"Congress's power to create rules governing the admission of non-U.S. nationals (aliens) has long been viewed as plenary. In the Immigration and Nationality Act (INA), as amended, Congress has specified grounds for the exclusion or removal of aliens, including on account of criminal activity. Some criminal offenses, when committed by an alien who is present in the United States, may render that alien subject to removal from the country. And certain criminal offenses may preclude an alien outside the United States from being either admitted into the country or permitted to reenter following an initial departure. Further, criminal conduct may disqualify an alien from certain forms of relief from removal or prevent the alien from becoming a U.S. citizen. In some cases, the INA directly identifies particular offenses that carry immigration consequences; in other cases, federal immigration law provides that a general category of crimes, such as 'crimes involving moral turpitude' or an offense defined by the INA as an 'aggravated felony,' may render an alien ineligible for certain benefits and privileges under immigration law."
CRS Report for Congress, R45151