Authors:

These are party-sanctioned debates. This is a presidential election, you show up at the debates. These are the rules. We have a series of unwritten rules of how campaigns are run, and everybody has followed those rules consistently over the decades. And no one has really even seriously thought about breaking them.

"Roundtable: Donald Trump's Media Tactics". "All Things Considered" with Michel Martin, www.npr.org. January 30, 2016.
These are party-sanctioned debates. This is a presidential election, you show up at the debates. These are the rules. We have a series of unwritten rules of how campaigns are run, and everybody has followed those rules