It should be an easy case to win.
Article 1 - The Legislative Branch
Section 10 -
Powers Prohibited of StatesNo State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts;
pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.
No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
http://en.wikipedia.org/wiki/Ex_post_facto_lawn ex post facto law (Latin for "from after the action" or "after the fact"), also called a retroactive law, is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law.
In criminal law, it may criminalize actions that were legal when committed;
it may aggravate a crime by bringing it into a more severe category than it was in when it was committed;