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Speech = Free: People have the right to say what they want. You can't keep a person from talking.
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Spending = Speech: People have the right to spend money about political issues. You can't keep a person from spending.
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- Corporations = People: A group of people have the same rights that an individual has.
Basically, Citizens United says that for-profit companies can't be restricted from political activities. They are free to buy elections.