Guelzo makes an interesting point. He believes the factor that made the U.S. Civil War "thinkable" was the contiguity of slave states.
What gave the question “political mass” was geography: Slavery had been outlawed throughout the North by the early 19th century, leaving 15 states where it was legal. “Because these slave states were all contiguous, they could look at a map and see themselves as a political unit.” Eleven did in 1860-61.Today's divisions don't have that same geographic clustering, most progressives being bicoastal plus a handful of mid-country large cities. Then Guelzo focuses on what he calls:
A party’s “political center.” For Democrats, it is “local”; for Republicans, “national.” Mr. Guelzo isn’t talking about policy. His argument is that Republicans think of themselves as Americans first, whereas today Democratic localism takes the form of subnational identity politics.If Trump voters think of themselves as "Americans," how long before they begin thinking of Democrats as something else, as not-American? Where do you suppose that thought can take us policywise? Some answers could be decidedly unattractive.
“A sense of belonging to an American nation is much more attenuated,” he says. “Do you identify yourself as being a woman, transgender, black, Latino—you go down the list—or do you identify yourself as an American? That has actually now become an issue. This would have been unthinkable two generations ago.