I plan to expand on this in the future - but in short, calling Donald Trump 'Trump' gives him too much credit and weight.

'Trump' is literally a heavy, ugly word. (Expansion will include a linguistic analysis.) The sounds, plus the situation, make him seem like a giant, hardened pile of dog poop we all have to go around. Like a crap pile of the gods. Which is what he wants, uses, revels in.

'Donald', on the other hand, is a snotty kid. A five-year-old whose mother can't seem to keep him from reaching into other peoples' bags randomly to see what he can find. 'Donald' is the prick in the meeting who knows if he keeps it going, he'll win because other people will get tired. 'Donald' is a jerk.

So call him Donald.