The mortgage market has paid close attention to a lawsuit brought by PHH Mortgage that challenges the constitutionality of the Consumer Financial Protection Bureau, and the Trump administration’s recent move to side with the mortgage lender. Now, the Treasury Department is making a similar argument that the structure of the Federal Housing Finance Agency is also unconstitutional. Both the CFPB and FHFA, the regulator of Fannie Mae and Freddie Mac, are independent agencies led by a single director whom the president can only fire for cause. In an advisory filed March 24, the Treasury backed...