Please be VERY careful as to citizenship. There is a difference between requiring "U.S. Persons" to work on a government contract and U.S. Citizens. To my knowledge, the Immigration and Nationality Act protects against citizenship discrimination against five separate groups: U.S. Citizens and nationals, refugees, asylees, and recent lawful permanent residents. There is a U.S. Department of Justice opinion letter that is useful as a reference on this topic: https://www.justice.gov/crt/file/837281/download
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