Republicans have much more discipline in their party than Democrats. The party comes first in their eyes and they are more apt to obey its dictates than Democrats obey their party, and it does keep out the riff-raff. For example, Republicans insist their Congressmen sign pledges to the party first rather than rely on a simple oath to the nation. As Will rogers once said, "I don't belong to any organized political party, I'm a Democrat."
You don't think there's a rift between the TPers and the establishment GOP? Or between the fiscal and social conservatives, with their differing priorities?