There are a few factors that contribute to differences in party perception.
Due to the constituencies, the excesses of the parties will differ. Democrats are the party of minorities and also the party of big government, so Republicans will be more likely to tolerate the bigotry of the majority. Democrats are more likely to support sending people to jail for refusing to perform certain duties, while Republicans are more likely to believe there's a free market solution.
Republicans are the party of uneducated white people, a group that it's socially acceptable to mock. Democrats are the party preferred by uneducated Afircan-Americans and Hispanics, who it's less socially acceptable to mock.
If we split most politicians on a
spectrum from far right to far left, with the right, the center-right, the center, the center-left and the left somewhere in between, someone on the center-left will be as close to the center-right as to the far-left, and will see the right-wing as completely out of bounds. The American media is generally center-left, if not more liberal, so they'll see Susan Collins or Charlie Baker as Bernie Sanders' ideological equivalents. They're more likely to understand where a liberal who says something outrageous (IE- Hillary Clinton wanting to put Terry Jones in jail for burning a Koran) is coming from, so it's less likely to get coverage.
Demographics do force some of the crazier liberals to broaden their appeal. An LGBT activist or militant atheist will have to appeal outside of their group in order to get votes. There are plenty of areas where white married Christians are the majority of the voters.