Writing at
The Federalist, Ben Domenech
identifies the four issues that fuel the Trump phenomenon, though for unclear reasons he calls them three issues plus a "cultural phenomenon." His 3 issues:
1. Immigration
2. Foreign policy
3. Elite corruption
His "cultural phenomenon:"
Animosity toward political correctness
Call 'em three or four issues, they're things about which the GOP base cares and with which the party establishment doesn't want to deal. Hence, Trump, a flawed messenger who speaks for the otherwise unrepresented.