This MLAT?
"101st Congress
1st Session
October 24, 1989, 5:15 p.m.
Page S-13889 Temp. Record
Vote No. 267
MLAT TREATY WITH CANADA/Ratification
...By unanimous consent, two amendments of understanding would be included in each of the MLAT resolutions of ratification. The first amendment would add that nothing in the treaty requires or authorizes legislation or other action by the United States that is prohibited by the U.S. Constitution... "
MLAT TREATY WITH CANADA/Ratification
"...nothing in the treaty requires or authorizes legislation or other action by the United States that is prohibited by the U.S. Constitution... "
Actually, you will find language like that in most MLAT's between major nations. That's because neither nation can strong arm the other. The real problems are the MLAT's that the US has intimidated or bribed many smaller nations into signing. By offering a small tax haven nation half of everything that they turn over to US authorities and threatening sanctions against them if they don't, the US authorities often force/entice the smaller nations to sign very one-sided MLAT's.
Remember that there are currently over 50 MLAT's in force between the US and other countries. Only a handful of those are with other large nations, that can stand up to the bullying tactics, that the US has become known for in recent years.
Only a small percentage of Americans try to legally shelter their assets in Canada. That's because it's much easier to recognize legal tax savings in a dozen or so Caribbean nations. Those are the ones that I would worry about. The fact is, that every MLAT is different. Some are not bad at all. But, most have one or more serious flaws and even those have different flaws. It isn't the 10 or so fairly reasonable MLAT's that are the problem. It's the 40 or so one-sided MLAT's between the US and the small nations most favored for legal tax shelters and privacy that should be of major concern.
I must agree with taxcontrol on the idea of an amendment that would specifically limit what treaties can be signed and what provisions of existing treaties might be ruled invalid, based upon the Constitution.