The reason for a state to do that would be allowing the people to legislate it in their areas. ANYONE who props up "let the states decide for themselves" has to acknowledge that letting each county decide for themselves is the same idea, just better.
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If not, then you're just being politically blind.
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It means more people are pleased with the outcome. Isn't that what it's all about? Take Texas - the state goes no abortions, angering majorities in the blue counties. But if each county decides, there are no angry majorities. Doing it state-by-state is like painting with a roller, but leaving it up to each county, you're painting with a brush.
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This is an honest-to-goodness middle-line, moderate idea I'm putting out there. I really don't see any problem with it.