Gallup reports that the number of people self-identifying as independents has increased dramatically since 2008:
There are two things to say about this. First, this trend most likely represents moderate Republicans who no longer want to identify with the modern tea-party-ized GOP, and are now calling themselves independents. But as John Sides points out, this doesn't mean much in terms of voting behavior. These folks might call themselves independents, but they mostly vote the same way they always have.
Second, it goes a long way toward explaining that Pew survey last week, which found that belief in evolution had plummeted from 54 percent to 43 percent among Republicans over the past four years. If you dig into the details of that poll, the decline is actually a little more moderate than it seems, and it's probably explained mostly by the fact that so many moderate Republicans have left the party. When you remove a big chunk of people who believe in evolution, the group that's left will have a higher percentage of deniers even though no one's beliefs have actually changed.
Bottom line: moderates are abandoning the Republican Party. The remaining rump is more conservative, and this certainly affects the behavior of Republican politicians in Washington. However, it doesn't mean that anyone's views have changed or that anyone's voting patterns have changed.