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Congress, congressional

Always capitalize Congress in U.S. context:
            the Congress, Tenth Congress, 11th Congress, 63rd Congress
            Congresswoman Christine Ullrich, the U.S. congresswoman
            member of Congress, congressman (although congressman/
                        congresswoman technically refer to any member of
                        Congress, the terms are often used as titles for representative)
joint meeting, joint session

Always capitalize in Civil War context:
            Confederate Congress, the Congress

Also capitalize with name of foreign country if congress is part of proper name:
            Argentine Congress, the congress
but     Guatemalan congress (Legislative Assembly)