We actually had an interesting candidate in my district that I felt gave insight into this issue. It was a young guy with no experience or thoughts on issues - basically he said that he was going to ask the people what they thought. He got 30% of the vote.
That leads me to believe that 50-60% of people are voting purely based on party. Sure there may have been some intentional votes, but his platform seems more appropriate for a third party aimed at freedom, etc - and third parties around here don't get more than 2-3% of the overall vote.
He was running as a Democrat, but I don't think that's relevant - I'd expect to see the same numbers if you ran him as a republican.