@evan My impression is that the US used to operate much like Canada where party membership was purely a private affair and parties themselves were purely private organizations. I think I heard that, to combat back-room power dealing within parties, some of those private functions were made public (e.g. the state running primaries).

It seems like this encouraged party membership and/or identification to the what, 20-30% level compared with Canada's 1-2% levels.

This encourages polarization.