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The word act, as used in the term Act of Congress, is a common, not a proper noun. The capitalization of the word act is deprecated by some dictionaries and usage authorities. Some writers, in particular the U.S. Code, capitalize Act.
If you are talking about the President of the United States, it's capitalized. If you are referring to President Kennedy, it's capitalized. It is not capitalized if you are saying, someday I want to be president, because you are not referring to the person who is a president.