Question - late in the thread. This applies to Dems and Reps

Does one define “mainstream” as the loudest voices, the most elected representatives, the most publicized characters, or the critical mass of voters?

I am hearing a lot of mainstream Republicans are like _____ and mainstream Democrats are like ____. Usually the blank is more radical when the opposition describes the party.

That is what I take issue with. Declaring that so and so is mainstream without defining what the critical mass of nutroots vs. adults is. If the definition is provided by opponents or proponents only then of course the definition becomes a partisan label.