The source is federal law. I didn’t intend that the source would be an issue because the link provides references to each of the federal laws that enable each emergency power listed. In this article the Heritage Foundation references the Brennan study and the 136 powers that were identified without question:
I think it’s named after one of my favorite childhood movie actors, the great Walter Brennan. No! Sorry that’s fake news, it is in fact named for former Supreme Court justice William J. Brennan, a liberal.
Because I’m not a lawfag, even reading federal laws need some kind of explanation or commentary for me. The Herigate Foundation article looks useful, thanks.