I remember some stat that the loudest voices are the 8% on either end of the political spectrum. I couldn't find where I heard that, but Pew has some data that mirrors that.

I think that 20% is coming from this:

"The overall share of Americans who express consistently conservative or consistently liberal opinions has doubled over the past two decades from 10% to 21%"[1]

[1] https://www.pewresearch.org/politics/2014/06/12/political-po...