Treaties take precedence over state rights.
Here is a legal analysis -
http://www.legalweekblogs.com/legalvillage/2008/03/medellin_case_shows_theres_no.html
Not being a lawyer I can’t be sure, but if it is a legal matter then its for the Courts to intervene, not Congress. And the Supreme Court has already struck down the attempt by the Executive Branch to intervene. So what’s left?