Democrats have attacked Fagg for acting like a candidate well before he filed his statement of candidacy with the FEC last month. That's the American Democracy Legal Fund's assertion, too -- that he was speaking about his candidacy in public and in the media while he was still legally in "testing the waters" mode.
So what distinguishes testing the waters from actual campaigning?
"There are two ways that you can become a candidate -- based on what you say and what you do," Toner said.
Announcing a candidacy is an obvious giveaway that someone is no longer testing the waters and is a candidate, subject to regular FEC reporting.
But becoming a candidate can also be more subtle, and sometimes, harder to prove.
According to the FEC, actions that trigger candidacy include: someone making a statement referring to themselves as candidate, using "general public political advertising to publicize their intention to campaign," taking action to qualify for the ballot or "conducting activities over a protracted period of time or shortly before the election."
One of the less obvious ways someone could become a candidate? Raising more money than "reasonably needed" to test the waters.
But the FEC doesn't specify how much is too much.
"Once an individual begins to campaign or decides to become a candidate, the testing the waters period ends," FEC guidelines say. A 2015 FEC advisory opinion clarifies that someone who's raised or spent more than $5,000 becomes a candidate "when he or she makes a private determination that he or she will run for federal office."
Democrats argue that Fagg's plan was to run for Senate all along.