The elected representatives of the Democratic party is overwhelmingly moderate. There are a handful of conservative democrats and equally many progressive democrats. Progressive democrats have been loosing as many primaries as they win, but overwhelmingly they are not up for election, instead a moderate democrat is the default choice.

The same can not be said about the Republican Party. This is not a both sides issue. Polarization is happening on the right wing in the American Party system, and the right wing alone.