment probably would have been the least of it), all this certainly means that these silly "Bush kept us safe" mantras should stop forthwith.
We'll never know if a less negligent Administration, one that actually listened to its own intelligence experts rather than pursuing rigid, ideologically predetermined operating assumptions, would have been able to stop 9/11. However, for a New Yorker such as me who saw the whole thing happen right in front of his face - this was no TV show for those of us in lower Manhattan on that horrible day - it is sickening to reflect on how close lower-level intelligence officials came to putting all the pieces together in time, even with zero help and encouragement from the top.
Returning to current political brouhahas, I have no idea whether Trump will pay at the polls for saying this in South Carolina. But anyone on the Republican side who thinks that the Jeb-Rubio side of this argument is something they want to get anywhere near discussing in the general election is living in a dream world. If this is ever openly litigated in public political debate, the verdict they get will be devastating, because the facts are so bad for them.