Andrew Sullivan sums it up well:
Historians will figure this out, but my own view is that Katrina did it. Katrina was the equivalent of Toto pulling back the curtain. Once Bush's passivity, indolence and arrogance were put on full display, once it was apparent that the government was not working, and that Bush was the reason, people figured out why the war in Iraq was such a shambles. And so the mystique required to sustain patriarchal authority was shattered. I think this is largely irreparable because it's about a basic assessment of a single man.
As much as our culture diminishes it, intellect matters.