They represent different sets of alliances. Mostly those that want to expand the state and those who want to contract it.
https://www.quora.com/unanswered/What-are-the-differences-between-the-political-parties-in-the-USA
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https://www.quora.com/unanswered/What-are-the-differences-between-the-political-parties-in-the-USA
Curt Doolittle (ericd)