It's a name that's way easier to write down than his actual full name, Cornelius Harvey McGillicuddy IV.
Yes, he's got a nickname for both his first and last names. But it's become much more than just his great-grandfather's nickname that an entire line of Connies Mack have appropriated for themselves -- it's how he's listed everywhere, including on actual Congressional legislation, where he is referred to as "Mr. Mack," like he's some kind of preschool teacher with a last name too hard for kids who just figured out how to talk. It's rebranding taken to the extreme, and it looks like it's totally OK to do.
Basically, regulations at numerous levels are worded in such a way that if you get elected to Congress, you can call yourself whatever you want, which opens the door to all kinds of potential silliness. If Cornelius McGillicuddy can go by "Connie Mack," what's to keep Sen. Marco Rubio from going by "Markie Rubzz"? I'd vote for a Rubzz. I don't care what his position on immigration is.
When it comes to campaign finance reporting, federal law refers only to "the individual's name" being required on file. The closest it looks like McGillicuddy has come to breaking a rule is in the instructions for the FEC form to declare candidacy for federal office, which reads (emphasis mine), "LINE 1. Print or type complete name and mailing address of the candidate."
An FCC spokesperson said, "Beyond this, I don't see any elucidation of the naming convention for candidates' names on FEC reports."
The House Office of the Clerk said congressmen simply designate what they want their official name to be when they're elected, and that's the name put on voting records and office doors and whatnot.
The Committee on House Administration is where incoming congressmen go to set all of that up. A call for comment to its press officer wasn't returned late last week.
One place Corny McGill can't hide his real name? Court. So when he gets a ticket for driving without a seat belt and having expired tags, you get a glimpse of his secret (actual) identity: