I don't even think trump is the main problem. He has been in power before, and while he did lots of stupid stuff, he didn't actually do anything of note (good or bad). It was all grandiose statements and photo ops to appeal to his "average Joe" supporters. The problem is he courted the extremists and refused to distance himself from them.
The worry for me is what comes after. I think there's only one likely scenario - His supporters turn on him when he doesn't deliver what he promised. Republicans start looking for the next trump; More aggressive, more offensive etc, to keep the supporters on side.