James E. Campbell has written an excellent book on this contested and…polarizing…topic. Here is just one of many good bits:
As they [some commentators] see it, party polarization has been asymmetrical. The Republican Party allegedly has been captured by right-wing zealots while the Democratic Party has remained a reasonable center-left party. The claim of asymmetrical party polarization is half-true and completely understandable. First, there should be no mystery to asymmetry. If the parties are very competitive, as they are, and the public is skewed to the conservative end of the ideological scale, the parties should be similarly skewed. In a center-right nation, the right-wing party should be further to the right than the left-wing party is to the left. If the two parties were equally ideological, the Democrats would be in a permanent minority. That said, the increased polarization of the parties cannot be entirely attributed to the Republican Party becoming more conservative. Before the Republicans began moving to the right, Democrats had moved further to the left. Party polarization followed the staggered nature of the realignment. In the 1970s, congressional Democrats moved significantly to the left, while there was little change in congressional Republicans. The Republican shift to the right came later and was augmented by the growth of conservatism in the public. The polarization of the parties was a two-step dance — maybe three big steps: One big step to the left and two smaller steps to the right.
There is also this:
A five- or ten-percentage-point shift in ideological preferences may seem like “small potatoes,” but a nation that is 40% moderate and 60% ideological (liberal or conservative) operates quite different politically from one that is a 50-50 split.
By the way, it is sometimes noted, or noticed, that left-leaning thinkers have become crazier lately. I think overall that is true. It may be a sign that America is switching from a center-right to a center-left nation, given Campbell’s analysis above.