But, if such a bill were enacted, would it be an ex post facto law? Such a law is prohibited by the United States Constitution - see Article I, Section 9, paragraph 3 which provides that - "No bill of attainder or ex post facto law shall be passed." What is an ex post facto law anyway? Can a law be retrospective without being ex post facto?
By way of a partial answer, take a look at Statler v. U.S. Savings & Trust Co. of Conemaugh, 192 A. 250 (Penn. 1937). A statute can be retrospective in nature and not be an ex post facto law where the statute in question does not impose a criminal penalty.
The court in Quinteros v. Hernandez, 419 F. Supp. 2d 1209 (C.D. Calif. 2006) may have summed up the overall state of the law best -
"Article I of the United States Constitution provides that neither Congress nor any state shall pass an ex post facto law. U.S. Const. Art. I, § 9, cl. 3; Art. I, § 10, cl. 1. “Although the Latin phrase ‘ex post facto’ literally encompasses any law passed ‘after the fact,’ it has long been recognized ... that the constitutional prohibition on ex post facto laws applies only to penal statutes which disadvantage the offender affected by them.” Collins v. Youngblood, 497 U.S. 37, 41, 110 S.Ct. 2715, 2718, 111 L.Ed.2d 30 (1990). The ex post facto clause is “aimed at laws that ‘retroactively alter the definition of crimes or increase the punishment for criminal acts.’ ” California Dep't of Corr. v. Morales, 514 U.S. 499, 504, 115 S.Ct. 1597, 1601, 131 L.Ed.2d 588 (1995) (citations omitted). Thus, an ex post facto law “punishes as a crime an act previously committed, which was innocent when done[,] which makes more burdensome the punishment for a crime, after its commission, or which deprives one charged with crime of any defense available according to law at the time when the act was committed....” Collins, 497 U.S. at 42, 110 S.Ct. at 2719 (quoting Beazell v. Ohio, 269 U.S. 167, 169-70, 46 S.Ct. 68, 68-69, 70 L.Ed. 216 (1925)); Stogner v. California, 539 U.S. 607, 612, 123 S.Ct. 2446, 2450, 156 L.Ed.2d 544 (2003).Do the bills pending in Congress impose criminal liability? No, only a new tax liability. Thus, the ex post facto clause does not pertain.