Article I, Section 8, Clause 18:
[The Congress shall have Power . . . ] To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.
Upon the advent of the twentieth century, the Supreme Court began to establish some outer limits on Congress’s seemingly unfettered power over immigration, particularly with respect to aliens physically present within the United States. But the Court’s jurisprudence repeatedly recognized that Congress retains broader power with respect to aliens seeking to enter this country.