Why is the Republican Party divided?

Republicans are definitely more divided than Democrats, and nothing has demonstrated this reality more than the ascendance of everybody's favorite Reality TV star, Donald Trump.
 
Before Trump, the GOP was already struggling to maintain unity among a number of factions.  Nativists, "Old School" Reaganites, Evangelicals, Neo-Cons, Cato/Paul Libertarians, etc.  But then suddenly, out of nowhere, Donald Trump dipped in his big toe and changed the whole soup (possibly indefinitely).
 
B.T. ("Before Trump"), I never thought a Republican presidential candidate could poke fun at a GOP war hero's record and get away with it.   B.T., I never thought a Republican presidential candidate could give FoxNews the finger and get away with it.
 
B.T., I couldn't imagine a Republican presidential hopeful who would even think about saying "No, GW Bush did NOT keep us safe!" (a la Bill Maher); and not back off an inch and still maintain frontrunner status.
 
I never imagined a top GOP candidate with extremely dubious social conservative credentials -- "I'm Presbyterian! I eat the crackers and all that!" -- getting away with profanity in public -- numerous times.
 
Bottom line, Donald Trump has single-handedly rearranged all of the cards in the deck and left the GOP with an ultimatum:  You are either with him -- or not.
 
Among Democrats, there are basically two factions as well:  Those who like Obama and those who think Obama is too centrist and/or too milquetoast.  But the overwhelmingly majority of Democrats will happily choose Hillary if Trump is on the other side.
 
But I don't know what pro-Trumps (or anti-Trumps) will do if things don't go their way.  Hence, a significantly more divided party.
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