One good answer is from Tim O’Neill:
People were generally very familiar with the Bible pre-1900, so the figures usually cited as the epitome of evil tended to be Judas Iscariot, Herod the Great or, most commonly, the Pharaoh of the story of Moses in Exodus. In Common Sense, Thomas Paine wrote: “No man was a warmer wisher for reconciliation than myself, before the fatal nineteenth of April, 1775 [the date of the Lexington massacre], but the moment the event of that day was made known, I rejected the hardened, sullen Pharaoh of England forever.” The Confederates referred to Abraham Lincoln as “the northern Pharaoh” and abolitionists in turn called slaveowners “modern Pharaohs”. Americans also referred to all tyrants by comparing them to King George III and Napoleon was often cited as the ultimate bogeyman in Britain. But generally it was Pharaoh who was used the way we use Hitler.
Did they have something akin to Godwin’s Law back then: “if you have to mention the Pharaoh, you’ve lost the argument!” Somehow I don’t think so. A link to the Quora forum is here.
Update: It seems Brian Palmer deserves credit for the information behind that answer.