I can only offer an anecdote here, but it's very interesting that I happened to have quoted a Union soldier from Connecticut writing home about the Civil War in a paper I wrote in college for an advanced Civil War elective. It's saved on my old computer, but if I remember I would really like to cite it in a few weeks when I am back at my parents' house.
Anyway, the jist of it was that this soldier effectively didn't see any reason why they shouldn't just let the South go, and that his views on slavery were closer to a Southerner's than the federal government by this point (I think the letter was written in the later stages of the war?). Considering CT seems to have been a lot less Republican than the rest of New England and close to a Democratic base in New York City, perhaps this wasn't an overly uncommon attitude in the state around this time?
I am on my phone but their was certainly a rural vs urban, pious vs mainline, and populist vs elite dynamic to the formation of Republican Party in New England. The painted picture of a smooth departure of Whigs fails to account for this rather bloody process, politically speaking. A number of establishment and/Websterite Whigs ended up politically homeless or even reluctantly backing Democrats.
Holt's book on the Whig party is pretty good read to get a good understanding of this chaotic transition.
