John McCain, until this election cycle, would've been considered by conservatives to be on the left side of the right side... placing him closer to the overall middle than many Republicans.

Obama is considered to be on the left of the left, placing him squarely on the left.

Hillary at one time would've been left of left, but throughout her campaign as she contrasted herself with Obama, she created an image that was more on the right side of the left.... putting her closer to the overall middle.

So at the end of the day, people may perceive Hillary to be closer to McCain in terms of 'centrist' than they do Obama, despite the fact that both HC and BO are on the left side of the paper. Couple that with the negative spin the Obama camp put on HRC during the primaries (by necessity) and you may have some voters who feel disenfranchised by those now controlling the Democratic ticket.... and who see McCain as a few inches closer to them than Huckabee or Romney would ever have been.

Were I to not vote party line, I'd be more apt to defect in a presidential race than a congressional one, simply because of the powers allocated to the majority in the House and Senate, and the desire to see those powers fall to the party with which I align myself. Since the executive branch is a stand-alone, you may see more swing voters there.