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It is a ruling that will fail.
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Here's why the ruling will fail - marriage has traditionally been a state matter, BUT common uniformity has met that many (over 1000) Federal Laws cover marriage. The Feds wade into (as in Utah with polygamy) the marriage fray to maintain consistency across the country.
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Now, Judge Moore might be on safer ground if he declared that same sex marriage was not valid for the purpose of the State of Alabama. For example some states still outlaw same sex married couples the right to adopt. Or he might be able to force Alabama same sex couples to file state taxes separately.
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But Judge Moore is grandstanding. Here is why he is making the ruling. His position is elected and he is just trying to make a political point. Oddly enough, it is one that will probably make his voters happy. But Alabama will end up paying millions in defending (and losing) this battle. And voters won't notice because it won't make headlines.
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He will probably get campaign donations out of this, which is the point.