From my perspective as an outsider, most of the field of constitutional law seems intellectually corrupt. It seems that almost everybody does the following:
1. Start with a political philosophy–a view of what you want the government to be able to do and what you want to the government to to be forbidden from doing.
2. Take the Constitution as a given.
3. Reverse engineer a theory of constitutional interpretation such that it turns out–happily!–that the Constitution forbids what you want it to forbid and allows what you want it to allow.
The rest is just as good.