Why were the Northern states Republican?

[King of Kensington]

Maybe the best "generalization" one can make, is that the groups of people that tend to support the GOP were once heavily located in New England, but are not any longer; and that the issues that the GOP stands for change over time, and what is considered "conservative" or "liberal" also changes over time.  Maybe that's the best way to summarize this all.

I think that sums it up best.