He's actually very mild mannered and I don't think you can argue that he's that much of a nutcase if he worked with Obama on some issues. This is something Obama has no problem admitting to either.
He's a "straight talker" which means he has a record of saying outrageous things that wear very badly. Think of what people claimed made Joe Biden unelectable, and then shackle them to what everyone would agree is a set of policy positions at one extreme of our spectrum. It's hard to get elected if you can get characterized as not a serious candidate.