Here's another explanation.  I think there's a college perfesser in there somewhere, but he might not be the right kind of perfesser, if you know what I mean (nudge, nudge, wink, wink - ).

Let me know.

https://www.livescience.com/34241-democratic-republican-parties-switch-platforms.html

'nother perfesser:

https://www.quora.com/Is-it-true-that-Democrats-used-to-be-the-conservative-party-and-Republicans-used-to-be-the-progressive-party-Im-sure-the-whole-truth-is-more-complex-than-two-parties-switching-their-main-ideologies-but-what-is-the-truth