Under Title 8, Section 1182 of the U.S. Code, the president has authority to use a proclamation to suspend the entry of “any aliens or of any class of aliens into the United States [who] would be detrimental to the interests of the United States,” for however long he deems necessary. This provision was included in the Immigration and Nationality Act of 1952.
(5) Labor certification and qualifications for certain immigrants
(A) Labor certification
(i) In generalAny alien who seeks to enter the United States for the purpose of performing skilled or unskilled labor is inadmissible, unless the Secretary of Labor has determined and certified to the Secretary of State and the Attorney General that—
(I) there are not sufficient workers who are able, willing, qualified (or equally qualified in the case of an alien described in clause (ii)) and available at the time of application for a visa and admission to the United States and at the place where the alien is to perform such skilled or unskilled labor, and
(II) the employment of such alien will not adversely affect the wages and working conditions of workers in the United States similarly employed.