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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.
No. constitutional law is not a proper noun and it should not be capitalised. Constitutional Amendment X is the name of the amendent, and would be capitalized. Likewise, Constitutional Law as the title of a course or book would be capitalized.