I was surprised by the northeastern states. Vermont??
The SPLC notes that in 2013, 939 different hate groups were active in the United States. Every state except Hawaii, had at least one hate group. California led the way in the total number of hate groups with 77, followed by Florida with 58, and Texas with 57.
However, if one measures hate groups per capita, Arkansas leads the way with one hate group for every 123,000 residents. Closely following Arkansas in hate groups per capita are the states of Montana, Mississippi, Vermont and Tennessee.
http://www.liberalamerica.org/2014/12/27/the-state-of-hate-in-every-state/
I don't know details about the developments in recent history, but the Southern Poverty Law Center has a wealth of information.