You are right, the GOP is a regional party. So is the Democratic party, which is all but shut out from about 20 States. Point is, America is now a bitterly geographically polarized country.
This is a predictable condition in a nation which uses a state-winner-take-all Electoral College system to elect President when it's two major parties become more ideologically focused than at any other time in our history.
If we elected the President by national popular vote, it would tend to nationalize both parties more.
