So, what's the Hatch Act? And did Comey really break it?
The Hatch Act was passed by Congress back in 1939. It's meant to prevent federal employees from engaging in some forms of political activity.
As a member of the FBI, Comey is prohibited under the act from using his authority to interfere with an election. Some of Comey's critics say handing Republican lawmakers political ammunition against Clinton days before the presidential election basically adds up to electoral interference.
But it'll be tough to make that case against Comey without some proof that the FBI director intended to tip the scales of this election.
Comey argued in an internal FBI memo that he sent the letter to keep his testimony before Congress accurate. He says he told Congress the investigation was finished several months ago, and he felt compelled to let lawmakers know that wasn't true anymore.
The Hatch Act isn't a criminal statute, so Comey won't be going to jail over this or anything. Enforcing the act is the job of the Office of Special Counsel, an independent federal agency.
The office most recently reprimanded Housing and Urban Development Secretary Julián Castro for breaking the Hatch Act by praising Clinton during an official interview. It didn't recommend any official punishment against Castro.
Incidentally, the Hatch Act doesn't apply to the president or the vice president. So President Obama's free to stump for Clinton as much as he wants.