Briefly, both parties have realized that you can use national-scale money to win local elections, which means that more states have become one party states. The Republicans are better at this, at least in part because they tend to like practical politics more than Democrats do.
This bit of background explains why we have rather sudden extreme changes happening at the state level-- same sex marriage and marijuana legalization (more or less) on one hand, and refusal to cooperate with Obamacare on the other.
I assume the national parties will run out of easy local victories, but I have no idea what's likely to happen after that.
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