Via Larry Bartels, here's an interesting chart from James Stimson that tracks the public's level of conservatism over the past 60 years. You can click the link to read about his methodology, but what this chart basically shows is that "the public tends to act as a 'thermostat,' shifting to the left when the political climate in Washington shifts to the right and to the right when policy shifts to the left." In other words, when the president is a Republican, the public tends to tire of conservatism and become more liberal. When the president is a Democrat, the public tends to tire of liberalism and become more conservative.
Until recently, the only real exception to this was the 60s. But now there's a second one: the George W. Bush administration. And this is interesting, I think, because it shows the enduring effect of the war on terror. My interpretation of this chart is that Bush managed to avoid the normal public backlash during his first three years in office, and the obvious explanation for this is 9/11 and the Iraq War. Eventually, the political mood did start to trend more liberal, but because of the initial 9/11 effect, the public ended up in 2008 about where it had started in 2000. As a result, Obama began his presidency with an unusually conservative public. Thus, instead of merely bouncing back from the previous presidency in the usual yo-yo fashion, the conservative backlash against Obama took us into the right-wing stratosphere.
There may be other explanations for this. Have at it in comments. But George Bush did everything he could to politicize 9/11, and it looks to me like it paid off. Whether we realize it or not, we're still living with the toxic legacy of 9/11 and the war on terror.