The Bernie Sanders being a Communist accusation is actually a very good comparison.
People are pretty much saying. Bannon calls himself alt-right. White nationalists call themselves alt right. Therefore Bannon is a White nationalist.
It's just like saying, Bernie is a socialist. Stalin called himself a socialist. Therefore, Bernie is a Stalinist.
Clearly, Bernie and Stalin have different definitions of socialism and Bannon and the White nationalists have different definitions of the alt right.
except alt-right isn't just an umbrella term that happens to include white nationalists, it's literally synonymous with white nationalists
