This map, from the New Historical Atlas of Religion in America, shows the largest religious denomination in each US county. This does not mean that the majority of the county belongs to the denomination, only that no other denomination is larger.
My guess is that Hancock County, Illinois (Nauvoo) will be the first purple county east of the Rockies. Maybe it already is (I don’t know the age of this map). What are the other contenders out east? Which will be the first purple county in California?