In Reynolds v. United States, in a dissent joined by his elephant-riding-buddy, Justice Scalia flags a potential nondelegation doctrine issue!

Indeed, it is not entirely clear to me that Congresscan constitutionally leave it to the Attorney General to decide—with no statutory standard whatever governing his discretion—whether a criminal statute will or will not apply to certain individuals. That seems to me sailing close to the wind with regard to the principle that legislative powers are nondelegable, see Whitman v. American Trucking Assns., Inc., 531 U. S. 457, 472–476 (2001); Loving v. United States, 517 U. S. 748, 776–777 (1996) (SCALIA, J., concurring in part and concurring in judgment), and “[i]t is our settled policy to avoid an interpretation of a federal statute that engenders constitutional issues if a reasonable alternative interpretation poses no constitutional question.” Gomez v. United States, 490
U. S. 858, 864 (1989).