"The Supreme Court has determined that inherent principles of sovereignty give Congress 'plenary power' to regulate immigration. The core of this power--the part that has proven most impervious to judicial review--is the authority to determine which aliens may enter the country and under what conditions. The Court has determined that the executive branch, by extension, has broad authority to enforce laws concerning alien entry mostly free from judicial oversight. Two principles frame the scope of the political branches' power to exclude aliens. First, nonresident aliens abroad cannot challenge exclusion decisions because they do not have constitutional or statutory rights with respect to entry. Second, even when the exclusion of a nonresident alien burdens the constitutional rights of a U.S. citizen, the government need only articulate a 'facially legitimate and bona fide' justification to prevail against the citizen's constitutional challenge."
CRS Report for Congress, R44969
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html