Pew seems to indicate that the democratic party (or at least the electorate) has moved significantly leftward, particularly after the 2016 election. It doesn't seem that polarization is unique to a particular party. Moreover, I would argue that a self described socialist becoming one of the most important voices in the party is actually evidence of this leftward movement.
[1] https://www.pewresearch.org/politics/interactives/political-...