Definition of Blue State
What is a "Blue State"? What is the definition of the term "blue state"?
A "blue state" is a state in which the voters tend to favor the Democratic party.
The opposite of a "blue state" is a "red state", which is a state where voters tend to favor the Republican party.

Examples of states that could be categorized as "blue states", at least based upon the last 4-5 elections - California, Oregon, New York and Washington.
Examples of traditionally "red" states - Texas, Georgia and Oklahoma.
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