Senator Schumer is drafting anti-inversion legislation that would reach back all the way to April 17, 1994 — more than 20 years ago — to impose adverse tax consequences on companies that moved their headquarters out of the U.S. Bloomberg News has the story. (Link via Economic Polices for the 21st Century.)
The Heritage Foundation guide to the Constitution has some useful commentary on the prohibition, in the Constitution's Article I, against the passage of "ex post facto" laws. The dominant interpretation seems to be that it applies only to criminal law, and not to taxation. But the spirit of the rule of law that led to the constitutional provision would certainly tend to argue against changing the tax law in a way that reaches backward to change the rules years after companies relied on those rules to organize their affairs. If Senator Schumer gets away with this one, in other words, what's to stop him from retroactively raising the income or capital gains taxes that applied to individuals during the George W. Bush presidency, or even reversing the Clinton-Gingrich capital gains tax cuts and sending taxpayers retroactive tax bills for the years 1998 or 1999? If Senator Schumer goes on in this vein, perhaps some of the donors who have supported him over the years will want to request, retroactively, their money back. They may need it to pay the taxes they thought they had saved but would now owe under his legislation.