I would be careful drawing too many conclusions about the 100 years prior to where I cut off the screen grab. The margins of victory were still relatively narrow for most elections, but there was still more volatility than we’ve seen over the past few cycles.
I think (but am not certain) that if I had an analogous exhibit documenting party membership vs elligible voters, it would show that the proportion of voters who are members of parties has been generally declining, although there would perhaps be some noise if states changed laws about eligibility to vote in primaries.
Admittedly, some of that decline would be the result of the decline of the big, corrupt party machines that dominated life in some cities and regions. There was a time when, if you wanted to succeed in certain businesses, in certain places, you needed to belong to and support the local Boss’ party (and grease the Boss’ peoples’ palms).
There’s certainly corruption today…but it’s not as openly, blatantly linked to parties.
In American politics there has always been a tendency for party strength to be linked to regionalism, and with certain exceptions that regionalism has had a divisive effect.
One way things are different now (I hesitate to say “new” because parts of this have occurred before, some form or fashion), I think, are the simple fact that demographics have caused “them” and “us” to have about equal strength, causing the two sides to have to work a little bit harder to eke out marginal gains.
Another difference, this one obvious, is the change in how we communicate. With more opportunities to have broad audiences now, the conditions are set for idiots and uncivil folks to get their message out, to find populated echo chambers to amplify their messages, etc. That makes the polarization more visible, promotes more extreme politicians who are able to capitalize on that polarization, etc. However, even this difference isn’t necessarily new.
It took more work, but I think you can find examples of the echo chamber phenomenon in American history in the 1760’s - 1780’s. The only reason the polarization that existed then doesn’t stick in our memories is that migration eased some of the pressure – new immigrants arriving in North America, folks moving out to and beyond the frontiers of European settlement, Tories/Loyalists moving to Québec and Nova Scotia (sometimes voluntarily, sometimes not)… It wasn’t talk radio and online social media, but dang there were a lot of local newspapers dominated by opinion and gossip moving throughout the 13 colonies, and lots of local groups organized to ensure that opinion and gossip were propagated and adhered to.
