In a letter he sent to Comey on Sunday, Reid alleged that the FBI director broke the Hatch Act, a federal law passed in 1939 that limits the political activities of federal employees.
"Your actions in recent months have demonstrated a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another," Reid wrote.
Reid also noted that all Justice Department employees were re-apprised of their responsibilities under the Hatch Act in a March 2016 memo.
The retiring Democratic leader concluded his letter by indicating that he has lost trust in Comey.
"When Republicans filibustered your nomination and delayed your confirmation longer than any previous nominee to your position, I led the fight to get you confirmed because I believed you to be a principled public servant," he wrote. "With the deepest regret, I now see that I was wrong."
The FBI did not immediately respond to a request for comment.
Comey told Congress of the decision in a letter Friday afternoon, later saying he felt compelled to do so because he had previously assured lawmakers and the public that the FBI investigation was "completed."