They describe themselves:
DE FIDE is a non-profit association founded specifically to use every available means of Canon Law to defend the Faith and Church from Heresy and other grievous crimes. To accomplish its mission, DE FIDE initiates lawsuits in Ecclesiastical Court to protect the rights of the faithful and unbaptized.In terms of defense I've noticed one thing already. De Fide only goes after pro choice politicians. They do not go after politicians which are pro death penalty or violate the just war doctrine.
In particular readers may want to focus on their very careful distinctions they make regarding the high bar required to establish a heresy vs. a simple act of disobedience. This has the making of a heresy defense.