You can buy Chapman's explanation -- and of course there is some truth in it.
Or you could notice that the last two Dim presidents, Siick Willy and BO, had huge advantages over their nerdy Repub opponents in the area of public charisma. And when you run a high-charisma candidate for pres., he will generally have wide coattails as he brings out the low-information voters.
The major-party candidate with the better public charisma has won every presidential election since 1980.
For all the whining we might do about the GOP's message, there is typically a bigger problem with the GOP's messenger.