"Mainstream America" seems to have nativist and white supremacist connotations; I guess that's why conservatives love it so much. The phrase is inherently exclusionary, because it implies an "out-of-the-mainstream America," and not positively.
I actually believe in a "mainstream America" but it is nothing like the stupid nostalgic one the Republicans have made up.
America is a land of immigrants from all places. There is no mainstream America except the Anglo-Saxon America fanatical conservatives wish to preserve.
LOL, The drones around here are so funny.