What kind of constitutional challenge?
An enumerated powers challenge is out since Congress is defining marriage for purposes of its spending powers and because, after Raich, commerce clause challenges are much tougher. Certainly since marriage is supposed to be a creature of state law, one could argue that Congress has overstepped its bounds. That sounds like a 10th Amendment challenge, but I don’t think a 10th Amendment challenge would work. That doctrine was never really all that established before Garcia v. San Antonio unestablished it. I’m not sure if its officially dead, but even the more conservative members of the court were moving away from the 10th Amendment in favor of the enumerated powers line of cases in Morrison and Lopez. But, again, after Raich thats a very tough argument to make. Can a Congressional law be challenged as violating the Full Faith and Credit clause? I’ve never heard of that, but I could see a potential argument that the Full Faith and Credit clause requires a state to recognize another state’
