From The Washington Post:

There are questions, however, about Congress’s authority to pass the bill, which seems to stretch the limits of the commerce power and of the 14th Amendment’s enforcement power, as discussed in posts by Josh Blackman and Joseph Blocher, among others. But there may be another way.

In a letter sent today, Stephen Sachs, Randy Barnett and I argue that Congress should not rely on the commerce power but should instead rely on the Full Faith and Credit Clause.