Peter Spiro has a post up over at opinio juris on a pending US constitutional challenge to FATCA, of interest. No surprise, I agree with him that the strongest case is likely to be found in the violation of the treaty power (and not just because he points to my own work on the subject!) He says:
The Treaty Clause argument is a plausible one, the doctrinal terrain at least unsettled. The FATCA agreements enjoy implied congressional authorization, at best, in the form of prior tax treaties. ...There is a lot of history behind sole executive agreements but not much judicial precedent. ... Could this be another platform for the Supreme Court to advance its formalist turn in foreign relations law?A very good question.