I think in the main it's three things
He once voted against the Bush tax cuts because there was no equivalent cut in spending (this used to be a conservative kinda thing.)
He "might' give "amnesty" to millions of illegal brown people.
He doesn't live in the same Jack Bauer fantasy land as many many wing nuts seem to.
After listening to RW radio callers for a while, it's the latter 2 more than the first.