Comey spent 15 minutes publicly chastising Hillary Clinton before announcing that she had done nothing illegal and the FBI weren't recommending an indictment. That was unprecedented, and he did it without informing anyone of his intentions ahead of time.
Then he goes and writes an incredibly vague letter that looks damning to Clinton, though having nothing to do with her, and releases it to a Republican Congress RIGHT as they're scrambling for ANYTHING to prevent a down-ticket catastrophe.
I'm sorry, but the bigger reach here is to argue that he's NOT playing politics.