Just to clarify a few matters, now that I see the Hill has picked up on this idea that somehow Senator Rand Paul is blocking something the folks at Global Financial Integrity are calling "FATCA implementation treaties":
1) There is no such thing as a FATCA implementation treaty. There are things called "intergovernmental agreements" (IGAs) which the US has been signing with some countries, which override FATCA, making the unilateral statute less onerous than it otherwise would be. Anyone who is "pro" anti-evasion legislation (is there anyone who isn't?) can be wildly enthusiastic about FATCA, more or less enthusiastic about IGAs (perhaps less since they tend to water FATCA down), and basically uninterested in double tax conventions, which do almost nothing to combat tax evasion. Conversely, anyone who doesn't like FATCA can be enthusiastic about IGAs because they water down FATCA and leave more room for non-compliance, and wildly enthusiastic about double tax treaties, since they do nothing, FATCA-wise.
2) Double tax conventions are not needed to implement FATCA. FATCA is a domestic US statute which needs no implementing anything of any kind. It is already the law, it is already in force, and it is already in action. So let us put to rest any notion that any international agreement could have any impact on the implementation of FATCA. FATCA is going to be difficult to enforce, no question about that. But it is existing law. Its administrative and above all political difficulties are what have led Treasury--after FATCA was already law--to start thinking about asking for help.
3) Double tax conventions are not needed to implement IGAs. The IGAs are Treasury's way of reducing the administrative and political burden Congress created for it with FATCA. An IGA would override the FATCA statute to make life easier for both the Treasury and foreign banks. But the IGAs are most certainly not treaties, in fact, just what exactly they are is muddled indeed. Treasury has suggested that to the extent the US undertakes anything in these IGAs (which is precious little, at best), they are "interpretive" in nature that is, they interpret existing tax treaties. Hence perhaps some confusion: if you want an IGA which involves the US giving you anything back, Treasury is suggesting you may need a treaty for the IGA to "interpret." To be sure, the idea that IGAs are interpretive in nature is a stretch: IGAs are really just sole executive agreements. Treasury is not seeking any kind of congressional approval for them. Therefore no one in congress-Sen Paul or otherwise-can block them by holding up any kind of international agreement. The only way to block an IGA would be through a direct challenge to their legitimacy given that they violate the treaty power, which is supposed to be shared with the senate. Perversely, perhaps, if the IGAs were legitimate treaties, they could be blocked by Sen. Rand. But they are not, so they cannot be.
4) Switzerland already has a double tax treaty with the USA so even if the Switzerland IGA was the kind that theoretically needs a DTT to "interpret" (namely, a Model 1-style), the treaty needed is already in place, with language on information sharing ready to be "interpreted" by an IGA. However, Switzerland has not signed a Model 1 IGA with the US, it has signed a so-called "Model 2" IGA, which is non-reciprocal and binds the US to virtually no action whatsoever. It is for this reason, perhaps, that Treasury has already claimed that Model 2 agreements do not even need a treaty to interpret: that is, the Treasury has suggested that even non-treaty countries can get a Model 2 IGA. So you never need a treaty with Switzerland to implement the proposed IGA. Blocking the Swiss treaty may do a lot of things, but stopping the FATCA IGA is most certainly not one of them
5) Hungary and Luxembourg also already have DTTs in place. They are both old (Hungary-1979, Lux-1962) but each has info exchange language (Hungary-art 23, Lux art 28) so Treasury could again "interpret" these with an IGA, again by the logic that the IGAs are interpretive in nature. Of course, there is no IGA yet for Hungary that I know of, and Luxembourg is still mulling things over. If either goes with model 1, fine, DTT already in place. If they go with model 2, see above re Switzerland. Therefore there is no blocking of FATCA, an no blocking of FATCA IGAs, with respect to either of these countries, either.
Summary: IGAs are wholly unconnected to DTTs. The mistaken connection being made between these two completely unrelated legal instruments is understandable: it has been borne of Treasury's attempt to cast the IGAs as interpretive in nature, which at best is an ad hoc attempt to fix a legal impossibility of congress' own creation, without actually explaining their legal pedigree. But DTTS are in no way necessary to the implementation of FATCA.