It's kind of circular. Clinton gets the hatred she does because she's a Democrat, and the GOP has been cultivating a base of really hateful people. Obama was equally hated (it's only receding now because he's no longer running for office--remember how popular Clinton was when she wasn't running for office), and any Democratic candidate, regardless of personal characteristics or policy positions, would be very hated by the GOP base. So you have that hard 40% or so (might be a little higher, might be a little lower).